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L’influence de la coupe partielle sur le parasitisme de l’arpenteuse de la pruche et de
la tordeuse des bourgeons de l’épinette
Mémoire
Martin Lukas Seehausen
Maîtrise en sciences forestières
Maître ès sciences (M.Sc.)
Québec, Canada
© Martin Lukas Seehausen, 2013
iii
Résumé
L’influence de la coupe partielle à deux intensités, soit 25% et 40% de réduction de la surface
terrière, sur le taux de parasitisme chez l’arpenteuse de la pruche, Lambdina fiscellaria (Guinée), et
la tordeuse des bourgeons de l’épinette, Choristoneura fumiferana (Clemens) a été étudiée. Un an
après les traitements, le taux de parasitisme chez les chrysalides de l’arpenteuse de la pruche a
significativement diminué dans les parcelles à 40% de coupe partielle, alors que l’année suivante,
aucune influence significative des traitements n’a été démontrée. Deux ans après les traitements, le
taux de parasitisme chez les larves de la tordeuse des bourgeons de l’épinette a significativement
diminué suite aux coupes partielles. Cependant, cet effet n’a pas été observé l’année suivante. En
outre, la présente étude démontre que l’utilisation de nourriture artificielle pour produire des larves
sentinelles de la tordeuse n’influence pas significativement les taux de parasitisme observés sur le
terrain.
v
Abstract
This study investigates the influence of partial cutting at two intensities, 25% and 40% stand basal
area removed, on parasitism of hemlock looper, Lambdina fiscellaria (Guinée), and spruce
budworm, Choristoneura fumiferana (Clemens). In plots with 40% partial cutting the parasitism of
hemlock looper pupae was significantly reduced one year after treatments but, the treatments did
not significantly influence parasitism the following year. Two years after treatments, parasitism of
fourth and fifth instar spruce budworm larvae was significantly reduced by partial cutting of both
intensities. However, once again, this influence disappeared in the following year. In addition, the
present study demonstrates that artificial rearing diet of the spruce budworm does not significantly
influence parasitism of exposed larvae in the field.
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Table des matières
Résumé ............................................................................................................................................... iii Abstract ............................................................................................................................................... v Liste des tableaux ............................................................................................................................... ix Liste des figures ................................................................................................................................. ix Avant-propos .................................................................................................................................... xiii Remerciements .................................................................................................................................. xv
Introduction ....................................................................................................................................... 1 Problématique ................................................................................................................................. 1 L’arpenteuse de la pruche et ses parasitoïdes ................................................................................. 2 La tordeuse des bourgeons de l’épinette et ses parasitoïdes ........................................................... 4 Les traitements sylvicoles comme outils de gestion des insectes défoliateurs ............................... 6
Short-term influence of partial cutting on hemlock looper parasitism ........................................ 9 Résumé ......................................................................................................................................... 10 Abstract ......................................................................................................................................... 11 Introduction................................................................................................................................... 12 Material and Methods ................................................................................................................... 13
Statistical analysis .................................................................................................................... 15 Results .......................................................................................................................................... 15 Discussion ..................................................................................................................................... 17
Conclusion ................................................................................................................................ 19 Acknowledgements ....................................................................................................................... 19 References..................................................................................................................................... 19
Does spruce budworm rearing diet influence larval parasitism? ............................................... 27 Résumé ......................................................................................................................................... 28 Abstract ......................................................................................................................................... 29 Acknowledgements ....................................................................................................................... 32 References..................................................................................................................................... 33
Influence of partial cutting on parasitism of endemic spruce budworm populations .............. 37 Résumé ......................................................................................................................................... 38 Abstract ......................................................................................................................................... 39 Introduction................................................................................................................................... 40 Material and Methods ................................................................................................................... 41
Statistical analysis .................................................................................................................... 42 Results .......................................................................................................................................... 42 Discussion ..................................................................................................................................... 43
Conclusion ................................................................................................................................ 45 Acknowledgments ........................................................................................................................ 46 References..................................................................................................................................... 46
Conclusion........................................................................................................................................ 53
Bibliographie ................................................................................................................................... 57
ix
Liste des tableaux
Table 1-1: Parasitism rates (%) by species attacking hemlock looper pupae and larvae. ... 23
Table 1-2: Influence of partial cutting on environmental temperature and relative humidity
(RH) (means ± SEM). ................................................................................................... 24
Table 2-1: Percent parasitism (mean and SEM, n=4x9) and statistical analysis of parasitism
of foliage-fed (n=325) and diet-fed (n=347) spruce budworm larvae. ......................... 35
Table 3-1: Parasitism rates (means (%) ± SEM) of species attacking 4th
and 5th
instar
spruce budworm larvae for control stands and two intensities of partial cutting (25 and
40% stand basal area reduced) in 2011 and 2012. ........................................................ 50
Liste des figures
Figure 1-1: Mean rates of overall attack (A, B) and attack by A. ontario (C, D) as a
function of exposure periods in 2010 and 2011 following partial cutting treatments, 0,
25 and 40% basal area reduction. Lines followed by the same letter do not differ
significantly at P < 0.05 according to a priori comparisons. ........................................ 25
Figure 1-2: Mean number of A. ontario females (A, B) and males (C, D) per trapping day
caught with Malaise traps as a function of time (ordinal date), one (2010) and two
(2011) years following partial cutting treatments, 0, 25 and 40% basal area reduction.
Lines followed by the same letter do not differ significantly at P < 0.05 according to a
priori comparisons. ....................................................................................................... 26
Figure 2-1: Mean rates of (A) overall attack; (B) attack by T. rostrale; (C) attack by E.
cacoeciae; and (D) attack by other parasitoid species, as a function of time (ordinal
date) in 2011 (black: foliage-fed larvae; dashed grey: diet-fed larvae). ....................... 36
Figure 3-1: Pupae were placed in each plot on balsam fir branches by binding together
several adjacent shoots and placing one single pupa in the centre. .............................. 51
Figure 3-2: Mean rates of overall attack (A, B), attack by T. rostrale (C, D), E. cacoeciae
(E, F) and other parasitoid species (G, H) as a function of time (ordinal date) in 2011
and 2012 for two intensities of partial cutting (25 and 40% stand basal area reduced)
and control stands. ........................................................................................................ 52
xi
“Knowledge is powerful, only if it is shared.” Henry Lickers
xiii
Avant-propos
Le présent mémoire de maîtrise est composé de trois chapitres écrit en anglais représentant des
articles scientifiques qui vont être soumis à des revues scientifiques pour publication dans les
prochains moins ainsi que d’une introduction et une conclusion écrit en français. L’ordre
d’énumération des auteurs des différents chapitres est choisi en fonction de leur contribution à la
réalisation du projet. L’auteur principal des trois chapitres est le candidat à la maîtrise du présent
mémoire, M. Lukas Seehausen. Pour la préparation de chacun des chapitres, j’ai planifié et réalisé
les expériences, j’ai analysé et interprété les résultats et j’ai écrit le manuscrit. Le Dr Éric Bauce et
le Dr Jacques Régnière ont contribués à la planification des expériences, à l’analyse et à
l’interprétation des données et à la rédaction des trois chapitres. Le Dr Richard Berthiaume a
participé à la planification des expériences du premier et du dernier chapitre et à la rédaction du
premier chapitre. Les détails concernant la contribution de chacun des auteurs et des autres
personnes impliquées dans le présent projet de maîtrise sont décrits dans la partie des
remerciements.
xv
Remerciements
J’aimerais remercier sincèrement mon directeur de recherche, le Dr Éric Bauce, pour m’avoir
accueilli dans son équipe de recherche. Je n’oublierai jamais sa tranquillité et sa serviabilité malgré
son poste de vice-recteur exécutif et au développement. Son attitude positive et toujours décidé m’a
donné la certitude et le soutien nécessaire pour réaliser le présent travail. Merci beaucoup de
m’avoir donné cette occasion exceptionnelle de réaliser un de mes rêves, une formation en
entomologie forestière. C’était un honneur de travailler avec toi Éric !
Également, j’aimerais chaleureusement remercier mon co-directeur de recherche, le Dr Jacques
Régnière, pour sa contribution exceptionnelle au présent projet. Grâce à de nombreuses discussions,
sa grande disponibilité et sa faculté de me ramener sur terre, il m’a permis d’acquérir des
connaissances indispensables pour réaliser ma recherche avec professionnalisme. Particulièrement,
sa grande sincérité dans le partage de ses connaissances avec moi m’a impressionné beaucoup ! Tu
es un grand modèle pour moi, Jacques !
Le Dr Richard Berthiaume, je tiens à remercier pour son travail comme coordonateur des projets de
recherche du consortium iFor, ce qui implique d’avoir activement participé à la rédaction de la
demande de subvention pour le présent projet et d’avoir contribué à l’établissement du dispositif
expérimental. Les discussions avec lui, ses commentaires et ses corrections pertinentes de l’article
sur l’arpenteuse de la pruche (premier chapitre) m’ont aidé à réfléchir sur les résultats d’une
nouvelle façon, ce qui m’a aidé pendant la rédaction de mon mémoire.
Sans l’aide des nombreux étudiants, le présent travail n’aurait jamais été possible. Pour cette raison,
j’aimerais exprimer ici ma profonde gratitude à Nicolas Giasson pour sa contribution exceptionnelle
tout au long de ce projet, pour son aide fiable et autonome sur le terrain et au laboratoire, ainsi que
pour les nombreuses corrections des textes en français. Merci beaucoup également à Michel Lemay,
Pascal Charron, David St-Pierre et Éric Tremblay-Bérubé pour leur aide sur le terrain et au
laboratoire, vous m’avez aidé beaucoup ! Également un gros merci à tous les étudiants qui ont
récolté les échantillons des pièges Malaises et Luminoc sur le terrain et à ceux qui ont contribué au
triage des échantillons des pièges Malaises. Spécialement, je remercie chaleureusement à Michelle
Addy pour les dessins et son aide sur le terrain.
Du laboratoire d’entomologie forestière de l’Université Laval (UL), j’aimerais spécialement
remercier Madame Paule Huron pour son aide technique au laboratoire et sur le terrain, pour
m’avoir introduit à ses contacts qui ont joué un rôle important dans mes travaux et pour son support
moral en tout temps. Également, Monsieur Martin Charest mérite des remerciements pour son aide
technique avec les stations météorologiques. Un gros merci aussi à mon ami et collègue de
laboratoire Alvaro Fuentealba pour la multitude de discussions enrichissantes, pour la révision de
plusieurs textes ainsi que pour son encouragement dans toutes les situations de la vie.
Plusieurs personnes au Centre de foresterie des Laurentides (CFL, Service canadien des forêts) qui
m’ont aidé mérite également un grand merci : Dans un premier temps le Dr Michel Cusson, pour
son accueil dans son laboratoire, pour certaines expériences sur le parasitoïde Tranosema rostrale et
pour sa sincérité dans la multitude de discussions fructueuses. Merci également au Dr Christian
Hébert, pour avoir partagé des informations sur l’arpenteuse de la pruche avec moi. Je tiens aussi à
remercier Monsieur Georges Pelletier pour l’identification de certains parasitoïdes et pour sa
patience lorsqu’il m’a montré la technique de photographie avec un microscope. Merci également à
Monsieur Yves Dubuc pour l’échantillonnage des pièges Malaise et des pièges Luminoc, ainsi qu’à
Pierre Duval pour m’avoir fourni des résultats du modèle saisonnier de la tordeuse des bourgeons
de l’épinette et aussi pour son aide avec la technique des hôtes sentinelles.
J’aimerais également souligner la contribution de certaines personnes du Ministère des Ressources
Naturelles et de la Faune du Québec (MRNFQ) : Merci à Monsieur Pierre Therrien pour avoir
accepté de faire identifier les parasitoïdes de la tordeuse des bourgeons de l’épinette dans son
laboratoire. Je n’oublierai jamais la grande contribution de Madame Guylaine Trudel et de Madame
Claudine Dussault identifiant avec une diligence incroyable les parasitoïdes ainsi que la
contribution de Monsieur Denis Simoneau, qui a fourni les larves de tordeuse des bourgeons de
l’épinette toujours avec une très grande fiabilité. Merci beaucoup !
Pour l’identification de certains parasitoïdes, je remercie le Dr Owen Lonsdale, le Dr Andrew
Bennett, le Dr José Fernandez et le Dr James E. O’Hara d’Agriculture et Agroalimentaire Canada.
Pour l’aide avec l’analyse statistique, j’aimerais remercier Madame Michèle Bernier-Cardou (CFL),
le Dr André Desrochers (UL) et Monsieur Stéphane Daigle (CEF, Université de Montréal).
Monsieur Pierre Racine (CEF, UL) m’a beaucoup aidé avec ArcGIS et avec la correction de divers
textes et la Dr Véronique Martel (CFL), Amélie Rivet (CEF, UL) ainsi que Shuva Hari Gautam
avec la correction de plusieurs textes de ce mémoire : merci beaucoup ! Merci également à Josh
Novac pour la révision de plusieurs textes et des discussions constructives ainsi qu’à Christian Roy
pour avoir partagé sa connaissance des modèles statistiques avec moi. Je remercie le Dr Sandy
Smith pour avoir accepté d’évaluer le présent mémoire.
xvii
Je tiens à remercier le Dr Michael Boppré, le Dr Tim Burzlaff et Madame Salzer, pour leur
introduction dans le fabuleux monde de l’entomologie et Madame Opfermann pour son excellent
cours de français à l’école secondaire sans lequel il aurait été beaucoup plus difficile d’étudier à
Québec !
Mes parents, Hilde et Udo Seehausen, j’aimerais bien chaleureusement remercier pour me donner la
possibilité de faire ma maîtrise à l’étranger, pour leur support dans mes décisions en tout temps et
aussi pour leur aide financière. Sans vous, il ne m’aurait pas été possible de compléter la maîtrise à
Québec ! Également, mille mercis aux Drs Wolfgang et Giota Zimmermann ainsi qu’à Madame
Irmgard Schnellbächer pour leur soutien financier et moral.
Je remercie spécialement et du fond du cœur ma chère conjointe Julieta Caballero, qui m’a
accompagné dans les bons comme dans les mauvais jours et qui m’a donné la force et l’inspiration
pour la finalisation du présent travail.
Finalement, ce projet n’aurait pas pu se réaliser sans le soutien financier de plusieurs organismes
partenaires du consortium de recherche iFor. Merci au Conseil de recherches en sciences naturelles
et en génie du Canada (CRSNG-NSERC), au MRNFQ, au Conseil de l’Industrie Forestière du
Québec (CIFQ), au Service Canadien des Forêts et à la Société de Protection des Forêts contre les
Insectes et les Maladies du Québec (SOPFIM).
1
Introduction
Problématique
L’arpenteuse de la pruche, Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae), et la
tordeuse des bourgeons de l’épinette, Choristoneura fumiferana (Clemens) (Lepidoptera:
Tortricidae), causent la grande majorité de la défoliation des conifères d’Amérique du Nord
(MacLean, 1990; National Forestry Database, 2012). Les épidémies de ces deux insectes peuvent
couvrir d’énormes superficies. Lors de la dernière épidémie majeure de la tordeuse des bourgeons
de l’épinette, celle-ci a causé de la défoliation sur environ 55 millions d’hectares (Blais, 1983) alors
que la plus importante épidémie d’arpenteuse de la pruche a couvert plus de 925 000 hectares de
forêt (Hébert & Jobin, 2001). Lors d’une infestation de la tordeuse des bourgeons de l’épinette, la
mort des arbres hôtes survient généralement après quatre ou cinq ans de défoliation répétée de la
pousse annuelle (MacLean, 1980; Blais, 1981), tandis que les conifères attaqués par l’arpenteuse de
la pruche peuvent mourir après seulement une à deux années de défoliation (Hudak et al., 1978)
puisque cet insecte consomme toutes les classes d’âge du feuillage (Dobesberger, 1989). En
conséquence, les infestations de ces deux défoliateurs entraînent une perte massive de volume
ligneux pour l’industrie forestière (Coulombe et al., 2004). Par exemple, la dernière épidémie de la
tordeuse des bourgeons de l’épinette au Québec a causé des pertes d’environ 500 million de m3 de
fibre de bois (Coulombe et al., 2004).
Pour limiter la défoliation et la mortalité des arbres afin de réduire les pertes de volume, il est donc
nécessaire de prendre des mesures de contrôle. Par le passé, des arrosages aériens avec des
insecticides chimiques ont été effectués pour lutter contre les épidémies de ces deux défoliateurs
(MacDonald & Webb, 1963; Prebble, 1975; Dorais et al., 1995; Kettela, 1995). Cependant,
l’utilisation de ces produits chimiques a été condamnée par le public et des méthodes de contrôle
alternatives ont été développées (Cunningham, 1985). L’arrosage avec l’insecticide biologique
Bacillus thuringiensis var. kurstaki (Btk) a été proposé comme outil de contrôle alternatif et est
toujours utilisé avec succès aujourd’hui pour limiter les dommages de ces deux ravageurs forestiers
(West et al., 1989, 1997; Dorais et al., 1995; Fournier et al., 2010). De plus, plusieurs tentatives
pour trouver, introduire et établir d’autres agents de lutte biologique ont été mises à l’essai, mais
sans succès (Wilkes, 1946; Miller, 1963b; Mills, 1985; West & Kenis, 1997). Les pratiques
sylvicoles ont également été suggérées par plusieurs auteurs pour réduire les dommages causés par
la tordeuse des bourgeons de l’épinette et l’arpenteuse de la pruche (Crook et al., 1979; Martineau,
1984; Bauce, 1996; D’Amato et al., 2011; Fuentealba & Bauce, 2012b). Cependant, l’influence des
2
traitements sylvicoles sur le parasitisme de la tordeuse des bourgeons de l’épinette et l’arpenteuse
de la pruche était inconnue jusqu'à ce jour.
L’arpenteuse de la pruche et ses parasitoïdes
L’arpenteuse de la pruche est une espèce univoltine indigène d’Amérique du Nord (Hébert & Jobin,
2001). Les adultes émergent entre la fin août et le début d’octobre, s’accouplent et les femelles
déposent les œufs après une courte période pré-oviposition (<2 jours) (Berthiaume et al., 2009).
L’insecte passe l’hiver au stade d’œuf et la larve émerge tard au printemps, après le débourrement
du sapin (Hébert & Jobin, 2001). Les jeunes larves commencent à se nourrir sur le feuillage de
l’année courante puis consomment le feuillage des années antérieures plus tard durant leur
développement (au stade 3) (De Gryse & Schedl, 1934; Watson, 1934; Carroll, 1956). Ces larves
ont un comportement gaspilleur lorsqu’elles s’alimentent car elles ne consomment que
partiellement les aiguilles. Ce comportement alimentaire particulier leur confère une importante
capacité destructrice du feuillage puisque les aiguilles partiellement consommées vont
éventuellement rougir et tomber au sol aggravant l’impact de la défoliation (Otvos et al., 1971).
Ainsi, cette espèce peut causer la mort des arbres après seulement une à deux années de défoliation
(Hudak et al., 1978). Il y a quatre à cinq stades larvaires chez l’arpenteuse de la pruche, selon
L’origine latitudinal la des populations (Berthiaume, 2007; Berthiaume et al., 2007). Au début du
mois d’août, les larves cherchent un endroit pour se transformer en chrysalide. Ces dernières se
retrouvent habituellement sur l’arbre hôte dans des crevasses, sous des lambeaux d’écorce, dans des
lichens (Watson, 1934), dans des vieilles souches (Carroll, 1956; Jobin & Desaulniers, 1981) et
dans les angles formés par les racines au niveau du sol (De Gryse & Schedl, 1934). Le
développement des chrysalides dure entre 16 à 20 jours (Carroll, 1956; Berthiaume, 2007). Les
mâles émergent quelques jours avant les femelles puisque leur nymphose se fait plus tôt en saison
(Carroll, 1956; Berthiaume et al., 2007).
Les épidémies de l’arpenteuse de la pruche ont été documentées surtout dans des peuplements
matures de sapins baumiers et de pruches du Canada, Tsuga canadensis ((L.) Carrière) (Otvos et al.,
1979; Trial, 1993; Hébert & Jobin, 2001). Cependant, la défoliation peut être observée sur d’autres
espèces de conifères ainsi que sur quelques espèces de feuillus dont l’épinette blanche, l’épinette
noire, le thuya occidental, Thuja occidentalis (L.), le pin blanc d’Amérique, Pinus strobus (L.),
l’érable à sucre, Acer saccharum (Marshall), le bouleau à papier, Betula papyrifera (Marshall) et le
peuplier faux-tremble, Populus tremuloides (Michaux) (Hébert & Jobin, 2001). Néanmoins, la
mortalité des arbres est plus importante dans les peuplements mûrs et surannés du sapin baumier
(De Gryse & Schedl, 1934; Carroll, 1956; Davidson & Prentice, 1967). Les régions les plus
3
affectées par des épidémies au cours du 20e siècle sont Terre-Neuve et certaines régions du Québec
(Hébert & Jobin, 2001). Des épidémies moins destructives ont également été rapportées en Ontario,
dans les provinces maritimes et aux États-Unis (Hébert & Jobin, 2001). En 2012, une nouvelle
épidémie d’arpenteuse de la pruche a été rapportée dans le parc des Laurentides au nord de la ville
de Québec.
Jusqu’au début des années 1980, les insecticides chimiques étaient utilisés pour contrôler les
épidémies d’arpenteuse de la pruche (Prebble, 1975; West et al., 1989). Par la suite, le Btk a été
utilisé avec succès comme agent de contrôle (West et al., 1989, 1997). Le climat, les virus et les
parasitoïdes peuvent aussi jouer un rôle important lors du déclin d’une épidémie (Otvos, 1973).
Cependant, une étude de Hébert et al. (2001) a démontrée que le parasitisme des œufs par
Telenomus spp. (Hymenoptera: Scelionidae) peut aussi être responsable pour empêcher le
déclenchement d’une infestation par l’arpenteuse de la pruche.
Les parasitoïdes des larves et des chrysalides peuvent également causés une mortalité importante
dans des épidémies d’arpenteuse de la pruche (Otvos, 1973; Turnquist, 1991). Dans des études
réalisées à Terre-Neuve (Carroll, 1956; Otvos, 1973), en Ontario (De Gryse & Schedl, 1934), au
Nouveau-Brunswick (Hartling et al., 1991), en Colombie-Britannique (Hopping, 1934; Turnquist,
1991) et en Alaska (Togersen, 1971), un total de 80 espèces de parasitoïdes ont été associés avec
l’arpenteuse de la pruche. La plupart de ces parasitoïdes émergent de l’hôte vers la fin du
développement larvaire ou durant le stade chrysalide (Hébert & Jobin, 2001). Certaines de ces
espèces sont considérées comme plus importantes au niveau du parasitisme, tels que les parasitoïdes
larvaires Apanteles flavovariatus (Muesebeck) (Hymenoptera: Braconidae) (Carroll, 1956, Otvos,
1973) et Winthemia occidentis (Reinhard) (Diptera: Tachinidae) (Otvos, 1979; Mills & Räther,
1990; Turnquist, 1991; Hébert & Jobin, 2001) ainsi que les parasitoïdes des chrysalides Itoplectis
conquisitor (Say) (Hymenoptera: Ichneumonidae) (Otvos, 1973, Hébert & Jobin, 2001), Apechthis
ontario (Otvos, 1973; Turnquist, 1991) et Aoplus velox (Cresson) (Hymenoptera: Ichneumonidae)
(Carroll, 1956; Togersen, 1971; Otvos, 1973). Cependant, la plupart des études sur le parasitisme
ont été réalisées avec des populations épidémiques et peu de données existent sur les parasitoïdes
dans des populations endémiques (Armstrong, 1996; Pardy, 2000).
4
La tordeuse des bourgeons de l’épinette et ses parasitoïdes
La tordeuse des bourgeons de l’épinette est une espèce indigène de l’Amérique du Nord. Il s’agit
d’un insecte univoltin. L’adulte émerge de la chrysalide au milieu de l’été et pond ses œufs sur le
feuillage des arbres hôtes après s’être accouplé. La larve du premier stade sort de l’œuf après
environ dix jours, construit son hibernaculum et mue au deuxième stade larvaire pour ensuite entrer
en diapause hivernale obligatoire (Han & Bauce, 2000). Le printemps suivant, la larve du deuxième
stade sort de son hibernaculum et commence à s’alimenter jusqu’au sixième stade larvaire. C’est
pendant ce dernier stade que la larve cause la majorité des dommages au feuillage en mangeant sept
à huit fois sa masse (Miller, 1977). La chrysalide est habituellement formée à partir de la mi-juillet
dans le refuge d’alimentation de la larve, où elle passe la totalité de sa période de développement,
soit de huit à douze jours (Miller, 1963a).
Les principales espèces hôtes de la tordeuse des bourgeons de l’épinette sont le sapin baumier Abies
balsamea (L.) Mill, l’épinette blanche Picea glauca (Moench) Vos, l’épinette rouge P. rubens Sarg
et l’épinette noire P. mariana (Mill.) BSP (Greenbank, 1963b). La vulnérabilité à la défoliation
varie entre les espèces hôtes et le sapin baumier serait plus vulnérable que les autres espèces
(Henninger et al., 2008). En outre, il a été démontré que les dommages les plus sévères causés par
la tordeuse des bourgeons de l’épinette ont eu lieu dans des peuplements mûrs et surannés
(Greenbank, 1963a).
Depuis plus de trois siècles, la tordeuse des bourgeons de l’épinette entre en épidémie sur une base
plus au moins régulière (Blais, 1965b; Morin & Laprise, 1990; Morin, 1994). Les débuts des
épidémies au Lac Saint-Jean, QC, Canada, ont été datés pour 1974, 1944, 1909 et, possiblement,
1832 (Morin & Laprise, 1990). La dernière grosse épidémie a commencé en 1968 à Aylmer en
Outaouais et s’est terminée en 1987 en Gaspésie (MRNF, 2005a). En outre, de 1995 à 2002, il y a
eu une épidémie plus restreinte en Outaouais (MRNF, 2005b). Récemment au Québec, les
populations de cette espèce ont atteint des niveaux épidémiques au Saguenay – Lac-Saint-Jean et
sur la Côte-Nord (MRNF, 2011), menant à l’hypothèse du début de la prochaine épidémie.
Plusieurs facteurs dépendant de la densité de la population ont été suggérés comme cause probable
de l’oscillation de ce défoliateur. Mis à part un important facteur de mortalité inconnu (Royama,
1984), plusieurs auteurs suggèrent le parasitisme comme facteur de contrôle naturel car celui-ci joue
un rôle important pendant le déclin d’une épidémie (Royama, 1984; Régnière & Nealis, 2007).
Cependant, l’efficacité des parasitoïdes est grandement réduite en l’absence d’autres mécanismes
limitant le développement de la tordeuse des bourgeons de l’épinette comme l’épuisement de la
nourriture et le climat (McGugan & Blais, 1959; Blais, 1960; Miller, 1963b; Blais, 1965a).
5
L’absence d’hôtes alternes pour les parasitoïdes peut aussi limiter le contrôle de la tordeuse des
bourgeons de l’épinette par le parasitisme. Presque tous les parasitoïdes attaquant cette espèce sont
connus ou soupçonnés d’être dépendants de la présence d’hôtes alternes pour compléter leur
développement (Miller, 1963b). Afin de pouvoir s’établir à un site donné, les hôtes alternes doivent
donc être présents (Blais, 1960; Miller, 1963b; Miller & Renault, 1976). Ainsi, lors d’une épidémie,
si le nombre d’hôtes alternes est relativement faible, la plupart des espèces de parasitoïdes auront un
impact limité sur la mortalité de la tordeuse des bourgeons de l’épinette. D’un autre côté,
l’abondance relative ainsi que l’impact de plusieurs parasitoïdes augmentent lorsque les densités de
la population de la tordeuse des bourgeons de l’épinette diminuent (Blais, 1965a). Par conséquent,
l’influence des différentes espèces de parasitoïdes sur la tordeuse des bourgeons de l’épinette
changent considérablement dans des populations endémiques (Blais, 1965a; Miller & Renault,
1976).
Par exemple, le braconide Meteorus trachynotus (Viereck) est un parasitoïde dépendant de la
tordeuse à bandes obliques, C. rosaceana (Harris) comme hôte hivernal afin de compléter son
développement (Maltais et al., 1989; Thireau & Régnière, 1995). Ce parasitoïde augmente en
abondance relative pendant les dernières années d’une épidémie de la tordeuse des bourgeons de
l’épinette et est considéré comme un ennemi naturel important pour accélérer le déclin des
épidémies (Blais, 1960; Miller, 1963b; Miller & Renault, 1976). Cependant, en situation
endémique, d’autres espèces de parasitoïdes gagnent en importance. L’ichneumonidae Tranosema
rostrale (Brischke), par exemple, est connu pour parasiter un nombre important de larves de la
tordeuse des bourgeons de l’épinette lorsque celles-ci sont rares (Cusson et al., 1998). Cependant, la
façon dont cette espèce passe l’hiver ainsi que sa possible dépendance à un hôte alterne pour
compléter son cycle restent toujours inconnues.
Trichogramma minutum Riley est le seul parasitoïde connu pour attaquer les œufs de la TBE (Tilles
& Woodley, 1984). Le taux de parasitisme par cette espèce peut atteindre 77% (Anderson, 1976)
mais il reste normalement plus bas que 15% chez la TBE (Miller, 1953; Quayle, 2003). En situation
naturelle, le taux de parasitisme de T. minutum n’est pas assez important pour avoir un impact sur
l’effondrement des épidémies de la TBE (McGugan & Blais, 1959; Smith et al., 1990). Cependant,
des relâchements de ce parasitoïde à la volée ont démontré une suppression significative des
populations épidémique de la TBE qui a eu pour conséquence une diminution de la défoliation
(Smith et al., 1990).
6
Les traitements sylvicoles comme outils de gestion des insectes
défoliateurs
Muzika et Liebhold (2000) indiquent que les approches sylvicoles comme outils de gestion contre
les insectes défoliateurs sont potentiellement intéressantes parce qu’elles sont peu coûteuses,
efficaces, durables et qu’elles ont peu d’impact sur l’environnement. Cependant, ces auteurs
concluent que peu d’évidences empiriques ont été apportées jusqu’à maintenant pour justifier
l’utilisation des pratiques sylvicoles pour contrôler les insectes défoliateurs. Théoriquement, les
traitements sylvicoles peuvent influencer les défoliateurs de différentes façons en modifiant leur
habitat et par conséquent les interactions avec les différents niveaux trophiques (Gottschalk, 1993;
Miller & Rusnock, 1993; Cappuccino et al., 1998; Muzika & Liebhold, 2000).
Par exemple, la coupe de récupération et la coupe d’assainissement peuvent être utilisées pour
prélever les arbres susceptibles aux attaques d’insectes (Gottschalk, 1995). Ainsi, cette approche
permet d’éviter la propagation de l’insecte dans les peuplements traités et permet également de
récupérer la valeur monétaire des arbres avant d’éventuels dommages par le ravageur considéré.
Ces et autres pratiques ont surtout été utilisées avec succès pour réduire la susceptibilité des arbres
face aux attaques des scolytes (Mitchell et al., 1983; Waring & Pitman, 1985; Belanger et al.,
1993). Il a d’ailleurs été démontré que la croissance des arbres ayant survécu à une défoliation de la
chenille à houppes du douglas, Orgyia pseudotsugata (McDunnough) (Lepidoptera: Lymantriidae),
était supérieure après une coupe de récupération (Wickmann, 1988). Fuentealba et Bauce (2012b)
ont montré qu’une coupe partielle peut entraîner une augmentation de la résistance des sapins
baumiers à la défoliation par la tordeuse des bourgeons de l’épinette due à une augmentation de la
production foliaire. À court terme, ce traitement peut toutefois réduire la résistance des sapins à la
tordeuse des bourgeons de l’épinette, en raison d’une réduction de la concentration de certains
monoterpènes qui se traduit par une augmentation de la prise alimentaire par les larves (Bauce,
1996; Fuentealba & Bauce, 2012a).
Récemment, la coupe partielle a attiré l’attention comme traitement sylvicole dans la forêt boréale
afin de favoriser le maintien et le développement d’attributs caractérisant les vieilles forêts et la
préservation des espèces sauvages (Vanderwel et al., 2009). Le glossaire du Service canadien des
forêts décrit le terme général « coupe partielle » comme « (…) toute coupe enlevant une partie des
arbres d’un peuplement. ». Dans un contexte où il faut à la fois tenir compte des intérêts
économiques, écologiques et sociaux, l’influence de ce traitement sur la flore et la faune des forêts a
été l’objet de plusieurs études (Kessler et al., 1992; Steventon et al., 1998; Deal, 2001; Vanderwel
et al., 2009; Jacobs & Work, 2012). De plus, le potentiel des coupes partielles pour augmenter la
7
résistance des peuplements face aux insectes défoliateurs, en particulier la tordeuse des bourgeons
de l’épinette, a été démontré par plusieurs études (Batzer, 1967; Bauce, 1996; D’Amato et al., 2011;
Fuentealba & Bauce, 2012b). Cependant, l’influence des coupes partielles sur les interactions tri-
trophiques, c’est-à-dire qui incluent les ennemis naturels des défoliateurs, est peu connue.
Plusieurs traitements sylvicoles ont été suggérées afin d’augmenter l’abondance ou l’activité des
ennemis naturels des insectes défoliateurs, en particulier les parasitoïdes (Simmons, 1975; Hébert et
al., 1990; Cappuccino et al., 1998, 1999). Par exemple, la diversification de la végétation afin
d’augmenter l’abondance et la diversité des ennemis naturels (Cappuccino et al., 1998) ainsi que
l’éclaircie permettant d’accroitre l’activité des parasitoïdes (Hébert et al., 1990). Par contre,
l’évidence selon laquelle les traitements sylvicoles peuvent, en pratique, augmenter la mortalité des
insectes défoliateurs due à l’action des parasitoïdes n’a pas été démontrée. La mortalité de la
spongieuse, Lymantria dispar (L.), (Liebhold et al., 1998) et de la tordeuse occidentale de
l’épinette, C. oxidentalis (Freeman) (Mason et al., 1992) par les parasitoïdes n’est pas influencée
par l’éclaircie. Cependant, dans une étude à grande échelle, Roland et Taylor (1997) ont déterminé
que le taux de parasitisme de trois espèces de parasitoïdes de la livrée des forêts, Malacosoma
disstria (Hübner) a été influencé négativement par une fragmentation de l’habitat.
Davantage d’études empiriques sont nécessaires afin de mieux comprendre l’influence des
traitements sylvicoles sur la performance des insectes défoliateurs (Simmons et al., 1975; Crook et
al., 1979; Roland & Taylor, 1997; Muzika & Liebhold, 2000; Fuentealba & Bauce, 2012a) mais
également sur les ennemis naturels associés avec ces derniers. Le présent travail a été effectué dans
le but de déterminer l’influence de la coupe partielle sur le taux de parasitisme de deux insectes
défoliateurs très importants, l’arpenteuse de la pruche et la tordeuse des bourgeons de l’épinette.
9
Short-term influence of partial cutting
on hemlock looper parasitism
Seehausen, M. Lukas1; Bauce, Éric1; Régnière, Jacques2 & Berthiaume, Richard1
1Centre d’Étude de la Forêt and Département des Sciences du Bois et de la Forêt, Faculté
de foresterie, de géographie et de géomatique, Université Laval, Québec, QC, Canada,
G1V 0A6
2Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre,
Québec, QC, Canada, G1V 4C7
For submission to: Bulletin of Entomological Research
10
Résumé
L’influence des coupes partielles sur le taux de parasitisme chez l’arpenteuse de la pruche,
Lambdina fiscellaria (Guenée), a été étudiée dans des peuplements ayant reçu deux intensités de
coupes partielles, soit 25% et 40% de réduction de la surface terrière, et des parcelles témoins sans
traitement sylvicole. Un an après les traitements sylvicoles, seul le taux de parasitisme chez les
chrysalides de l’arpenteuse de la pruche a été significativement réduit dans les parcelles à 40% de
coupe partielle. Cependant, l’année suivante, aucune influence significative des traitements sur le
parasitisme tant chez les larves que chez les chrysalides n’a été démontrée. Les captures du
parasitoïde le plus abondant, Apechthis ontario (Cresson), dans les pièges Malaise ne pouvaient pas
expliquer la réduction du parasitisme un an après les traitements. Seules les plus hautes
températures maximales dans les parcelles avec traitement ont été corrélées négativement avec le
parasitisme. Ainsi, les principaux mécanismes expliquant l’influence des coupes partielles sur le
parasitisme restent inconnus.
11
Abstract
The hemlock looper, Lambdina fiscellaria (Guenée), is one of the most important lepidopteran
defoliators in the conifer forest of eastern North America. Silvicultural treatments have been
suggested as an option to control insect defoliators but treatment effects on parasitism remain
widely unknown. Therefore, the influence of partial cutting on hemlock looper parasitism was
studied in mature balsam fir / white birch forests using two intensities of treatments, and control
stands. During the two years following partial cutting, laboratory reared hemlock looper pupae were
periodically exposed in each stand to determine parasitism rates and Malaise traps as well as
meteorological data loggers were installed in each stand. Additionally, wild hemlock looper larvae
were collected in teach stand two years after treatments. Hemlock looper pupae were significantly
less parasitized in stands with the higher partial cutting intensity one year after treatments.
However, the following year, an influence of the treatments was neither detected on pupal nor on
larval parasitism. No correlation between pupal parasitism and Malaise trap samples of Apechthis
ontario (Cresson), the most important parasitoid in the present study, was found and only a weak
negative correlation between higher maximum temperature and parasitism was observed. Thus, the
main reason for the reduced parasitism in the short term remains unknown. However, to sustain
parasitism rates in forest stands vulnerable to hemlock looper defoliation at naturally high levels,
the authors recommend refraining from partial cutting or to conduct this treatment at intensities
lower than 40%.
12
Introduction
Manipulation of forest habitats using silviculture has been repeatedly proposed as an attractive
approach to manage defoliating insects because it may be effective, long-lasting, inexpensive and
have low environmental impact (Muzika & Liebhold, 2000). For example, thinning has been
suggested to reduce damage caused by spruce budworm, Choristoneura fumiferana (Clemens),
(Crook et al., 1979; Bauce, 1996; Bauce et al., 2001; Fuentealba & Bauce, 2012b) and western
spruce budworm, Choristoneura occidentalis Freeman, (Mason et al., 1992). Fuentealba & Bauce
(2012b) showed that thinning can reduce negative impacts of the spruce budworm in the long term
(three years after treatments) through an increase of foliar production. However, in the short term
(one year after treatments), tree resistance to spruce budworm defoliation was found to be reduced
as a result of its negative impact on host foliar monoterpenic defensive compounds (Bauce, 1996;
Fuentealba & Bauce, 2012a). Altering stand structure may also affect the relationship between
defoliating insects and their natural enemies. Several authors have recommended silvicultural
procedures to increase natural enemy abundance and/or activity (Simmons, 1975; Hébert et al.,
1990; Cappuccino et al., 1998; 1999) but few studies on the practical application have been
conclusive (Mason et al., 1992; Liebhold et al., 1998).
The hemlock looper, Lambdina fiscellaria (Guinée) (Lepidoptera: Geometridae), is one of the most
economically important defoliators in the coniferous forests of eastern North America (Otvos et al.,
1979; MacLean, 1990). Its outbreaks have mainly been reported on balsam fir Abies balsamea (L.)
Miller, and eastern hemlock Tsuga canadensis (L.) Carrière (Otvos et al., 1979; Trial, 1993; Hébert
& Jobin, 2001), where tree mortality is mainly restricted to mature and overmature stands (De
Gryse & Schedl, 1934; Carroll, 1956; Davidson & Prentice, 1967). However, defoliation can take
place on other conifers and deciduous trees (Hébert & Jobin, 2001; Berthiaume, 2007). The areas
most affected by severe infestations during the 20th century were mainly concentrated in regions
characterized by a maritime climate, more specifically in Newfoundland and in certain Québec
areas while less destructive outbreaks were reported in Ontario, New-Brunswick and Nova-Scotia
(Hébert & Jobin, 2001).
The hemlock looper is univoltine and overwinters in the egg stage (Carroll, 1956). Eggs hatch in
late May and early June, shortly after the budbreak of balsam fir (Hébert & Jobin, 2001). In late
July, larvae seek for pupation places, usually on host trees under pieces of bark or in lichens
(Watson, 1934; Hébert et al., 2001b). The pupal stage lasts between 16 and 20 days and adults
emerge from mid-August to early October (Carroll, 1956; Hébert & Jobin, 2001, Berthiaume,
2007). Many parasitoid species attack all subadult life stages of this defoliator (De Gryse & Schedl,
13
1934; Hopping, 1934; Carroll, 1956; Torgersen, 1971; Otvos, 1973; Hartling et al., 1991; Turnquist,
1991). Together with diseases and climatic conditions, parasitism can play an important role in
population dynamics of the hemlock looper and during outbreak collapses (Otvos, 1973; Turnquist,
1991; Hébert et al., 2001a). However, most of the studies on hemlock looper parasitism were done
in outbreak populations and little data exist on parasitism in low density populations (Armstrong,
1996). Studies about the effect of stand structure on hemlock looper parasitism are also very scarce
(Pardy, 2000).
The main objective of the present study was to examine the short-term influence of partial cutting
on pupal and larval parasitism of hemlock looper populations to evaluate this silvicultural approach
as an alternative control tool against this important defoliator.
Material and Methods
The experiments were conducted during the summers of 2010 and 2011 in the Montmorency
experimental forest of Laval University. This boreal forest is located in the Laurentian Mountains,
70 km north of Quebec City, Quebec, Canada, in a balsam fir - white birch forest. Twelve four-
hectare plots were chosen in mature rich stands with a mesic-with-seepage drainage class. They
varied in elevation from 650 to 800m above sea level and were all growing on a north-eastern slope.
Partial cutting treatments were done in autumn 2009 in eight plots using two intensities (25 and
40% stand basal area removed) with four replicates each. Four untreated plots were used as
controls. Thinning was done with mechanical harvesters leaving 3.3±0.2m skid trails representing
15% stand basal area, and 22±3m residual forest strips in which selected trees were harvested to
achieve the desired percentage removal.
Hemlock looper pupae for this experiment were obtained from a laboratory colony at Laval
University originating from wild insects collected in the Québec City region (46°53’N; 71°18’W) in
2001 (Berthiaume, 2007). This colony was enriched with wild insect every three to four years to
limit genetic drift within the population. Larvae had been reared on mature balsam fir branches
under laboratory conditions (20 ±1°C, 40% R.H. and 16L:8D photoperiod) and eggs overwintered
in a field insectary located at the Laurentian Forestry Center in Québec City, exposed at natural
winter conditions. After overwintering, hatching larvae were reared until pupation on balsam fir
branches collected in the Montmorency forest in 6-L plastic boxes (KIS Omni Plastic Shoe Box)
with small screened windows for ventilation. The boxes were placed in growth chambers (40%
R.H. and 16L:8D photoperiod) and temperature was adjusted continuously to synchronize the larval
development with the wild population in the Montmorency forest. Synchronisation was done by
14
repeated field observations of the larval development of wild hemlock looper in the study area.
Only living individuals that had pupated within 48 hours before the exposure period were used for
this experiment. Pupae selected for field exposition were kept at 10±1°C in a growth chamber and
in cooling boxes for transport to reduce their development before arriving in the study area.
Hemlock looper pupae were placed in the study area on balsam fir branches at breast height (1.3m)
by binding together three adjacent shoots and placing one single pupa in the center. To avoid
excessive host clustering, only five pupae were installed on any given balsam fir tree and trees were
at least 20m apart from each other. Hemlock looper pupae are commonly the prey of birds (Otvos,
1973) and therefore bird exclusion cages were installed around each branch bearing the pupae. A
construction consisting of a metal hoop held by a wooden stake in front of a branch was used to
support a nylon net around the branch. The mesh width of 2.5 x 2.5cm was large enough to not
interfere with searching parasitoids but to prevent predation by birds. In 2010, a total of 540 pupae
were exposed to parasitism in three periods from 19 July to 9 August and in 2011, a total of 1015
pupae were exposed in four periods from 29 July to 26 August. After a period of seven days,
exposed pupae were recovered and transported in a cooling box to the laboratory for diagnosis.
Pupae were kept individually at 23 ±1°C, 60% R.H., 16L:8D photoperiod, and checked every other
day for moth or parasitoid emergence. Pupae dying before moth or parasitoid emergence were
excluded from the analysis.
In 2011, an increase of the hemlock looper population in the study area allowed the collection of
221 late instar larvae (15-23 larvae per plot) over two days (20 and 22 July). These wild larvae were
collected from balsam fir branches at eye level in the plots and were transported to the laboratory in
a cooling box, placed in individual containers containing a balsam fir branch and held in a growth
chamber under the same conditions as described above for the pupae. Foliage was replaced twice a
week until pupation or parasitoid emergence.
One 1.8 x 1.8m Malaise trap (Townes, 1972) was placed in every plot at ground level oriented to
the south. In plots with partial cutting treatments, traps were placed in residual forest stripes,
whereas in control plots, traps were placed randomly in the forest. All traps were checked and
emptied on a weekly basis from 11 June to 25 August in 2010 and from 15 June to 2 September in
2011. However, the duration of trapping periods varied in some months. A 70% alcohol solution
was used in the Malaise traps to kill and preserve insects. Samples were analysed in the laboratory
using a binocular to identify, sex and count the most abundant pupal parasitoid (Apechthis ontario
(Cresson)) of the hemlock looper.
15
Environmental temperature and relative humidity were taken in each plot with HOBO® Micro
Station Data Loggers (H21-002). They were always placed in residual forest stripes for treated plots
and randomly in the forest for control plots. Data were recorded every half an hour by one Temp
Smart Sensor (S-TMA-006) for environmental temperature and one Temperature/RH Smart Sensor
(S-THB-M002) for temperature and relative humidity, placed in solar radiation shields (M-RSA).
Maxima, minima and means were calculated on a daily basis for both temperature and relative
humidity. Only data from 2010 are available because numerous weather stations malfunctioned in
2011.
Statistical analysis
Pupal and larval parasitism was analyzed using logistic regression with random effects on the plots
nested in the treatments. Exposure periods of hemlock looper pupae were introduced as repeated
measures. Effects of silvicultural treatments were tested with a priori comparisons between controls
and treated plots as well as between the two levels of partial cutting. For pupal parasitism, p-values
were adjusted for multiplicity (PROC GLIMMIX, SAS Institute Inc., 2004). Numbers of A. ontario
wasps in Malaise traps per plot and trapping period were submitted to analysis of variance using the
Poisson distribution for the error term (PROC GLIMMIX, SAS Institute Inc., 2004). Because the
duration of the trapping periods varied in some months, it was used as an offset. The effect of
partial cutting on environmental temperature and relative humidity was analysed with an analysis of
variance using random effects due to plots nested in the treatments and sampling periods (days) as
repeated measures. Tukey adjustments were used subsequently for multiple comparisons (PROC
MIXED, SAS Institute Inc., 2004). Simple regressions were used to measure the impact of
temperature and humidity on parasitism and Malaise trap samples. (PROC REG, SAS Institute Inc.,
2004).
Results
Three species of generalist ichneumonid wasps, A. ontario, Aoplus velox (Cresson) and Itoplectis
conquisitor (Say), emerged in both years from exposed hemlock looper pupae (Table 1-1). In 2010,
overall parasitism was 36.88% and was significantly lower in stands with high levels (40%) of
partial cutting (Fig. 1-1A; F2, 25.53=4.48; P=0.0215). Most of this parasitism was done by A. ontario
(table 1). The parasitism rate by this species was also significantly lower in 40% partial cutting
stands (Fig. 1-1C; F2, 24.95=5.01; P=0.0148). In 2011, overall parasitism (14.74%) was lower than in
2010 (Table 1-1). Partial cutting had an effect neither on overall parasitism (Fig. 1C; F2, 33.97=0.65;
P=0.5299) nor on parasitism by A. ontario alone (Fig. 1-1D; F2, 38.34=2.70; P=0.0800). Parasitism by
16
the other two species in 2010 and 2011 was too low to be submitted to statistical analysis (Table 1-
1).
Parasitism of wild hemlock looper larvae was attributed to two ichneumonids, Mesochorus
discitergus (Say) and Phobocampe sp., one braconid, Protapantheles paleacritae (Riley), and two
tachinids, Campylocheta semiothisae (Brooks) and Madremyia saundersii (Williston) (Table 1-1).
Overall parasitism was only 5.45% and did not differ significantly between the silvicultural
treatments (F2, 9=3.03; P=0.0988). None of the larval parasitoid species was abundant enough to be
analyzed separately (Table 1-1).
Adult A. ontario females were captured in all sample periods in 2010 (11 June – 25 August) and the
mean number of captured individuals increased significantly throughout the season (F4, 26.25=5.70;
P=0.0019). Numbers of females caught in 2010 were significantly higher in stands with partial
cutting when compared to the control stands (Fig. 1-2A; F2, 11.54=7.50; P=0.0082). In 2011, adult
females were captured in the first four (15 June – 10 August) of five sample periods. The number of
captured females in 2011 (n=15) was much lower than in 2010 (n=248), and varied significantly
neither throughout the season nor between the silvicultural treatments (Fig. 1-2B). Adult A. ontario
males were captured in Malaise traps only early in the season. In 2010, males were only found
during the first sample period (11 – 30 June, Fig. 1-2C) and in 2011 during the first two sample
periods (15 June – 13 July, Fig. 1-2D). However, significantly fewer males were captured in the
second sample period (30 June - 13 July) than in the first (15 - 30 June) (F1, 5.57=13.68; P=0.0116).
In 2011, considerably fewer males were captured (n=111) than in 2010 (n=251). Differences in the
number of captured individuals were found between the silvicultural treatments neither in 2010 (F2,
9=0.51; P=0.6152) nor in 2011 (F2, 11.08=1.05; P=0.3807).
During the Malaise trapping period and hemlock looper pupal exposure (20 July - 9 August) in
2010, partial cutting increased significantly the daily mean and daily maximum temperature (Table
1-2). A significant decrease in daily mean relative humidity could only be detected during the pupal
exposure period but the daily minimum relative humidity decreased significantly in stands with
partial cutting in both periods (Table 1-2). Among environmental parameters measured in 2010,
only maximum temperature was significantly correlated with both overall parasitism (R2=0.17;
P=0.0164) and parasitism by A. ontario (R2=0.17; P=0.0172). Parasitism decreased with increasing
maximum temperature. However, none of the measured environmental parameters were
significantly correlated with Malaise trap catch.
17
Discussion
The first season after treatments, parasitism in plots with the higher intensity of partial cutting (40%
stand basal area removed) was significantly reduced to less than half of that in control stands.
However, the lower partial cutting treatment had no significant influence on parasitism. Higher
levels of partial cutting may have led to a higher degree of disturbance of the environment in the
treated stands. A 40% basal area removal leads to larger openings in a forest stand than 25% partial
cutting. Open areas can have very different environmental conditions compared to continuous
forests (Aussenac, 2000). These in turn may affect parasitoid movement and alter parasitism
(Roland & Taylor, 1997). Longer distances between residual patches (Bernstein et al., 1999) and
higher temperatures (Denis et al., 2011) can lead to increased travel costs for parasitoids, which
decrease their efficiency to find and attack hosts within patches. The daily maximum and mean
temperature were significantly higher in stands with partial cutting. However, no significant
differences in temperature between the two partial cutting intensities during the pupae exposure
period were found, whereas parasitism was only reduced in stands with high partial cutting
intensities. Maximum temperature was negatively correlated with parasitism, although this
correlation only explains a small portion of variation in parasitism, indicating that other factors
must be involved. Very high levels of relative humidity have already been shown to negatively
influence parasitism by A. ontario attacking the greater wax moth, Galleria mellonella (L.), in the
laboratory (Ryan, 1974). However, only significant differences in mean and minimum relative
humidity were found between treated and control stands but not between the two partial cutting
intensities. Furthermore, relative humidity was not significantly correlated with parasitism in our
study.
The analysis of parasitism rates by pupal parasitoids in 2010 indicated that the reduced parasitism at
40% basal area reduction was due to the ichneumonid A. ontario. However, pupal parasitism and
numbers of captured A. ontario females in the exposure period of hemlock looper pupae in 2010
were not related, as parasitism was reduced only in stands with 40% partial cutting whereas more
female parasitoids were caught in stands with both partial cutting intensities. Malaise traps measure
flight activity and flight behaviour (e.g. Hébert et al., 1990) and not exclusively abundance of
insects. It is possible that increased numbers of parasitoids caught in stands with partial cutting
treatments indicate an increased flight activity due to longer distances that have to be covered
between trees in search for hosts. Climatic factors are also known to influence parasitoid flight
activity and therefore numbers of captured insects with Malaise traps (Waage & Hassel, 1982).
18
However, even if higher temperatures were measured in stands with partial cutting, none of the
measured environmental parameters influenced significantly the Malaise trap catches in this study.
In previous studies, no influence of thinning on parasitism of the gypsy moth, Lymantria dispar (L.)
(Liebhold et al., 1998) or the western spruce budworm (Mason et al., 1992) was found. The results
of the present study suggest that the influence of thinning on parasitism may vary between
parasitoid-host communities. However, Roland & Taylor (1997) suggest that parasitoid species may
each respond to forest fragmentation at a characteristic spatial scale, as they demonstrated for four
parasitoids of the forest tent caterpillar, Malacosoma disstria Hübner. Partial cutting treatments can
be seen as a forest fragmentation on a relatively small scale, as skid trails separate residual forest
strips from each other. As demonstrated in the present study, 40% basal area reduction on only four
hectare are sufficient to significantly influence parasitism by A. ontario attacking hemlock looper
pupae.
No influence of partial cutting treatments on pupal parasitism was found in 2011. This result may
indicate that the negative effect of high partial cutting intensities on parasitism is only present in the
very short term (first season after treatments). However, forest fragmentation through the skid trails
persisted until 2011 and beyond, and should therefore continue to influence parasitism. A reduced
abundance of A. ontario females on the landscape level may be related to the missing effect of
silvicultural treatments on parasitism in 2011. In all plots, numbers of A. ontario females captured
in Malaise traps were considerably lower in 2011 compared to 2010. Simultaneously, the parasitism
rate of A. ontario was also very low in 2011, leading to a reduction in overall parasitism to less than
half of that in 2010. This reduction of female parasitoids in the study area may have caused the
effect of partial cutting on parasitism to recede into the background. Interannual changes in the
abundance of wild hemlock looper and alternative hosts for the parasitoids in the study area may
also have led to changes in parasitism rates between plots and years. For example, A. ontario was
found to attack the rusty tussock moth, Orgyia antiqua L., which was very abundant in both years in
the study area. Experiments over a longer time period are necessary to analyze the temporal
development of the influence of partial cutting on parasitism of hemlock looper.
Overall parasitism of hemlock looper larvae was low, which has already been reported in several
other studies (Togersen, 1971; Moody, 1993; Hall, 1995; West & Kenis, 1997). Furthermore, no
differences in larval parasitism were found between silvicultural treatments. These results suggest
that in comparison to pupal parasitism, larval parasitism plays a minor role as a cause of mortality
in this species and is also less important to take into account for forest management implications.
19
Conclusion
As demonstrated in the present study, parasitism of hemlock looper pupae is significantly reduced
one year after high levels of partial cutting. However, the main reason for this reduction remains
unknown. Reduced pupal parasitism may lead to a better performance of the hemlock looper
population and thus to more defoliation of its host trees the following season. To sustain parasitism
rates in forest stands vulnerable to hemlock looper defoliation at naturally high levels, it may be
refrained from partial cutting in those stands or it may be conducted at intensities lower than 40%
stand basal area removal. Alternatively, high levels of partial cutting may be accompanied with
aerial spraying of the microbial insecticide Btk at least one year after treatments. The use of this
biological insecticide was shown to be efficient to decrease hemlock looper populations and to
reduce defoliation by this species (West et al., 1989; 1997). However, two years after the partial
cutting treatments, no significant influence on parasitism was found. Further research is necessary
to evaluate the influence of partial cutting on parasitism in the medium- and long-term.
Acknowledgements
We are grateful to N. Giasson, P. Huron, M. Charest, P. Charron and M. Lemay for help in the
laboratory and in the field and to A. Fuentealba for reviewing an earlier version of this paper. We
also thank G. Pelletier from the Canadian Forestry Service (CFS) and, O. Lonsdale, A. Bennett, J.
Fernandez and J.E. O’Hara from Agriculture and Agri-Food Canada for the identification of the
parasitoids. M. Bernier-Cardou (CFS) provided important statistical support. Financial support was
provided to the iFor Research Consortium by the Natural Sciences and Engineering Research
Council of Canada (NSERC-CRSNG), the Ministère des Ressources Naturelles et de la Faune du
Québec (MRNFQ), the Conseil de l’Industrie Forestière du Québec (CIFQ), the Canadian Forest
Service and the Société de Protection des Forêts contre les Insectes et les Maladies du Québec
(SOPFIM).
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23
Table 1-1: Parasitism rates (%) by species attacking hemlock looper pupae and larvae.
*Pupal parasitism was analyzed in 2010 (n=385) and 2011 (n=726), whereas larval parasitism was only
analyzed in 2011 (n=202)
2010* 2011
Pupal parasitoids
Ichneumonidae
Apechthis ontario (Cresson) 35. 58 10. 33
Aoplus velox (Cresson) 1. 04 3. 86
Itoplectis conquisitor (Say) 0. 26 0. 55
Larval parasitoids
Ichneumonidae
Mesochorus discitergus (Say) - 0. 50
Phobocampe sp. - 0. 99
Braconidae
Protapantheles paleacritae (Riley) - 0. 99
Tachinidae
Campylocheta semiothisae (Brooks) - 0. 99
Madremyia saundersii (Williston) - 0. 99
Unidentified - 0. 99
24
Table 1-2: Influence of partial cutting on environmental temperature and relative humidity (RH)
(means ± SEM).
Parameter partial cutting intensity
F df P 0% 25% 40%
Malaise trap sampling period*
Mean temperature 14.94±0.19a 15.35±0.22
b 15.26±0.20
b 12.78 2, 8 0.0032
Maximum temperature 19.44±0.21a 20.25±0.26
a 20.73±0.24
b 10.25 2, 8 0.0062
Mean RH 81.21±1.52a 77.98±1.58
a 79.78±1.52
a 3.99 2, 8 0.0627
Minimum RH 60.12±0.97a 54.92±1.12
b 55.21±1.03
b 12.02 2, 8 0.0039
Pupae exposure period†
Mean temperature 14.56±0.29a 14.93±0.33
b 14.97±0.29
b 20.49 2, 8 0.0007
Maximum temperature 18.61±0.33a 19.43±0.42
b 19.87±0.36
b 11.84 2, 8 0.0041
Mean RH 83.56±0.83a 80.17±1.04
b 81.83±0.86
a,b 4.72 2, 8 0.0443
Minimum RH 63.07±1.42a 57.03±1.69
b 57.66±1.46
b 10.45 2, 8 0.0059
* June 11 - August 25 2010; † July 20 - August 9 2010
Values in each row followed by the same letter do not differ significantly at P < 0.05 according to Tukey’s
range test.
25
Figure 1-1: Mean rates of overall attack (A, B) and attack by A. ontario (C, D) as a function of
exposure periods in 2010 and 2011 following partial cutting treatments, 0, 25 and 40% basal area
reduction. Lines followed by the same letter do not differ significantly at P < 0.05 according to a
priori comparisons.
26
Figure 1-2: Mean number of A. ontario females (A, B) and males (C, D) per trapping day caught
with Malaise traps as a function of time (ordinal date), one (2010) and two (2011) years following
partial cutting treatments, 0, 25 and 40% basal area reduction. Lines followed by the same letter do
not differ significantly at P < 0.05 according to a priori comparisons.
27
Does spruce budworm rearing diet influence
larval parasitism?
Seehausen, M. Lukas1; Régnière, Jacques2 & Bauce, Éric1
1Centre d’Étude de la Forêt and Département des Sciences du Bois et de la Forêt, Faculté
de foresterie, de géographie et de géomatique, Université Laval, Québec, QC, Canada,
G1V 0A6
2Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre,
Québec, QC, Canada, G1V 4C7
Accepted by: The Canadian Entomologist
28
Résumé
Dans la présente étude, des larves de la tordeuse des bourgeons de l’épinette, Choristoneura
fumiferana (Clemens) (Lepidoptera: Tortricidae), ont été élevées en laboratoire sur nourriture
artificielle ainsi que sur du feuillage de sapin baumier, Abies balsamea (L.) Miller (Pinaceae), pour
ensuite être introduites en forêt afin de comparer leur parasitisme. Par ailleurs, un test de choix en
laboratoire a été effectué avec le parasitoïde Tranosema rostrale (Brishke) (Hymenoptera:
Ichneumonidae). Aucune influence significative de la diète sur le parasitisme n’a été démontrée sur
le terrain. Cependant, un nombre significativement plus grand de larves élevées sur le feuillage ont
été attaquées par T. rostrale en laboratoire. Malgré la préférence pour les larves élevées sur le
feuillage en laboratoire, nous concluons que des larves élevées sur la diète artificielle peuvent être
utilisées dans la détermination des taux de parasitisme sur le terrain sans risque de biaiser les
résultats obtenus.
29
Abstract
Artificial diet is commonly used to rear the spruce budworm, Choristoneura fumiferana (Clemens)
(Lepidoptera: Tortricidae), in the laboratory. While its effect on spruce budworm performance is
relatively well studied, no information exists about the influence of rearing diet on larval parasitism.
In this study, spruce budworm larvae reared in the laboratory on artificial diet or balsam fir, Abies
balsamea (L.) Miller (Pinaceae), foliage were introduced in the field to compare parasitism.
Additionally, a laboratory choice test was conducted with the larval parasitoid Tranosema rostrale
(Brishke) (Hymenoptera: Ichneumonidae). No significant influence of spruce budworm rearing diet
on parasitism in the field was found. However, in the laboratory, T. rostrale attacked significantly
more foliage-fed larvae. We conclude that even if initial differences in parasitism may exist
between diet-fed and foliage-fed larvae in the laboratory, spruce budworm larvae reared on artificial
diet can be used in field studies investigating parasitism of wild spruce budworm populations
without concern that the food source would affect parasitism.
30
The eastern spruce budworm Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae) is
commonly reared on artificial diet (McMorran, 1965) for both laboratory and field experiments.
Certain differences exist between foliage-fed and artificial diet-fed larvae, such as higher survival of
larvae, increased pupal weight, adult longevity, fecundity and development times (McMorran,
1965). Poirier and Borden (2000) observed that foliage-fed larvae did not respond to the oral
exudates of artificial diet-fed larvae, whereas exudates from foliage-fed larvae had a deterrent effect
on the feeding behavior of both foliage- and artificial diet-fed larvae. Several studies used spruce
budworm larvae reared on artificial diet for exposure in the field to investigate parasitism and
parasitoid behavior in endemic budworm populations (Doucet and Cusson, 1996a, 1996b; Fidgen
and Eveleigh, 1998; Fidgen et al., 2000; Cusson et al., 2002).
It is well known that variation in the food consumed by herbivores can change their chemical
composition and affect their susceptibility to predator attack (reviewed by Price et al. 1980). For
instance, several plant compounds such as toxins assimilated by lepidoptera can alter parasitoid
performance (reviewed by Ode, 2006). However, few studies have dealt with influences of artificial
diets compared to natural diets of lepidoptera on parasitism (but see Song et al., 1997; Havill and
Raffa, 2000). So far only the effect of host diet on parasitism of spruce budworm eggs had been
tested (Song et al., 1997) but no attempt has been made to test if larval parasitoids of the spruce
budworm are influenced by the artificial diet ingested by its host. Therefore, both field and
laboratory experiments were conducted to test the use of diet-fed spruce budworm larvae for
parasitism studies.
The field experiment was conducted in summer 2011 in the context of a larger study on the
influence of partial cutting on parasitism (Seehausen, 2013), in Laval University’s Montmorency
experimental forest (47°20’N, 41°30’W). This boreal forest is located in the Laurentian Mountains,
70 km north of Quebec City, Quebec, Canada, in a balsam fir - white birch forest. Four plots, each
approximately four hectare in size, were located in unaltered mature forests. Plot elevations ranged
from 650 to 800 m on a north-eastern slope.
Diapausing second instar spruce budworm larvae were obtained from the Canadian Forest Service,
Sault Ste. Marie, Ontario, Canada, and reared at 23°C and a 16-h photoperiod on either artificial
diet (McMorran, 1965) or foliage of balsam fir, Abies balsamea (L.) Miller (Pinaceae), harvested in
the study area. Foliage was stored at 4°C for not longer than one week in buckets filled with water
and offered to larvae as small branches in water-filled tubes. Branches in rearing containers were
31
exchanged twice a week to provide larvae with fresh food. Larvae were held in plastic containers
with food ad libitum until the desired instar for the experiment was reached (7 to 20 days).
To compare parasitism rates of budworm larvae, fourth to sixth instar foliage- and diet-fed larvae
were implanted twice a week from 21 June to 19 July 2011 in each of the four plots. Before
implantation, larvae were held in small plastic containers without food for 24 hours at 4°C in
growth chambers. They were transported to the study area in cooling boxes and installed on balsam
fir current year shoots at eye level in groups of five larvae with the same treatment distributed on
two to five young trees. Trees and branches were marked with coloured flags to facilitate recovery
of larvae after exposure. Every group was randomly distributed in each plot and at least 15 meters
apart from each other. To synchronize the implantation of the larvae with their natural occurrence
and the occurrence of the parasitoids in the study area, a model of spruce budworm seasonality
(Régnière et al., 2012) was used. Larvae were recovered at nine dates after a seven-day exposure
period (Fig. 1). Shoots containing the exposed budworms were cut so that immature stages of
parasitoids (larvae or cocoons) hidden in the foliage in the vicinity of their host were also collected.
After transport to the laboratory in a cooling box, larvae were placed in individual containers on
artificial diet, reared at room temperature and checked three times a week for parasitoid emergence.
The proportion of parasitized recovered larvae was analyzed using logistic regression, with
treatments (diet) as fixed effects, plots as random effects, and time as repeated measures (PROC
GLIMMIX, SAS Institute Inc., 2004). Repeated measures were needed because of the likely
autocorrelation of the plots effect error. Individuals dying from causes other than parasitism were
not included in the analysis.
A laboratory choice test was conducted in 2011 and 2012 with a total of 33 female specimens of the
parasitoid Tranosema rostrale (Brishke) (Hymenoptera: Ichneumonidae) obtained from field
implanted spruce budworm larvae in and near the study area. One foliage-fed and one diet-fed larva
of the same instar (fourth or fifth) and size were placed at opposite sides in a 100 x 15 mm petri
dish. Using an aspirator, one inexperienced wasp (a wasp which had never before parasitized a
larva) was transferred to the petri dish for ten minutes or until it had attacked one host larva. A larva
was considered as attacked if the wasp’s ovipositor was clearly inserted in its skin. If no parasitism
occurred, the larvae were transferred to a new petri dish where the experiment was repeated using
another inexperienced wasp. The choice test was analyzed using a two tailed sign test (PROC
UNIVARIATE, SAS Institute Inc., 2004).
32
The overall parasitism rate in the field experiment was 86.2%, most of which was done by two
species: T. rostrale and Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae). Phytodietus
sp. (prob. vulgaris Cresson) (Hymenoptera: Ichneumonidae) and a few unidentified individuals
were also present (hereafter referred to as other species; Table 1; Fig. 1). No effect of larval diet on
parasitism was found, whether overall or by each species (Table 1). Overall parasitism decreased
significantly throughout the season (F8,19.67=2.94; P=0.0244) although this effect was not significant
when parasitism was analysed separately for each parasitoid species (Fig. 1). These changes over
time reflect the different phenology of each species, T. rostrale being more active early in the
summer, and E. cacoeciae later (JR, unpublished data).
In the choice test with T. rostrale females, spruce budworm larvae fed on balsam fir foliage were
chosen more than twice as often (70%, 23/33) as diet-fed larvae (30%, 10/33; P=0.0351). While the
sample sizes in these tests are small, the difficulty of obtaining sufficient biological material
precluded access to larger number of female parasitoids. We hypothesize that T. rostrale may use
olfactory cues from the budworm’s host plant to find its host, such as volatiles liberated by feeding
larvae or their frass, and therefore find or recognize foliage-fed larvae more readily. It is possible
that in the first few hours of exposure in the field, foliage-fed larvae are also attacked more often
than diet-fed larvae by T. rostrale females. However, an exposure period of seven days as described
above would mask such an effect as the diet-fed larvae feed on foliage.
We conclude that implanting in the field of spruce budworm larvae reared on McMorran (1965)
artificial diet can be used without concern that the food source used in rearing would affect
parasitism rates.
Acknowledgements
We thank M. Cusson (Canadian Forest Service) for assistance in the laboratory choice test, and N.
Giasson, M. Lemay and P. Huron for assistance with the field work. Thanks to P. Therrien, G.
Trudel, C. Dussault and D. Simoneau (Ministère des Ressources Naturelles du Québec) for insect
rearing and parasitoid identification and to M. Bernier-Cardou (Canadian Forest Service) for
statistical advice. Financial support was provided through the iFor Research Consortium by the
Natural Sciences and Engineering Research Council of Canada (NSERC), the Ministère des
Ressources Naturelles du Québec, the Conseil de l’Industrie Forestière du Québec, the Canadian
Forest Service and the Société de Protection des Forêts contre les Insectes et Maladies.
33
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(Hymenoptera: Eulophidae), an ectoparasitoid of spruce budworm larvae, Choristoneura
fumiferana (Lepidoptera: Tortricidae). The Canadian Entomologist. 130: 215-229.
Fidgen, J.G., Eveleigh, E.S., and Quiring, D.T. 2000. Influence of host size on oviposition
behaviour and fitness of Elachertus cacoeciae attacking a low-density population of spruce
budworm Choristoneura fumiferana larvae. Ecological Entomology, 25: 156-164.
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herbivores and parasitoids: implications for tritrophic interactions. Ecological Entomology, 25:
171-179.
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(Lepidoptera: Tortricidae). The Canadian Entomologist, 97: 58-62.
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interactions. Annual Review of Entomology, 51: 162-185.
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larval oral exudate in spruce budworms (Lepidoptera: Tortricidae). The Canadian
Entomologist, 132: 81-89.
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pruche et de la tordeuse de bourgeons de l’épinette. Master thesis, Université Laval, Quebec,
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84: 41-47.
35
Table 2-1: Percent parasitism (mean and SEM, n=4x9) and statistical analysis of parasitism of
foliage-fed (n=325) and diet-fed (n=347) spruce budworm larvae.
Species % parasitism of
foliage-fed larvae
% parasitism of diet-
fed larvae F df P
Tranosema rostrale 55.75 ±3.69 59.04 ±3.57 0.38 1, 46.19 0.5409
Elachertus cacoeciae 17.28 ±2.75 17.69 ±2.73 0.01 1, 24.96 0.9084
other species 9.29 ±1.64 5.92 ±1.30 2.40 1, 48.80 0.1275
total parasitism 88.70 ±2.11* 83.93 ±2.37 2.03 1, 46.16 0.1611
* Two larvae were multiparasitized by T. rostrale and E. cacoeciae; they were excluded from the species-
specific analysis but included in total parasitism.
36
Figure 2-1: Mean rates of (A) overall attack; (B) attack by T. rostrale; (C) attack by E. cacoeciae;
and (D) attack by other parasitoid species, as a function of time (ordinal date) in 2011 (black:
foliage-fed larvae; dashed grey: diet-fed larvae).
37
Influence of partial cutting on parasitism of
endemic spruce budworm populations
Seehausen, M. Lukas1; Bauce, Éric1; Régnière, Jacques2 & Berthiaume, Richard1
1Centre d’Étude de la Forêt and Département des Sciences du Bois et de la Forêt, Faculté
de foresterie, de géographie et de géomatique, Université Laval, Québec, QC, Canada,
G1V 0A6
2Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre,
Québec, QC, Canada, G1V 4C7
For submission to: Agricultural and Forest Entomology
38
Résumé
L’influence des coupes partielles sur le parasitisme chez la tordeuse des bourgeons de l’épinette a
été étudiée dans des sapinières à bouleau blanc. Des larves et des chrysalides de la tordeuse des
bourgeons de l’épinette ont été introduites dans des parcelles ayant subi deux intensités de coupes
partielles, soit 25% et 40% de réduction de la surface terrière, et des parcelles témoins sans
traitements pour mesurer le taux de parasitisme. Les chrysalides n’ont pas été suffisamment
attaquées pour comparer le taux de parasitisme entre les traitements. Deux ans après les traitements
sylvicoles, le taux de parasitisme chez les larves du quatrième et cinquième stade a été
significativement réduit dans les parcelles avec coupe partielle, peu importe l’intensité du
traitement. Cependant, cette influence n’a pas été observée l’année suivante au niveau du
parasitisme larvaire. Les mécanismes expliquant ce phénomène restent inconnus mais quelques
hypothèses sont élaborées dans le présent travail.
39
Abstract
Silvicultural treatments such as thinning have been suggested as management tools against spruce
budworm, Choristoneura fumiferana (Clemens). Among other things, parasitoids are also proposed
to be influenced by silvicultural procedures, but the effect of thinning on spruce budworm’s natural
enemies has not been tested yet. In this study, the influence of partial cutting on parasitism of
endemic spruce budworm populations has been investigated in mature balsam fir/white birch
forests. Two intensities of partial cutting (25% and 40% stand basal area reduced) were conducted
in 2009 and parasitism of introduced spruce budworm larvae and pupae was determined during
three years following these treatments. Pupal parasitism was too low for comparison between
treatments. However, two years following treatments, parasitism of fourth and fifth instar larvae
was significantly reduced in plots with both intensities of partial cutting, which was attributed to the
parasitoid Tranosema rostrale (Brishke). Three years after treatments, no significant influence of
partial cutting on parasitism of spruce budworm larvae was found. The explanation of the
mechanisms behind the reduction in larval parasitism is beyond the scope of this study but several
hypotheses are proposed. We conclude that practitioners should refrain from partial cutting in forest
stands threatened by imminent spruce budworm defoliation or alternatively, treatments may be
accompanied with application of insecticides for at least the first two years to prevent an increased
risk of defoliation.
40
Introduction
For at least the past three centuries, populations of the spruce budworm, Choristoneura fumiferanae
(Clemens) (Lepidoptera: Tortricidae) have reached epidemic densities over broad areas of North
American conifer forests on a more or less regular basis (Blais, 1965b; Morin, 1994). The last major
outbreak in eastern Canada reached its peak in 1975, covering some 57 million hectares of forest in
eastern Canada and the United States (Kettela, 1983). During this outbreak, the SBW caused losses
of more than 238 million cubic metres of wood fibre through tree mortality in Quebec’s forests and
approximately the same volume through growth reduction of trees (Coulombe et al., 2004).
Recently, populations of this defoliator have reached outbreak levels in several regions of north-
western Quebec, Canada (MRNF, 2011).
Mortality by parasitoids is one important factor driving local oscillations of spruce budworm
populations (Royama, 1984). However, their increased impact at the end of outbreaks has been
linked to prior host population reductions (Régnière & Nealis, 2007) due to other factors that might
include fluctuations in alternate host populations (McGugan & Blais, 1959; Blais, 1960; Miller,
1963; Blais, 1965a). Only limited data exist about the influence of parasitoids on endemic spruce
budworm populations (Miller & Renault, 1976; Cusson et al., 1998; J.R. unpublished data).
Currently, the most commonly used control measure against the spruce budworm is aerial
application of the microbial insecticide Bacillus thuringiensis var. kurstaki (Btk) (van
Frankenhuyzen, 1995, Fournier et al., 2010). Manipulation of forest habitats through silviculture is
an attractive alternative approach to manage defoliating insects because it may provide long-lasting,
inexpensive control with low environmental impact (Muzika & Liebhold, 2000). It has been
suggested that thinning could reduce damage caused by spruce budworm (Crook et al., 1979;
Bauce, 1996; Bauce et al., 2001). Fuentealba and Bauce (2012b) found increased resistance to
spruce budworm in balsam fir trees three years after stand thinning, mostly as a result of an increase
in foliar production in residual trees. It is not clear however how such increased foliar production
could lead to reduced damage during a spruce budworm outbreak, where insect numbers are limited
by food supply. In the shorter term (first year), tree resistance to defoliation was reduced through
reduction in foliar monoterpenes (Bauce, 1996; Fuentealba & Bauce, 2012a).
Several authors have suggested that silvicultural treatments could increase the abundance or impacts
of spruce budworm’s natural enemies (Simmons, 1975; Hébert et al., 1990; Cappuccino et al.,
1998, 1999). However, to our knowledge, no studies have been conducted to test whether thinning
can affect the impact of parasitoids on spruce budworm populations. The main objective of the
41
present study was to investigate the influence of different partial cutting intensities on spruce
budworm parasitism during the endemic period to further evaluate the use of this silvicultural
approach to reduce stand susceptibility to spruce budworm defoliation.
Material and Methods
The experiments were conducted during spring and summer from 2010 to 2012 in the Montmorency
experimental forest of the Laval University. This boreal forest is located in the Laurentian
Mountains, 70 km north of Quebec City, Quebec, Canada, in a region dominated by balsam fir and
white birch. Twelve four-hectare plots were chosen on a north-eastern slope varying in elevation
from 650 to 800 m, all situated in mature rich stands with a mesic-with-seepage drainage class. In
eight plots, partial cutting treatments of two intensities (25 and 40% stand basal area removed) were
done in autumn 2009. Four untreated plots were used as controls. Thinning was realised with
mechanical harvesters leaving 3.3±0.2m skid trails in the treated plots, which represented 15%
stand basal area removal. To achieve the desired percentage removal, selected trees were removed
on either side of the skid trails. Remaining forest strips were 22±3m broad.
Because the spruce budworm population in the study area was on a very low level, sentinel hosts
were introduced to measure parasitism. To this end, overwintering second instar larvae of the spruce
budworm were obtained from the insect rearing facility of the Canadian Forest Service (Great Lakes
Forest Research Centre, Sault Ste. Marie, Ontario) and reared on artificial diet (McMorran, 1965) in
growth chambers at 23°C and a 16-h photoperiod until the desired instar. The appropriateness of
using artificial diet-fed larvae for field implantation to measure parasitism was tested elsewhere
(Seehausen et al., accepted). To synchronize the development of implanted larvae with that of the
natural populations, a model of spruce budworm seasonality was used (Régnière et al., 2012).
A total of 540 and 585 pupae were introduced in the study area in 2010 and 2011, respectively. For
this purpose, trees were chosen in remaining forest strips for plots with silvicultural treatments and
randomly in the forest for control plots. Pupae were placed on balsam fir branches at breast height
(1.3m) by binding together several adjacent shoots and placing one single pupa in the centre (Fig. 3-
1). Only five pupae were installed on any given tree and trees were at least 15 meters apart from
each other to avoid excessive host clustering. Because spruce budworm pupae are commonly the
prey of birds (Crawford & Jennings, 1989; Venier & Holmes, 2010; J.R., unpublished data),
exclusion cages were installed around each branch bearing the pupae. These cages consisted of a
nylon net supported by a metal hoop fastened to a wooden stake in front of the branch. The 2.5-cm
mesh was large enough to not interfere with searching parasitoids but prevented birds from reaching
42
the pupae. Pupae were installed twice a week and recovered after seven days. They were
transported to the laboratory in a cooling box, placed in individual containers and reared at room
temperature until moth or parasitoid emergence.
A total of 2015 and 2340 fourth to sixth instar larvae were installed in 2011 and 2012, respectively,
on balsam fir current-year shoots at eye level. Groups of two to five young trees were chosen in
each plot to install five larvae per group. Groups were at least 15 meters apart from each other.
Larvae were installed twice a week and recovered after an exposure period of seven days. To
recover exposed larvae and also immature stages of parasitoids (larvae or cocoons) in the foliage in
the vicinity of their host, whole shoots containing the exposed budworms were cut. After transport
to the laboratory in a cooling box, larvae were placed in individual containers on artificial diet
(McMorran, 1965) and reared at room temperature until moth or parasitoid emergence.
Statistical analysis
Parasitism was analyzed using logistic regressions with binomial distributions and random effects
on the plots nested within the treatments. Exposure periods were introduced as repeated measures.
Effects of silvicultural treatments were tested with a priori comparisons between controls and
treated plots as well as between the two levels of partial cutting; P-values were adjusted for
multiplicity (PROC GLIMMIX, SAS Institute Inc., 2004). Larvae and pupae dying from causes
other than parasitism were excluded from the analysis.
Results
Pupal parasitism was very low in both sampling years (2010 and 2011); only two of the recovered
pupae were parasitized, both by Itoplectis conquisitor (Say) (Hymenoptera: Ichneumonidae).
Overall parasitism of fourth and fifth instar spruce budworm larvae was 76.6 and 53.9% in 2011
and 2012 respectively. Most of this parasitism was done by Tranosema rostrale (Brishke)
(Hymenoptera: Ichneumonidae) and Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae).
In both years, a few Phytodietus sp. (prob. vulgaris (Cresson)) (Hymenoptera: Ichneumonidae) and
some unidentified species were also present (hereafter referred to as other species) (Fig. 3-2G and
H).
In 2011, overall parasitism of fourth and fifth instar larvae was significantly lower in plots with
partial cutting, regardless of its intensity (Table 3-1), but did not vary significantly over the season
(Fig. 3-2A; F5, 25.19=0.64; P=0.6689). However, in 2012, partial cutting had no significant effect on
overall parasitism of fourth and fifth instar larvae (Table 3-1), but decreased significantly over the
43
sampling period (Fig. 3-2B; F5, 25.76=8.61; P<0.0001). The same tendency between the two years
was found for parasitism of fourth and fifth instar larvae when parasitism rates of T. rostrale were
submitted to statistical analysis. Parasitism was significantly lower in plots with partial cutting of
both intensities in 2011 but no significant influence of partial cutting on parasitism by this species
was found in 2012 (Table 3-1). Parasitism by E. cacoeciae and other species did not differ
significantly between the silvicultural treatments in both years (Table 3-1).
A significant decrease in the parasitism rate by T. rostrale over the sampling period was found in
2011 (Fig. 3-2C; F5, 26.37= 7.79; P=0.0001) as well as in 2012 (Fig. 3-2D; F5, 24.66=5.84; P=0.0011).
In contrast, parasitism by E. cacoeciae increased significantly over the sampling period in 2011
(Fig. 3-2E; F5, 26.44=5.67; P=0.0011) but in 2012, spruce budworm larvae were only attacked in the
first four sample periods by this species, during which its parasitism rate did not differ significantly
over time (Fig. 3-2F; F3, 19.81=0.63; P=0.6072). Parasitism by other species was significantly higher
in the beginning of the sampling period in 2011 (Fig. 3-1G; F5, 24.99=2.70; P=0.0441) but did not
vary significantly over time in 2012 (Fig. 3-2H; F2, 26.11=1.55; P=0.2081).
In 2011, not enough sixth instar larvae were collected after the exposure period to be submitted to
statistical analysis. However, parasitism of sixth instar larvae in 2012 (22.0%, n=436) was rather
low compared to parasitism of fourth and fifth instar larvae. Parasitism was done by T. rostrale
(16.7%), E. cacoeciae (0.2%) and some unidentified parasitoid species (5.1%). Partial cutting had
an effect neither on overall parasitism (F2, 29.67=1.12; P=0.3406) nor on T. rostrale attacking sixth
instar larvae (F2, 29.51=0.72; P=0.4961). Parasitism by E. cacoeciae and other species was too low to
be submitted to statistical analysis.
Discussion
Partial cutting significantly decreased parasitism of fourth and fifth instar spruce budworm larvae in
2011. Several hypotheses can be proposed to explain this reduction in parasitism. For instance, skid
trails, used by the harvesting machinery as pathways and to move trunks, constitute a fragmentation
of the forest which may influence parasitoids in several ways. Roland and Taylor (1997) suggest for
example that forest fragmentation may alter parasitism through the effects of habitat structure on
parasitoid movement. Openings of the forest canopy through silvicultural treatments like
commercial thinning can also cause substantial microclimatic changes (Aussenac, 2000; Thibodeau
et al., 2000) which in turn may influence parasitoids. For instance, higher temperatures as can be
found in stands with partial cutting (Seehausen, 2013) can result in less efficient exploitation of
patches which can result in a lower parasitism rate (Denis et al., 2011). Possibly modified humidity
44
(Gol’Berg, 1982; Ryan, 1974) and higher wind speeds (Messing et al., 1997; Gu & Dorn 2001)
resulting from partial cutting treatments may have similar effects on parasitism.
The analysis of parasitism rates of each parasitoid species revealed that only parasitism by T.
rostrale was negatively influenced by partial cutting in 2011. Thus, the negative influence of this
silvicultural treatment on overall parasitism is due to the influence on the parasitism rate of this
species. Roland & Taylor (1997) suggest that parasitoid species may each respond to forest
fragmentation at a characteristic spatial scale, depending on the size of the parasitoid species. In this
study, only one other parasitoid species, E. cacoeciae, was abundant enough to be analysed
separately. E. cacoeciae is smaller than T. rostrale and was significantly influenced by partial
cutting neither in 2011 nor in 2012. However, with only two species, it is beyond the scope of this
study to assess the reason for the differences of the influence of partial between the species.
No significant influence of the silvicultural treatments on larval parasitism was found in 2012,
which may indicate that the negative impact of partial cutting on parasitism detected in 2011 is
ephemeral. However, if partial cutting only influences parasitism in the short term, the previously
raised hypotheses about how the treatments may influence parasitism would be invalidated, because
the skid trails persist until 2012 and beyond, and should therefore continue to influence parasitism.
In a related study about the influence of partial cutting on pupal parasitism of the hemlock looper,
Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae), a similar phenomenon has been
described (Seehausen, 2013). While parasitism was negatively affected by partial cutting one year
after the silvicultural treatments, no significant influence has been detected in the following year. In
this case, Malaise trap samples showed an important reduction in the abundance and activity of the
mainly affected parasitoid in the same year in which no influence of partial cutting was detected,
which was also reflected in a reduced parasitism rate of the same species. Seehausen (2013)
hypothesises that this interannual variation in the abundance of adult parasitoids in the study area
may be related to the interannual variations in the influence of the silvicultural treatment. In the
present study, no Malaise trap samples were evaluated. However, an important reduction in
parasitism by T. rostrale and E. cacoeciae was found in 2012 (Table 3-1), indicating that
interannual variations in the abundance of adult parasitoids may also affect the influence of partial
cutting on parasitism rates.
The high overall parasitism of the fourth and fifth instar larvae in both years shows the importance
of parasitoids as natural control agents in the endemic phase of spruce budworm populations. High
parasitism rates by T. rostrale in both years emphasizes the efficiency of this species in finding
45
scarce hosts and its potential contribution in maintaining spruce budworm abundance at low levels.
However, the parasitoid complex of the spruce budworm and its influence on spruce budworm
survival changes considerably in outbreak conditions (Blais, 1965a; Miller & Renault, 1976).
Therefore, results of this study apply specifically to endemic spruce budworm populations.
Sixth instar larvae and pupae of the spruce budworm were only occasionally parasitized. Because
parasitism of sixth instar larvae was only measured in 2012 and information about parasitism in
endemic spruce budworm populations is very scarce, it remains unknown if the low parasitism of
these instars is only related to conditions found during the present study or if this is a general
observation for low density populations. However, an important reduction of parasitism by E.
cacoeciae was observed in 2012 compared to 2011. This exoparasitoid is known to prefer fifth
instar spruce budworm larvae as host but is also an important parasitoid of the sixth instar (Fidgen
et al., 2000). Our data for parasitism by E. cacoeciae under field conditions agree with these
observations. During the exposure periods of fourth and fifth instar larvae in 2011, parasitism by E.
cacoeciae increased over time, indicating the preference to attack fifth instar spruce budworm
larvae. In contrast, during the same exposure periods, parasitism by T. rostrale increased over time,
suggesting its preference for fourth instar larvae under field conditions. The reason for the reduced
parasitism rates of both species in 2012 remains unknown but may explain the low parasitism of
sixth instar larvae. Several parasitoid species are known to attack spruce budworm pupae in the
endemic phase (Miller & Renauld, 1976; Cappucino et al., 1998) but many also fall prey to birds
(J.R., unpublished data). Thus, low mortality of budworm pupae from parasitism may be
compensated by increased bird predation.
Conclusion
The present study demonstrated a negative influence of partial cutting on parasitism of spruce
budworm larvae the second year after treatments. Bauce (1996) and Fuentealba & Bauce (2012a)
found that in the short term partial cutting reduced tree resistance to spruce budworm through
reduction in foliar monoterpenes, resulting in enhanced budworm feeding performance and a higher
defoliation. A simultaneously reduced parasitism may further improve spruce budworm
performance and could consequently increase defoliation, causing serious damage in susceptible
forest stands. Therefore, it may be advisable to refrain from partial cutting in stands threatened by
imminent spruce budworm defoliation. Alternatively, partial cutting may be accompanied with
application of insecticides for at least the first two years, to prevent increased defoliation damage in
the short term. However, we found that parasitism was not significantly influenced by partial
46
cutting three years after treatments. More research is necessary to determine the long term effect of
partial cutting on parasitism and on stand resistance to spruce budworm defoliation.
Acknowledgments
We are grateful to N. Giasson, P. Huron, M. Lemay, P. Charron and É. Tremblay-Bérubé for help in
the laboratory and in the field and to M. Addy for drawing figure 1. We also thank P. Therrien, G.
Trudel, C. Dussault and D. Simoneau of the Ministère des Ressources Naturelles et de la Faune du
Québec (MRNFQ) for insect rearing and parasitoid identification and G. Pelletier from the
Canadian Forestry Service (CFS) for the identification of some parasitoids. M. Bernier-Cardou
(CFS) provided important statistical support. Financial support was provided to the iFor Research
Consortium by the Natural Sciences and Engineering Research Council of Canada (NSERC-
CRSNG), the MRNFQ, the Conseil de l’Industrie Forestière du Québec (CIFQ), the CFS and the
Société de Protection des Forêts contre les Insectes et les Maladies du Québec (SOPFIM).
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Table 3-1: Parasitism rates (means (%) ± SEM) of species attacking 4th and 5
th instar spruce budworm larvae for control stands and two intensities
of partial cutting (25 and 40% stand basal area reduced) in 2011 and 2012.
control 25% 40% F df p
2011
number of analyzed larvae 190 162 174
total parasitism 93 .08 ±2.58 a 71 .11 ±4.91 b 65 .25 ±5.02 b 9 .23 2, 52.82 0 .0004
Tranosema rostrale 71 .12 ±4.24 a 42 .95 ±5.02 b 34 .09 ±4.63 b 14 .63 2, 53.62 <0 .0001
Elachertus cacoeciae 11 .80 ±3.08 a 16 .85 ±4.06 a 16 .64 ±3.94 a 0 .82 2, 47.23 0 .4445
others 5 .01 ±1.74 a 1 .67 ±0.95 a 4 6.45 ±2.11 a 2 .61 2, 21.79 0 .0961
2012
number of analyzed larvae 245 239 240
total parasitism 56 .51 ±5.60 a 47 .24 ±5.66 a 48 .81 ±5.68 a 0 .72 2, 55.99 0 .4889
Tranosema rostrale 40 .16 ±4.44 a 34 .54 ±4.31 a 35 .48 ±4.37 a 0 .48 2, 37.36 0 .6247
Elachertus cacoeciae* 5 .10 ±1.72 a 3 .78 ±1.50 a 4 .46 ±1.64 a 0 .19 2, 23.51 0 .8322
other species 11 .57 ±2.48 a 8 .21 ±2.12 a 9 .03 ±2.24 a 0 .61 2, 41.79 0 .5458
*For E. cacoeciae in 2012, only parasitism rates of the first four sample periods were analyzed and are presented here.
Numbers followed by the same letter do not differ significantly at P < 0.05 according to a priori comparisons.
51
Figure 3-1: Pupae were placed in each plot on balsam fir branches by binding together several
adjacent shoots and placing one single pupa in the centre.
52
Figure 3-2: Mean rates of overall attack (A, B), attack by T. rostrale (C, D), E. cacoeciae (E, F)
and other parasitoid species (G, H) as a function of time (ordinal date) in 2011 and 2012 for two
intensities of partial cutting (25 and 40% stand basal area reduced) and control stands.
53
Conclusion
Le présent travail a mis en évidence une influence négative à court terme de la coupe partielle sur le
taux de parasitisme chez l’arpenteuse de la pruche et chez la tordeuse des bourgeons de l’épinette.
Cette influence a été observée un an après traitement chez les chrysalides de l’arpenteuse de la
pruche et deux ans après traitement pour le quatrième et le cinquième stade larvaire de la tordeuse
des bourgeons de l’épinette. Toutefois, l’année suivant l’observation de cet effet négatif des coupes
partielles, aucune influence significative des traitements n’a pu être démontrée pour les deux
espèces étudiées. Les mécanismes expliquant ce phénomène restent inconnus, et cela même si des
données supplémentaires rassemblées dans le cadre de cette étude permettent de soulever quelques
hypothèses.
En 2010, les températures moyenne et maximale sont significativement plus élevées dans les
parcelles où les coupes partielles ont été réalisées. À l’opposé, l’humidité relative a été
significativement diminuée dans les parcelles avec traitements sylvicoles. Les deux facteurs peuvent
influencer le comportement et le succès d’attaque des parasitoïdes (Hance et al., 2007; Kalyebi et
al., 2005; Ryan, 1974). Cependant, les différences observées entre les parcelles témoins et les
parcelles ayant subies la coupe partielle, bien que significatives, ne sont pas très importantes. De
plus, aucune différence significative n’a été établie entre les deux intensités de traitement, sauf pour
la température maximale pendant la période d’échantillonnage des pièges Malaise. Seule la
température maximale a été faiblement corrélée avec le taux de parasitisme des chrysalides de
l’arpenteuse de la pruche en 2010. Ainsi, ces résultats indiquent que d’autres facteurs expliquant
l’influence des coupes partielles sur le parasitisme doivent être en cause.
Les échantillons des pièges Malaise en 2010 ont démontré que les femelles du principal parasitoïde
attaquant les chrysalides de l’arpenteuse de la pruche, A. ontario, étaient significativement
capturées davantage dans les parcelles avec coupe partielle, peu importe l’intensité du traitement.
Ce résultat pourrait indiquer une plus grande activité de vol dans les parcelles avec coupe partielle
due à un espacement plus grand entre les arbres résiduels. Toutefois, cette hypothèse n’explique pas
le taux de parasitisme chez les chrysalides de l’arpenteuse de la pruche. En effet, l’abondance des
femelles dans les pièges Malaise était plus élevée dans les parcelles ayant subies la coupe partielle,
alors qu’une diminution du taux de parasitisme n’a été observée que dans les parcelles montrant
une réduction de 40% de la surface terrière. Le très faible nombre de captures de femelles d’A.
ontario en 2011 peut cependant expliquer en partie un taux de parasitisme inférieur à celui observé
l’année précédente.
54
L’influence de la coupe partielle sur le parasitisme était différente pour les deux défoliateurs
étudiés. En effet, une diminution du taux de parasitisme des chrysalides de l’arpenteuse de la pruche
a uniquement été observée dans les parcelles ayant une réduction de 40% de la surface terrière,
alors que le taux de parasitisme des larves de la tordeuse des bourgeons de l’épinette était influencé
dans toutes les parcelles ayant subies la coupe partielle peu importe le niveau d’intensité. Roland et
Taylor (1997) ont démontré que l’influence de la fragmentation de la forêt sur des parasitoïdes peut
être reliée à la taille du corps du parasitoïde, qui limite sa capacité de déplacement dans une forêt
fragmentée. La coupe partielle peut être considérée comme une fragmentation de l’habitat à petite
échelle, puisque les chemins de débardage séparent les parcelles de la forêt résiduelle. Les deux
espèces dominantes de parasitoïdes qui sont responsables de la réduction du taux de parasitisme
après coupe, A. ontario chez l’arpenteuse de la pruche et T. rostrale chez la tordeuse des bourgeons
de l’épinette, diffèrent considérablement en taille. La taille du corps de T. rostrale est inférieure à
celle d’A. ontario. En prenant en compte les constatations de Roland et Taylor (1997), la diminution
du parasitisme chez la plus petite espèce, T. rostrale, dans toutes les parcelles ayant subies une
coupe partielle, pourrait s’expliquer par une capacité de déplacement plus limitée chez cette espèce.
À l’opposé, il est possible que le taux de parasitisme chez l’espèce relativement plus grosse, A.
ontario, ait seulement été influencé par l’intensité de coupe la plus élevée en raison d’une meilleure
performance de vol, liée à la taille de son corps. Cependant, le parasitoïde de la tordeuse des
bourgeons de l’épinette E. cacoeciae est plus petit que T. rostrale et n’a pas été influencé par les
traitements sylvicoles. Un cas similaire est décrit dans l’étude de Roland et Taylor (1997), où le
plus petit parasitoïde est positivement influencé par la fragmentation de la forêt. Dans le cas de E.
cacoeciae, l’hypothèse peut être soulevée que cette espèce n’est pas capable de migrer entre les
parcelles résiduelles mais étant donné que la taille de son domaine vital est naturellement très petite,
il peut rester dans les parcelles résiduelles sans être négativement influencé par la fragmentation.
Cependant, les relations proposées restent strictement hypothétiques dans le cadre de ce travail et
nécessite davantage d’investigations afin d’être validée.
Le fait que le taux de parasitisme a seulement été influencé un an après les traitements pour
l’arpenteuse de la pruche et deux ans après les traitements pour la tordeuse des bourgeons de
l’épinette suppose que la coupe partielle perturbe uniquement à court terme les relations hôte-
parasitoïde. Davantage de recherches sont requises pour évaluer l’effet à long terme de ce
traitement sylvicole et également pour trouver les véritables mécanismes impliqués dans la
diminution du parasitisme démontrée dans le présent travail à court terme.
55
La population de tordeuse des bourgeons de l’épinette dans la Forêt Montmorency est présentement
endémique. En 2012, des peuplements ont été fortement défoliés par l’arpenteuse de la pruche à la
Forêt Montmorency. Cependant, la population de l’arpenteuse de la pruche dans les parcelles
expérimentales n’était pas assez importante pour être considérée comme épidémique. Comme
mentionné dans l’introduction, le complexe et l’influence des parasitoïdes peut changer
considérablement pendant une épidémie (Miller & Renault, 1976; Blais, 1965a). Par exemple, T.
rostrale est une espèce liée aux populations endémiques de la tordeuse des bourgeons de l’épinette
et joue un rôle mineur pendant les épidémies (Cusson et al., 1998; Miller & Renault, 1976). Pour
cette raison, il est important de mentionner que l’effet négatif de la coupe partielle sur le
parasitisme, tel qu’observé dans cette étude, n’est pas nécessairement transférable à une situation
épidémique de ces deux défoliateurs. Des expériences sur l’influence des coupes partielles sur le
parasitisme en situation d’épidémie devraient être effectuées pour définir l’influence de ce type
d’aménagement lorsque les populations du ravageur forestier sont à un niveau épidémique.
Néanmoins, il est important de prendre en considération les résultats obtenus dans les décisions de
gestion sylvicole pour la prévention des dommages causés par des insectes défoliateurs. Pour garder
le niveau de parasitisme élevé dans ces peuplements, il est recommandé de ne pas effectuer de
coupes partielles si le peuplement fait prochainement face à une défoliation par la tordeuse des
bourgeons de l’épinette, alors que pour l’arpenteuse de la pruche, une coupe partielle ne dépassant
pas 25% de réduction de la surface terrière pourrait permettre de conserver l’effet bénéfique des
parasitoïdes. Cependant, la rentabilité économique d’une coupe partielle de 25% doit être évaluée.
Si des coupes partielles sont effectuées dans des peuplements vulnérables à la défoliation par l’un
des deux défoliateurs, il serait possible d’appliquer des outils de contrôle comme l’insecticide
biologique Btk pour réduire la vulnérabilité des ces peuplements lors des deux premières années
suivant les coupes partielles.
Par ailleurs, les expériences effectuées dans le cadre du présent mémoire peuvent apporter une
meilleure compréhension des relations hôte-parasitoïde dans les populations endémiques de la
tordeuse des bourgeons de l’épinette et dans les populations en croissance de l’arpenteuse de la
pruche, puisque la littérature sur le parasitisme chez ces types de populations est rare. Les
expériences sur l’arpenteuse de la pruche ont démontré un important taux de parasitisme chez les
chrysalides alors que les larves de cette espèce ne sont parasitées qu’occasionnellement. À l’opposé,
le taux de parasitisme le plus élevé chez la tordeuse des bourgeons de l’épinette a été observé
pendant le quatrième et le cinquième stade larvaire. Le sixième stade larvaire chez cette espèce a été
moins parasité et le parasitisme des chrysalides a été presque absent.
56
En outre, l’expérience sur l’influence de la diète des larves de tordeuse des bourgeons de l’épinette
sur le taux de parasitisme a mis en évidence que, les larves élevées sur le feuillage de sapin baumier
et celles élevées sur la diète artificielle McMorran (1965) avant l’exposition aux parasitoïdes, sont
autant attaquées l’une que l’autre sur le terrain lors d’une exposition de sept jours. Cependant, des
tests de choix en laboratoire avec le parasitoïde T. rostrale ont démontré une préférence
significative pour les larves élevées sur le feuillage de sapin baumier. Cette observation suggère
l’importance d’une période d’alimentation de la larve sur le feuillage durant l’exposition sur le
terrain et offre un aperçu du comportement du parasitoïde T. rostrale dans la recherche de ses hôtes.
Ces résultats ne valident pas seulement les méthodes appliquées dans le présent travail, mais
également l’utilisation des larves élevées sur la diète artificielle McMorran (1965) comme hôte
sentinelle sur le terrain dans les études passées (Doucet & Cusson, 1996a, 1996b; Fidgen &
Eveleigh, 1998; Fidgen et al., 2000; Cusson et al., 2002) et futures sur les parasitoïdes des
populations endémiques de la tordeuse des bourgeons de l’épinette.
57
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