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DENSITY, PARASITISM, AND DISEASE INCIDENCE OF LARVAE OF THE BERTHA ARMYWORM, MAMESTRA CONFIGURATA WALKER (LEPIDOPTERA: NOCTUIDAE), IN MANITOBA, 1973–1986’1

Published online by Cambridge University Press:  31 May 2012

W. J. Turnock
W. J. Turnock
Affiliation:
Agriculture Canada Research Station, 195 Dafoe Road, Winnipeg, Manitoba, Canada R3T 2M9
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Abstract

Populations of larvae of the bertha armyworm, Mamestra configurata Wlk., in four physiographic regions of Manitoba showed similar trends over time: a decline from the outbreak of 1971–1972 to very low densities in 1975–1977, an increase to a peak during the years 1979–1981, and a subsequent decline. During the period of peak larval populations, brief (1 or 2 years) outbreaks [at least some fields with > 20 larvae per square metre) occurred at five locations in two regions, the Swan River Plain and the Valley River Plain, but not in the Western Uplands or the Manitoba Lowlands. In the first two regions, larval densities rose rapidly (from < 1.6 to > 13.8/m2) in 1 year. Although the general trend of population density was similar, there were differences in density among and within regions, and in the timing, severity, and duration of peak populations. Two parasitoids (Banchus flavescens Cress., Athrycia cinerea (Coq.)) and two pathogens (a nuclear polyhedrosis virus (NPV) and fungi of the Entomophthorales) occurred regularly in larval populations. Of these, B. flavescens had the highest constancy among collections and may help to keep bertha armyworm populations at low densities. NPV was rarely found among larvae from low-density populations but appeared in all populations that reached outbreak levels. No single biotic agent could be associated with the population declines because of multiple parasitism and the difficulty in partitioning mortality when only a single sample could be taken. The rapid increase of bertha armyworm larvae from very low to outbreak levels in 1 year will prevent predictions of outbreaks from being based on larval densities in the preceding year.

Résumé

Les populations de larves de la légionnaire bertha, Mamestra configurata Wlk., dans quatre régions physiographiques du Manitoba montrent des tendances analogues dans le temps, soit un déclin à partir du foyer d’infestation de 1971–1972 jusqu’à de très faibles densités en 1975–1977, suivi par une remontée jusqu’à un sommet au cours des années 1979–1981, et par un déciin subséquent. Au cours de la période du pic de densité des populations larvaires, de brefs (1 ou 2 ans) foyers (au moins quelques champs affichant plus de 20 larves/m2) se sont produits à cinq endroits dans deux régions, soit la plaine de la Swan River et celle de la Valley River, mais pas dans les terres hautes de l’Ouest ni les terres basses du Manitoba. Dans tes deux premières régions, les densités larvaires ont augmenté rapidement (< 1,6 à > 13,8/m2) en l’espace d’un an. Bien que la tendance générale de la densité de population était comparable, on a constaté des différences de densité entre les régions et à l’intérieur d’une même région, ainsi que dans la synchronisation, la gravité et la durée des pics de densité de population. Deux parasitoïdes (Banchus flavescens Cress., Athrycia cinerea (Coq.)) et deux pathogènes (un virus à polyhédrose nucléaire (NPV) et des champignons du genre Entomophthorales) se rencontraient régulièrement dans les populations larvaires. Parmi ceux-ci. B. flavescens affichait la plus grande constance dans les prélèvements et pourrait contribuer à maintenir faibles les densités de population de la légionnaire bertha. Le NPV se retrouvait rarement parmi les larves des populations de faible densité, mais apparaissait dans toutes les populations ayant atteint des niveaux de foyer d’infestation. Aucun agent biotique agissant isolément ne pouvait être associé an déclin de la population à cause du parasitisme multiple et de la difficulté de répartir la mortalité lorsqu’il n’était possible de prélever qu’un seul échantillon. L’augmentation rapide du nombre de larves de la légionnaire jusqu’à des niveaux de foyer d’infestation en l’espace d’un an empêchera de fonder la prédiction des foyers sur les densités larvaires observées l’année précédente.

Type
Articles
Information
The Canadian Entomologist , Volume 120 , Issue 5 , May 1988 , pp. 401 - 413
Copyright
Copyright © Entomological Society of Canada 1988

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