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DOSAGE RESPONSES OF THE LARVAL INSTARS OF THE BERTHA ARMYWORM, MAMESTRA CONFIGURATA (LEPIDOPTERA: NOCTUIDAE), TO A NATIVE NUCLEAR POLYHEDROSIS1

Published online by Cambridge University Press:  31 May 2012

G. E. Bucher  and
W. J. Turnock
G. E. Bucher
Affiliation:
Agriculture Canada, Research Station, Winnipeg, Manitoba R3T 2M9
W. J. Turnock
Affiliation:
Agriculture Canada, Research Station, Winnipeg, Manitoba R3T 2M9
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Abstract

The dosage–mortality relationship of a multi-embedded nuclear polyhedrosis virus was determined for each instar of the bertha armyworm by probit analysis of mortality following the ingestion of polyhedral inclusion bodies (PIBs) placed on the surface of an artificial diet. Susceptibility to infection decreased as larvae aged; the median exposure doses (LE50) increased from 18 PIBs/larva for the first larval instar (L1) to 21 × 106 PIBs/larva for half-grown L6. When given equivalent doses, the time for infected insects to die increased with the age of the larvae. Within any given instar the length of time to death shortened with increasing doses. Larvae that survived to become adults showed no signs of infection. There was no evidence that any infected individual could recover from infection. From the data presented, calculations show that maximum production, 2.90 × 109 PIBs/treated insect, is achieved by feeding 10,000–20,000 PIBs/larva to young L5. Larvae dying as L6 have about 40% of their normal dry weight of essential tissue converted to PIBs. Although this MNPV currently would not be cost effective as a spray for protecting crops from bertha armyworm damage, its potential as a biocontrol agent, applied in small amounts at an early stage in bertha armyworm outbreaks, should be evaluated.

Résumé

Le rapport dosage-mortalité d'une polyédrose nucléaire à inclusions multiples a été déterminé pour chaque stade de développement de la légionnaire bertha par l'analyse (méthode des probits) de la mortalité après ingestion de corps d'inclusions polyédriques (PIB) placés à la surface d'une ration artificielle. La sensibilité à l'infection diminue avec l'âge des larves; les doses d'exposition médianes (LE50) augmentent de 18 PIB/larve pour le premier stade larvaire (L1) à 21 × 106 PIB/larve pour la larve à moitié développée (L6). Pour des doses équivalentes, l'intervalle entre l'infection et la mortalité augmente avec l'âge des larves. Pour n'importe quel stade, l'intervalle diminue avec l'accroissement de la dose. Les larves qui survivent jusqu'à l'âge adulte ne manifestent aucun symptôme d'infection. D'après les données fournies, les calculs révèlent que la production maximale de PIB (2.90 × 109 PIB/insecte traité) est atteinte en servant 10 à 20,000 PIB/larve à de jeunes larves L5. Les larves qui meurent au stade L6 ont environ 40% de leur poids sec normal de tissu essentiel transformé en PIB. Bien que ce moyen de lutte ne serait actuellement pas rentable en pulvérisation pour protéger les cultures contre la légionnaire, ses possibilités comme agent de lutte biologique, appliqué en petites quantités à un stade précoce de foyers d'infestation du ravageur, devraient être évaluées.

Type
Articles
Information
The Canadian Entomologist , Volume 115 , Issue 4 , April 1983 , pp. 341 - 349
Copyright
Copyright © Entomological Society of Canada 1983

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References

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