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  • J.R. Byers (a1), D.S. Yu (a1) and J.W. Jones (a2)


During an outbreak of army cutworm in southern Alberta in the spring of 1990, the overall incidence of parasitism by the polyembryonic parasitoid, Copidosoma bakeri (Howard), was 61% in samples from seven fields. The incidence of parasitism in samples of army cutworms collected on five dates from a single location, during the spring of 1991, increased from about 20% in the early samples to about 50% in the later samples. Cutworms parasitized by C. bakeri feed for a longer time than unparasitized ones; therefore estimates of the incidence of parasitism by C. bakeri, based on samples of late-instar cutworms, are misleadingly high. Parasitized cutworms also grow considerably larger than unparasitized ones and may have a supernumerary instar. Larger hosts support larger broods of C. bakeri and apparently a successful strategy of C. bakeri is to prolong host development so as to maximize an acquired resource. Because cutworms parasitized by C. bakeri feed more and longer than unparasitized cutworms, a high rate of parasitism can exacerbate crop damage and complicate control recommendations. The life cycles of army cutworm and C. bakeri are asynchronous and it is likely that high rates of parasitism are dependent on the presence of intermediary hosts.

Au cours d’une infestation de Légionnaires grises dans le sud de l’Alberta au printemps de 1990, le taux de parasitisme par le parasitoïde polyembryonnaire Copidosoma bakeri (Howard) a été évalué à 61% dans des échantillons provenant de sept champs. Au cours d’un programme d’échantillonnage effectué en un endroit au printemps de 1991, l’incidence du parasitisme a augmenté d’environ 20% dans les premiers échantillons à environ 50% dans les échantillons recueillis plus tard. Les légionnaires parasitées par C. bakeri se nourrissent plus longtemps que les légionnaires saines, et les estimations de l’importance de l’infection basées uniquement sur les échantillons de légionnaires de dernier stade sont beaucoup trop élevées. Les légionnaires parasitées atteignent également une taille beaucoup plus grande que les légionnaires non parasitées et subissent peut-être une mue supplémentaire. Les hôtes les plus gros supportent des fardeaux de parasites plus grands et il semble que le parasite utilise une stratégie avantageuse en prolongeant le développement de l’hôte, maximisant ainsi la durée de son utilité. Comme les légionnaires parasitées par C. bakeri se nourrissent plus abondamment et plus longtemps que les légionnaires saines, un taux élevé de parasitisme peut exacerber les dommages aux récoltes et complexifier les problèmes de contrôle. Les cycles biologiques de la légionnaire et du parasite sont asynchrones et il est fort possible que des taux élevés de parasitisme soient reliés à la présence d’hôtes intermédiaires.

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  • J.R. Byers (a1), D.S. Yu (a1) and J.W. Jones (a2)


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