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Interactions between Bacillus thuringiensis and parasitoids of late-instar larvae of the spruce budworm (Lepidoptera: Tortricidae)

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We investigated interactions between Bacillus thuringiensis Berliner var. kurstaki and parasitoids that attack late instars of the eastern spruce budworm, Choristoneura fumiferana (Clemens). In a petri-dish arena, females of Tranosema rostrale rostrale (Brishke) (Hymenoptera: Ichneumonidae) were able to discriminate between untreated fourth instars and fourth instars that had been given a known dose of a commercial product (Foray 48B). When the choice tests were conducted before host mortality due to B. thuringiensis had occurred among treated larvae (24 h post ingestion), the parasitoid attacked untreated larvae more readily. When females were given a choice between control larvae and treated larvae that were still alive 72 h post ingestion, they were able to discriminate between the two only when the larvae had been treated with at least a 50% lethal dose. Under laboratory conditions, female T. r. rostrale were thus able to detect and avoid treated larvae that exhibited a lethal response to the pathogen, and to a lesser extent larvae that had survived pathogen exposure. The ability of the latter was not apparent under field conditions. When treated and untreated larvae were exposed for 1 week to a complex of indigenous parasitoids in the field, there was no difference between treatments in the rates of parasitism by either T. r. rostrale or Actia interrupta Curran (Diptera: Tachinidae). Parasitism averaged 91% for larvae in the control treatment compared with 92% for larvae treated with Foray 48B. The field data suggest that spruce budworm larvae that survive exposure to B. thuringiensis are just as likely to be parasitized as unexposed, healthy larvae. This means that prolonged development of late-instar spruce budworm larvae after treatment with B. thuringiensis could possibly result in increased attack rates by parasitoids.

Nous avons étudié les interactions entre Bacillus thuringiensis Berliner var. kurstaki et les parasitoïdes qui s'attaquent aux larves des derniers stades de la Tordeuse des bourgeons de l'épinette, Choristoneura fumiferana (Clemens). Dans l'enceinte d'un vase de pétri, les femelles de Tranosema rostrale rostrale (Brishke) (Hymenoptera : Ichneumonidae) ont été capables de faire la distinction entre des larves de quatrième stade non traitées et des larves qui avaient reçu une dose connue d'une préparation commerciale (Foray 48B). Dans des tests de choix, avant que la mortalité des hôtes causée par le bacille ne se soit manifestée chez les larves traitées (24 h après ingestion), les parasitoïdes ont attaqué les larves non traitées plus souvent que les larves traitées. Devant un choix entre des larves témoins et des larves traitées encore vivantes 72 h après l'ingestion, les femelles ont été capables de discrimination seulement lorsque les larves traitées avaient reçu une dose au moins égale à la dose létale 50 %. En laboratoire, les femelles de T. r. rostrale ont donc pu reconnaître et éviter les larves traitées qui ont eu une réaction létale au pathogène, et, à un degré moindre, les larves qui ont survécu au pathogène; ce dernier phénomène n'a pas été observé dans des conditions naturelles. Lorsque des larves traitées et des larves non traitées ont été exposées pendant 1 semaine à un complexe de parasitoïdes indigènes en nature, nous n'avons pas relevé de différence entre les larves traitées et les larves non traitées quant aux taux de parasitisme de T. r. rostrale ou d'Actia interrupta (Curran) (Diptera : Tachinidae). Le parasitisme affectait en moyenne 91 % des larves dans le traitement témoin, comparativement à 92 % des larves traitées au Foray 48B. Les données recueillies sur le terrain indiquent que les larves de la Tordeuse des bourgeons de l'épinette qui survivent au bacille sont tout aussi susceptibles d'être parasitées que les larves saines, non exposées au bacille. Cela signifie que le développement prolongé des larves de derniers stades de la Tordeuse des bourgeons de l'épinette après le traitement au bacille pourrait donner lieu à une augmentation des taux d'attaque des parasitoïdes.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: September 1, 2001

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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