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

We investigated interactions between Bacillus thuringiensis Berliner var. k urstaki and parasitoids that attack late instars of the eastern spruce budw orm, Choristoneura fumiferana (Clemens). In a petri-dish arena, females of Tranosema rostrale rostrale (Brishke) (Hymenoptera: Ichneumonidae) were abl e to discriminate between untreated fourth instars and fourth instars that had been given a known dose of a commercial product (Foray 48B). When the c hoice tests were conducted before host mortality due to B. thuringiensis ha d occurred among treated larvae (24 h post ingestion), the parasitoid attac ked untreated larvae more readily. When females were given a choice between control larvae and treated larvae that were still alive 72 h post ingestio n, 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 tha t exhibited a lethal response to the pathogen, and to a lesser extent larva e that had survived pathogen exposure. The ability of the latter was not ap parent under field conditions. When treated and untreated larvae were expos ed for 1 week to a complex of indigenous parasitoids in the field, there wa s no difference between treatments in the rates of parasitism by either T. r. rostrale or Actia interrupta Curran (Diptera: Tachinidae). Parasitism av eraged 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 parasiti zed as unexposed, healthy larvae. This means that prolonged development of late-instar spruce budworm larvae after treatment with B. thuringiensis cou ld possibly result in increased attack rates by parasitoids.