INTERACTIONS BETWEEN BACILLUS THURINGIENSIS BERLINER AND APANTELES FUMIFERANAE VIER. (HYMENOPTERA: BRACONIDAE), A PARASITOID OF THE SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (CLEM.) (LEPIDOPTERA: TORTRICIDAE)

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Abstract
Interactions between Bacillus thuringiensis Berliner and Apanteles fumiferanae Viereck as mortality factors of the spruce budworm, Choristoneura fumiferana (Clemens), were investigated by placing parasitized and nonparasitized budworm larvae on foliage with and without spray deposits of a commercial formulation of B. thuringiensis. The effect of larval age (and, thus, the timing of spray applications) was examined by using peak third-instar and peak fourth-instar larvae. We demonstrated that parasitized larvae are more likely to survive exposure to B. thuringiensis because they feed less than non-parasitized larvae and are thus less likely to acquire a lethal dose of the bacterium. Bacillus thuringiensis nevertheless reduced parasitoid populations by 50–60% by killing their hosts before parasitoid emergence. This negative impact of B. thuringiensis on parasitoid survival was decreased when exposure of budworm larvae to spray deposits was delayed from peak third to peak fourth instar. The enhanced survival of parasitoids offset the lower mortality as a result of B. thuringiensis in the delayed sprays. We conclude that B. thuringiensis applied when budworm larval populations are at peak fourth instar or later would complement rather than interfere with the beneficial effects of A. fumiferanae. The implications for budworm management strategies are discussed.

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