Effects of bacillus thuringiensis and destruxins (Metarhizium anisopliae mycotoxins) combinations on spruce budworm (Lepidoptera: tortricidae)

Laboratory bioassays were performed in order to assess the efficacy of a combination of a commercial preparation of Bacillus thuringiensis var. kurstaki (B.t.k.) (Thuricide) with destruxins (Metarhizium anisopliae mycotoxins) on the fifth instar of Choristoneura fumiferana Clemens. Lethal doses were determined for each microbial agent and used as a basis for the bioassays involving a combination of the two agents. Interaction or the lack of interaction was determined by comparing the observed and the theoretically expected mortality rates. Seven out of 10 different combinations demonstrated synergism between the two agents. The modelization applied on results allowed us to establish the following general equation: LDmixture = 1.259(LDDx) + 1.129(LDBt) - 0.016(LDDx)(LDBt) + 12.196. Such an equation explains the relationship which exists between the two lethal agents (R2 = 0.99). Our results suggest that the two agents contribute to the synergism in the system and that a combination of both could be an efficient means of controlling C. fumiferana populations and of reducing the dose of B.t.k. which is usually required for such a control. Copyright 1998 Academic Press.