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 v ar. kurstaki (B.t.k.) (Thuricide) with destruxins (Metarhizium anisopl iae mycotoxins) on the fifth instar of Choristoneura fumiferana Clemen s. Lethal doses were determined for each microbial agent and used as a basis for the bioassays involving a combination of the two agents. In teraction or the lack of interaction was determined by comparing the o bserved and the theoretically expected mortality rates. Seven out of 1 0 different combinations demonstrated synergism between the two agents . The modelization applied on results allowed us to establish the foll owing 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 (R-2 = 0.99). Our results sugges t that the two agents contribute to the synergism in the system and th at 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 us ually required for such a control. (C) 1998 Academic Press.