INTERACTION OF CAFFEINE AND RELATED-COMPOUNDS WITH BACILLUS-THURINGIENSIS SSP KURSTAKI IN BERTHA ARMYWORM (LEPIDOPTERA, NOCTUIDAE)

Increased susceptibility of the bertha armyworm, Mamestra configurata Walker, to Dipel, a commercial formulation of Bacillus thuringiensis ( Berliner) ssp. kurstaki, indicated by low LC50 was observed when fourt h instars were reared on artificial diet containing several lethal con centrations of B. thuringiensis ssp. kurstaki supplemented with sublet hal concentrations of 1,3,7-trimethylxanthine (caffeine), amidoindole 3-proprionic acid, theophylline 1,3-dimethylxanthine, or 3-isobutyl-1- methylxanthine. Surviving treated larvae either gained no weight or lo st weight after 7 d compared with controls which gained (mean +/- SD) 235 +/- 35 mg per larva; the effect suggests cessation of feeding acti vity in treated larvae. The data showed that bertha armyworm larvae we re weakly susceptible to commercial B. thuringiensis ssp. kurstaki alo ne applied to canola plants Brassica napus L. Caffeine at 0. 1% wt/vol increased toxicity of the pathogen to the larvae 3.5-fold in artifici al diet and 9.2-fold in a simulated field application on canola plants . Surviving larvae on the plants treated with B. thuringiensis ssp. ku rstaki + caffeine gained 91 +/- 59 mg per larva after 10 d, compared w ith 339 +/- 73 mg per larva on untreated plants. The construction and calibration of a spray chamber designed to apply known doses of microb ial insecticides such as B. thuringiensis ssp. kurstaki to agricultura l plants is described. We conclude that caffeine at 0.1% in B. thuring iensis ssp. kurstaki spray suspensions increases the toxicity of the m icroorganism and reduces feeding activity of bertha armyworm larvae on canola plants with little additional treatment cost.