Development of Cotesia marginiventris (Hymenoptera: Braconidae) in tobacco budworm (Lepidoptera: Noctuidae) larvae treated with Bacillus thuringiensis and thiodicarb

Bacillus thuringiensis var kurstaki and thiodicarb were evaluated for their im pact on Cortesia marginiventris (Cresson) survival in exposed Heliothis virescens larvae. B. thuringiensis tests were conducted at concentrations of 0, 10, 50, and 250 ppm in artificial diet with initial exposure at parasitization or 24 or 48 h following parasitization. Thiodicarb tests were conducted at concentrations of 0, SO, 100, and 200 ppm in artificial diet with initial pesticide exposure at parasitization or 72 h following parasitization. H. virescens mortality was directly related to B. thuringiensis concentration. Mortality trends were similar with respect to time of initial B. thuringiensis exposure. Exposure of H, virescens lanae to the combination of B. thuringiensis and C. marginiventris parasitization increased larval mortality over that of either alone. Emergence of C. marginiventris was inversely related to B. thuringiensis concentration and directly related to timing of B. thuringiensis exposure. In all instances, greater C. marginiventris emergence was noted when host exposure to B. thuringiensis was delayed for 48 h following parasitization. Mortality in thiodicarb-exposed H. virescens was influenced by both insecticide rate and parasitization. A delay in initial insecticide exposure significantly increased early larval mortality in both parasitized and nonparasitized groups. Overall, results suggest that use of both thiodicarb and B. thuringiensis would preclude maintenance of natural field populations of C. marginiventris.