Microplitis croceipes (Hymenoptera : Braconidae) development in tobacco budworm (Lepidoptera : Noctuidae) larvae treated with Bacillus thuringiensis and thiodicarb

Bacillus thuringiensis Berliner var. kurstaki and thiodicarb were evaluated in laboratory and field assays to determine the effect on tobacco budworm larvae, Heliothis virescens (F.), and the parasitoid Microplitis croceipes Cresson. Laboratory trials were conducted using B. thuringiensis concentrat ions of 0, 10, 50 and 250 ppm and thiodicarb concentrations of 0, 12.5, 25, 50, 100, and 200 ppm in the diet. The test using field-treated cotton squa res was conducted using B. thuringiensis and thiodicarb, independently and in combination, at rates recommended for resistance management in Arkansas. Laboratory tests indicated that tobacco budworm mortality was directly rel ated to B. thuringiensis and thiodicarb concentrations, although B. thuring iensis only significantly increased tobacco budworm mortality at the highes t concentration of exposure in the absence of parasitization. Parasitizatio n increased host mortality at all B. thuringiensis experimental rates after 6 and 14 days. Although host mortality increased linearly with increasing thiodicarb concentration, parasitization did not significantly increase hos t mortality over thiodicarb alone until day 14. Emergence of M. croceipes w as inversely related to B. thuringiensis and thiodicarb concentration. In a ssays using squares from field-sprayed cotton, thiodicarb, and thiodicarb/B . thuringiensis mixtures provided significantly greater tobacco budworm mor tality than did B. thuringiensis application alone. In addition, no signifi cant advantage was determined for tank mixtures with B. thuringiensis as co mpared to thiodicarb application alone. However, neither B. thuringiensis n or thiodicarb, alone or in combination, caused a high mortality of early th ird instar tobacco budworm in the absence of parasitization by M. croceipes .