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 concentrations 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 squares 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 related to B. thuringiensis and thiodicarb concentrations, although B. thuringiensis only significantly increased tobacco budworm mortality at the highest concentration of exposure in the absence of parasitization. Parasitization 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 host mortality over thiodicarb alone until day 14. Emergence of M. croceipes was inversely related to B. thuringiensis and thiodicarb concentration. In assays using squares from field-sprayed cotton, thiodicarb, and thiodicarb/B. thuringiensis mixtures provided significantly greater tobacco budworm mortality than did B. thuringiensis application alone. In addition, no significant advantage was determined for tank mixtures with B. thuringiensis as compared to thiodicarb application alone. However, neither B. thuringiensis nor thiodicarb, alone or in combination, caused a high mortality of early third instar tobacco budworm in the absence of parasitization by M. croceipes.