Effect of Bacillus thuringiensis Application Interval on European Corn Borer (Lepidoptera: Pyralidae) Control in Sweet Corn

Ground-applied treatments of two commercial Bacillus thuringiensis subsp. kurstaki formulations (MVP and Dipel ES) and tank-mixes with a pyrethroid (Ambush 2E) were evaluated for control of European corn borer, Ostrinia nubilalis (Hübner), larvae in sweet corn. Treatments were applied at average intervals of 3.4, 5, 7, and 10 days to determine field persistence. Manual infestations of first-instar O. nubilalis were used to augment natural populations. During both years, there were no significant interactions between application interval and treatment for all dependent variables tested, including late instars per ear, percent marketability, yield, and predator density. Regardless of application interval, MVP provided greater larval control than Dipel ES. However, the decline in efficacy of the encapsulated MVP formulation occurred at the same rate as that of the non-encapsulated Dipel ES formulation over the 3.4 to 10-d intervals. Tank-mixes of B. thuringiensis + low-rate permethrin provided no additional control compared with low-rate permethrin alone. Given the infestation levels present in this test, neither B. thuringiensis formulation provided control sufficient to maintain current processor standards of 5–10% infested ears at harvest.