ON-FARM EFFICACY OF AERIALLY APPLIED BACILLUS-THURINGIENSIS FOR EUROPEAN CORN-BORER (LEPIDOPTERA, PYRALIDAE) AND CORN-EARWORM (LEPIDOPTERA, NOCTUIDAE) CONTROL IN SWEET CORN

Aerial applications of Bacillus thuringiensis subsp. kurstaki and tank-mixes with synthetic insecticides were evaluated for control of Ostrinia nubilalis (Hubner) and Helicoverpa zea (Boddie) in Minnesota sweet corn during 1990-1992. In addition to natural infestations, sweet corn was manually infested with first-instar O. nubilalis to investigate optimum timing of applications relative to larval hatch. Efficacy of B. thuringiensis was significantly enhanced when applied 2 d before larval hatch, compared with 2 d after hatch. Treatments with low-rate permethrin + B. thuringiensis or full-rate permethrin followed by B. thuringiensis provided control of O. nubilalis equal to full rates of permethrin or micro-encapsulated methyl parathion. Of the B. thuringiensis treatments, only the tank-mix of low-rate permethrin + Dipel ES significantly reduced H. lea larval infestations and yielded more marketable ears than the untreated check. Based on active ingredient per hectare, low-rate permethrin + B. thuringiensis reduced synthetic insecticide use 66%, compared with full-rate permethrin, or 92% compared with full-rate methyl parathion. Using a four-application program, permethrin followed by three B. thuringiensis applications reduced synthetic insecticide use 75% compared with full-rate permethrin, or 94% compared with full-rate methyl parathion. Disadvantages of the use of B. thuringiensis for late-season insect management in sweet corn include application timing, incidence of H. zea, and potential for increased risk of larval contaminants in marketable product.