Residual Activity of Granular Starch-Encapsulated Bacillus thuringiensis

Residual insecticidal activity of Bacillus thuringiensis Berliner encapsulated B in cornstarch was determined by using two separate tests. In the first test, granules with various additives were placed in whorls of corn plants in the field, collected over time, and bioassayed against neonate Ostrinia nubilalis (Hubner). During a wet year (1990) when 10 cm rain fell during the exposure time, insecticidal activity of B. thuringiensis encapsulated in starch granules was higher than that of B. thuringiensis in a commercial formulation. In a dry year (1989), we observed no significant differences in activity. In 1991, when 3 cm rain fell midway through the study, the commercial formulation had less activity than two of the starch formulations. In the second test, field efficacy was measured at two locations. In this study, we examined the effect of an early versus a late application of toxicants when whorl-stage plants were infested with laboratory-reared O. nubilalis larvae over a 10-d period. Granules were applied either on the 1st d of insect infestation (early) or after the 7th d of infestation (late). The late application was significantly more efficacious than the early application. B. thuringiensis granules consistently prevented damage by O. nubilalis as well as or better than a chemical insecticide for the length of the study.