Residual Activity of Granular Starch - Encapsulated Bacillus thuringiensis

Residual insecticidal activity of Bacilltls thtlringiensis Berliner encapsulated in cornstarch was detern1ined by using two separate tests. In the first test, granules with various additives were placed in whorls ofcorn plants in the field, collected over time, and bioassayed against neonate Ostrinia ntlbilalis (Hubner). During a wet year (1990) when 10 em rain fell during the exposure time, insecticidal activity of B. tlltlringiensis encapsulated in starch granules was higher than that of B. tlwringiensis in a commercial formulation. In a dry year (1989), we observed no significant differences in activity. In 1991, when 3 em rain fell midway through the study, the commercial fornmlation 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. ntlbilalis 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. tlwringiensis granules consistently prevented damage by O. ntlbilalis as well as or better than a chemical insecticide for the length of the study.