Timing of the Initial Spray Application of Bacillus thuringiensis for Control of the Colorado Potato Beetle (Coleoptera: Chrysomelidae) in Potatoes

Field experiments were conducted in 1989 and 1990 to evaluate the timing of the initial spray application of Bacillus thuringiensis subsp. tenebrianis, relative to egg development in the field, for control of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say) in potatoes. Effective potato beetle control was achieved with B. thuringiensis if the initial spray was applied during the period beginning when an average of 1-30% of marked egg masses had hatched, extending to 4 days after 30% egg hatch, of the initial flush of eggs. Significantly greater defoliation, numbers of third and fourth instar CPB and lower yields occurred in plots when the initial B. thuringiensis application was delayed until 6 days after 30% egg hatch, compared with plots treated at 30% egg hatch. In 1990, no improvement in CPB control was gained by application of the initial B. thuringiensis spray at first observation of CPB egg deposition or at first egg hatch, compared with application at 30% egg hatch. Recommendations for proper timing of B. thuringiensis sprays for effective CPB management are presented.