Starch- and flour-based sprayable formulations: effect on rainfastness and solar stability of Bacillus thuringiensis

Microbial pesticides such as those based on Bacillus thuringiensis Berliner exhibit short residual activity when applied as foliar sprays. Formulation ingredients may be able to counter the effects of environmental factors by offering protection from rainfall or sunlight or both. We report on the use of pregelatinized cornstarch and corn flour as formulation ingredients for sprayable B. thuringiensis preparations. In 2 field tests, residual activity was measured by feeding treated leaves to diamondback moth, Plutella xylostella (L.), larvae. In the 1st test (1989), pregelatinized starch mixed with equal amounts of sucrose and tank mixed at a total of 4% solids (4 g/100 ml) provided protection of B. thuringiensis on cabbage leaves for up to 5 d under sunny field conditions. A commercial B. thuringiensis product lost activity after 3 d. In the 2nd study (1991), pregelatinized flour mixed with sucrose also provided protection in the presence of rainfall. In this study, 1, 2, and 4% solids were used to test the effects of amounts of formulation materials required to achieve protection of B. thuringiensis. In the presence of rain, 4% solids was required for optimum protection. Treatments that included 1 or 2% solids did not provide rainfastness as measured against a commercial product. Inclusion of sun screening agents did not affect longevity of activity. In both studies, overall efficacy of all B. thuringiensis preparations against 3 Lepidoptera species was excellent. Laboratory tests demonstrated the protective effects of the flour formulations against artificial sunlight. Formulations with only 0.5% solids protected B. thuringiensis equally well as formulations with 4% solids. Artificial rainfall tests, however, did not support results obtained in the field. All formulations were washed equally from cotton plants in response to 6 cm rain applied over a 1-h period. These experiments, considered together, reinforce the proposition that formulation ingredients such as corn flour can increase residual activity of B. thuringiensis.