STARCH-BASED AND FLOUR-BASED SPRAYABLE FORMULATIONS - EFFECT ON RAINFASTNESS AND SOLAR STABILITY OF BACILLUS-THURINGIENSIS

Microbial pesticides such as those based on Bacillus thuringiensis Ber liner exhibit short residual activity when applied as foliar sprays. F ormulation ingredients may be able to counter the effects of environme ntal factors by offering protection from rainfall or sunlight or both. We report on the use of pregelatinized cornstarch and corn flour as f ormulation ingredients for sprayable B. thuringiensis preparations. In 2 field tests, residual activity was measured by feeding treated leav es to diamondback moth, Plutella xylostella (L.), larvae. In the Ist t est (1989), pregelatinized starch mixed with equal amounts of sucrose and tank mixed at a total of 4% solids (4 g/100 mi) provided protectio n of B. thuringiensis on cabbage leaves for up to 5 d under sunny fiel d conditions. A commercial B. thuringiensis product lost activity afte r 3 d. In the 2nd study (1991), pregelatinized flour mixed with sucros e 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 formu lation materials required to achieve protection of B. thuringiensis. I n 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 a gents did not affect longevity of activity. In both studies, overall e fficacy of all B. thuringiensis preparations against 3 Lepidoptera spe cies was excellent. Laboratory tests demonstrated the protective effec ts of the flour formulations against artificial sunlight. Formulations with only 0.5% solids protected B. thuringiensis equally well as form ulations with 4% solids. Artificial rainfall tests, however, did not s upport results obtained in the field. All formulations were washed equ ally from cotton plants in response to 6 cm rain applied over a l-h pe riod. These experiments, considered together reinforce the proposition that formulation ingredients such as corn flour can increase residual activity of B. thuringiensis.