Effects of alkaline gluten on the insecticidal activity of Bacillus thuringiensis

Insecticidal activity of Bacillus thuringiensis subsp. kurstaki Berliner decreases rapidly when exposed to field conditions, but degradation can be minimized by improving the formulation. Sulubilized wheat-gluten, added to sprayable formulations, forms a film upon drying that resists wash-off. Because gluten requires a pH 10 to solubilize, we examined the effects of alkaline pH (with and without gluten) on the insecticidal activity of B. thuringiensis. Efficacy of various formulations was measured by the mortality of neonate European corn borer, Ostrinia nubilalis (Hubner), fed on treated cotton leaves. In solutions with pH from 5 to 11, B. thuringiensis (13,800 IU/ml) caused greater than 90% larval mortality. Mortality declined significantly (P 0.05) at pH 12, indicting a loss of insecticidal activity. However, adding 1% (wt:vol) gluten to the formulations provided 2 benefits that retained the activity of B. thuringiensis. First, formulations of B. thuringiensis with gluten caused 97% mortality of larvae at pH 13 compared with 68% mortality for larvae feeding on leaves treated with the same formulation without gluten. Second, the addition of gluten helped to resist wash-off of B. thuringiensis by 5 cm of simulated rain. Response surface regression analysis indicated that higher concentrations of gluten (up to 1.5% wt:vol) and less alkaline formulations (down to pH 10.5) improved resistance to wash-off. These results indicate that residual activity of B. thuringiensis in the presence of rain may be improved by the addition of gluten to the spray mixture