Extending the residual toxicity of Bacillus thuringiensis with casein-based formulations

We evaluated the extension of the residual insecticidal activity of Bacillus thuringiensis Berliner with 2 sprayable formulations based on casein. One formulation contained native casein in a basic solution (pH > 8.5); the other had a water soluble ammonium salt of casein in the presence of ammonium zirconium carbonate as a cross-linking agent. Formulations were applied to cotton leaves, Gossypium hirsutum L. 'DES 119', in a greenhouse and subjected to simulated rain in a modified spray chamber and artificial sunlight from a simulator. Activity was assessed based on mortality of neonate European corn borer, Ostrinia nubilalis (Hubner), that were fed treated cotton leaves. Performance of some formulations was also evaluated against beet armyworm, Spodoptera exigua (Hubner). Leaves treated with the casein formulations (0.5% wt:vol) of B. thuringiensis resisted wash-off, often retaining >60% of the original insecticidal activity of unexposed treatments compared with <20% of the original activity for unformulated and commercially formulated B. thuringiensis preparations. The casein formulations also provided some protection from light-induced degradation compared with unformulated B. thuringiensis, although the amount of protection was less than that provided by other experimental formulations.