Inhibition of the Bacillus thuringiensis var. israelensis toxin by dissolved tannins

The mortality of Aedes aegypti (L.) larvae treated with Bacillus thuringiensis var. israelensis was reduced by tannic acid at concentrations as low as 0.25 mM (425 mg/liter). The LC sub(50)'s for B. thuringiensis var. israelensis treatments were 45 ng/ml in deionized water and 233 ng/ml in 1.5 mM tannic acid. Soaking the bacteria in tannic acid for 48 h before the exposure of test insects resulted in a lower LC sub(50) value of 140 ng/ml and steeper log-probit regression slope. Although tannic acid lowers the gut pH of A. aegypti larvae, it is evident from the retention of B. thuringiensis var. israelensis toxicity after gut acidification in solutions of citric, hydrochloric, and phosphoric acid that this change did not mitigate the toxin's activity. The presence of tannins in mosquito breeding sites may require higher application rates for B. thuringiensis var. israelensis when used by mosquito abatement personnel as a larvicide.