Interactions between tannic acid and Bacillus thuringiensis subsp. kurstaki KB100 against Spodoptera exigua

Bacillus thuringiensis subsp. kurstaki KB100 isolated from the domestic soil has the most effective activity against larva of Spodoptera exigua. The tannic acid as protease inhibitor might be increased the efficacy of sublethal concentrations of B. thuringiensis. We identified tannic acid as protease inhibitor that increased the efficacy of sublethal concentrations of B. thuringiensis. Mixture of B. thuringiensis and tannic acid was investigated the mortality of S. exigua larva in the laboratory and field. When B. thuringiensis treated to 2nd larva of S. exigua, mortality was shown 54.4%. However, mixtures of B. thuringiensis with 4 and 40 mM tannic acid were increased mortalities to 2nd larva of S. exigua as 64.0 and 95.5%, respectively. In the welsh onion field, the accumulated mortalities of 3 times replicated with mixture of B. thuringiensis and 40 mM tannic acid were 83.9, 89.4 and 66.8% compare with 61.8, 80.4 and 47.3% as only B. thuringiensis treatment, respectively. Excessive degradation of activated toxin was observed in S. exigua midgut juice detected by SDS-PAGE. But tannic acid successfully prevented the excessive degradation of protoxin in S. exigua midgut juice by SDS-PAGE.