INCREASED EFFICACY OF BACILLUS-THURINGIENSIS SUBSP KURSTAKI IN COMBINATION WITH TANNIC-ACID

We identified tannic acid as an inexpensive additive that increased th e efficacy of sublethal concentrations of Bacillus thuringiensis subsp . kurstaki (Berliner). Tannic acid mimicked the active constituents co ntained in an aqueous, tannin-rich extract of Taxus baccata (L.) bark that retarded development of Heliothis virescens (F.) larvae at 10,000 ppm; most larvae remained in first and second stage when treated with 250-10,000 ppm of tannic acid. Instar development of Trichoplusia ni (Hubner) larvae was affected in a concentration-dependent manner by 2. 5-500 ppm of tannic acid. In subsequent bioassays, tannic acid at 25-5 00 ppm in combination with B. thuringiensis (1.63 mu g [AI]/ml diet) y ielded mean mortalities of 57-75%, whereas treatments with B. thuringi ensis alone produced 10% mortality. Mean mortalities in the 3.0, 4.5, and 6.75 mu g (AI) B. thuringiensis per milliliter of diet treatments (5.5, 8.0 and 30%, respectively) were significantly higher in the pres ence of 250 and 2,500 ppm tannic acid; in these treatments we observed 78-94% mortality. Addition of tannic acid increased the activity of c oncentrations of 3-4.5 mu g (AI) B. thuringiensis per milliliter of di et to approximately that of a concentration of 13 mu g (AI) B. thuring iensis per milliliter of diet alone (85-95% mortality). Although death s caused by a formulation of B. thuringiensis + tannic acid occurred m ore slowly than with high rates of B. thuringiensis alone, such formul ations would have the advantages of arresting development, minimizing foliar damage, and decreasing the concentration of B. thuringiensis us ed.