We identified tannic acid as an inexpensive additive that increased the efficacy of sublethal concentrations of Bacillus thuringiensis subsp. kurstaki (Berliner). Tannic acid mimicked the active constituents contained 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 25-500 ppm in combination with B. thuringiensis (1.63 micrograms [AI]/ml diet) yielded mean mortalities of 57-75%, whereas treatments with B. thuringiensis alone produced 10% mortality. Mean mortalities in the 3.0, 4.5, and 6.75 micrograms (AI) B. thuringiensis per milliliter of diet treatments (5.5; 8.0, and 30%, respectively) were significantly higher in the presence of 250 and 2,500 ppm tannic acid; in these treatments we observed 78-94% mortality. Addition of tannic acid increased the activity of concentrations of 3-4.5 micrograms (AI) B. thuringiensis per milliliter of diet to approximately that of a concentration of 13 micrograms (AI) B. thuringiensis per milliliter of diet alone (85-95% mortality). Although deaths caused by a formulation of B. thuringiensis + tannic acid occurred more slowly than with high rates of B. thuringiensis alone, such formulations would have the advantages of arresting development, minimizing foliar damage, and decreasing the concentration of B. thuringiensis used.
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