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The sunflower moth, Homoeosoma electellum (Hulst), a major pest of sunflower (Helianthus annuus L.), is susceptible to Bacillus thuringiensis Berliner, but field control has been inconsistent. Our study was done to determine whether phenolics in damaged sunflower tissue are in a conjugated or free form and whether they influence the toxicity of B. thuringiensis to the sunflower moth. Larvae fed diets containing B. thuringiensis had reduced survival and growth. The LD50 of B. thuringiensis was 52 international units of potency per gram of diet. Phenolic acids are general biocides. They were found in sunflower hybrids largely in the free (nonconjugated) form after tissue injury. The phenolics were probably converted from the conjugated form to the free form after release of glycosidases. By themselves and at the doses tested, specific phenolic acids did not affect sunflower moth survival and growth. When cinnamic or p-coumaric acids were combined with B. thuringiensis in artificial diet, growth (measured as width of sunflower moth head capsules) of larvae was reduced compared with that of larvae fed diets containing B. thuringiensis. High phenolic sunflower might increase the effectiveness of B. thuringiensis as an agent for sunflower moth control.