Toxicity of Free Fatty Acids on the Larval Surfaces of Two Lepidopterous Insects trowards Beauveria bassiana (BALS.) VUILL. and Paecilomyces fumoso-roseus (WIZE) BROWN et SMITH (Deuteromycetes : Moniliales)

Free fatty acids (C : 8 to C : 20) were detected qualitatively and quantitatively from lipids present on external surfaces of the silkworm (Bombyx mori) and the fall webworm (Hyphantria cunea). The major component of the fatty acids was linoleinic acid (C : 18-3), and the amount of short-chain fatty acids was small. The total contents of free fatty acids and the composition of each free fatty acid in the integument of the silkworm changed quantitatively with larval growth. The short-chain fatty acids (C : 6 to C : 12) inhibited conidial germinations and hyphal growths of Beauveria bassiana and Paecilomyces fumoso-roseus, whereas some long-chain fatty acids supprted the growths of both fungi. Though these short-chain fatty acids acted fungistatically to conidial germination, the conidia treated with capric acid (C : 10) lost their pathogenicity toward the silkworm larvae.