Increased mortality of Acanthoscelides obtectus by alkane-grown Beauveria bassiana

Abstract

The effect of alkane-growth induction of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin (Deuteromycotina: Hyphomycetes), on the ability to kill the bean weevil Acanthoscelides obtectus Say (Coleoptera: Bruchidae), was tested. Adult insects were sprayed with an 0.01% Tween 20 aqueous suspension of 4 × 10⁶ conidia/ml. The performance of fungi grown in complete agar medium containing glucose as carbon source (FS₀) was compared to that of alkane-grown fungi (FS₁) with n-hexadecane as the only carbon source. Mortality increased (p < 0.05) from 22 ± 4.5% to 44 ± 11.4% at day 7, and from 26 ± 5.5% to 60 ± 7.1% 14 days after treatment with FS₀ or FS₁ respectively. The insect epicuticular hydrocarbons were analysed by capillary gas chromatography (CGC); major components were saturated hydrocarbons, 27 to 29 carbons in length. A variety of methyl-branched isomers of C27 were the prevailing structures, and nC27 was the major straight chain component. Whole insect hydrocarbons were qualitatively identical to those of the epicuticular surface. Oleic, linoleic and palmitic acids accounted for almost 88% of the fungal fatty acids, irrespective of the carbon source used for growth; however, the unsaturated/saturated ratio diminished markedly from 4.32 (FS₀) to 2.47 (FS₁). These results indicate that alkane supplementation of culture media might be a tool to improve the virulence of some mycoinsecticides.