Attraction of a Flying Nitidulid (Carpophilus humeralis) to Volatiles Produced by Yeasts Grown on Sweet Corn and a Corn-Based Medium

Abstract

Attraction to microbial volatiles was examined for the sap beetle, Carpophilus humeralis, which is a pest of maize. Using 54 pure yeast and bacterial cultures, we evaluated differences in volatile emissions among species of microorganisms and whether these differences were associated with insect attraction. On a sterile corn-based medium, both yeasts and bacteria generally multiplied well and produced detectable volatile metabolites. The yeasts ranged from inactive to highly attractive, but no bacterial cultures attracted beetles above control levels. A variety of alcohols, esters, ketones, acids, and phenolic compounds were identified from the headspace above yeast cultures. Growth, volatile production, and, ultimately, attractiveness to beetles depend strongly on the ability of the yeasts to assimilate and/or ferment the carbohydrates present. Abundant volatile production on sweet corn was observed only with yeasts that are able to ferment sucrose and/or maltrose. Saccharomyces cerevisiae (ferments glucose, sucrose, and maltose) and Candida shehatae (ferments glucose and maltose) produced considerably more attractive volatiles than Candida guilliermondii, which only ferments glucose. Yeast volatiles important for beetle attraction included typical fermentation-associated substances (ethanol, acetaldehyde, 2-methyl-1-propanol, 1-propanol, ethyl acetate, 3-methyl-1-butanol and 2-methyl-1-butanol), and also 3-hydroxy-2-butanone, whose presence was not correlated with the occurrence of fermentation. Using aqueous mixtures of synthetic components that produced headspace compositions simulating those of attractive yeasts, it was shown that the typical fermentation volatiles are attractive but not essential for attractiveness. 3-Hydroxyl-2-butanone is sufficient but not necessary, although its attractiveness is enhanced by the presence of fermentation volatiles such as ethanol and 2-methyl-1-proponol. In nature, the beetles could take advantage of a variety of different microbial metabolic processes to locate hosts. The laboratory bioaasays in this study involved flight and therefore were particularly relevant to host-finding behavior in the field.