Volatiles from Fusarium verticillioides (Sacc.) nirenb. and their attractiveness to nitidulid beetles

It is known that sap beetles (Coleoptera: Nitidulidae) can vector the fungus Fusarium verticillioides (Sacc.) Nirenb. (= F. moniliforme Sheldon), which causes an important ear-rot disease in corn and also produces fumonisin mycotoxins. The volatiles produced by this fungus were studied to establish whether they could attract sap beetles. Such an association would suggest more than just an incidental role in transmission of the fungus by the beetles. F. verticillioides consistently produces a blend of five alcohols (ethanol, 1-propanol, 2-methyl-1-propanol, 3-methyl-1-butanol, and 2-methyl-1-butanol), acetaldehyde, and ethyl acetate. Ethanol is the most abundant alcohol. The fungus also produces four phenolic compounds (the most abundant of which is ethylguaiacol), a series of presently unidentified sesquiterpene hydrocarbons, and an unidentified compound that is probably a 10-carbon ketone. Solid-phase microextraction was the key technique used in volatile analysis. The volatile profiles change over time and differ somewhat among fungal strains: The alcohols, aldehyde, and ester always appeared first and were present for each strain. Production of the phenolics lagged by several days, and in some strains these compounds were barely detectable. Volatile production eventually diminished in all strains. All strains were attractive to the sap beetle, Carpophilus humeralis (F.), in wind-tunnel bioassays. Attraction was correlated primarily to the presence of the alcohols, acetaldehyde, and ethyl acetate, rather than to the phenolics. To verify that the identified culture volatiles were responsible for beetle attraction, cultures were quantitatively simulated with synthetic chemicals, and the cultures and corresponding synthetic mixtures were then compared by bioassay. The comparisons were favorable. Volatile emission patterns from cultures were fairly robust with respect to inoculum level or incubation temperature, but some manipulation was possible. For example, after freeze-drying and rehydrating (a rapid simulation of winter/spring conditions), F. verticillioides produced ethyl acetate and other esters at unusually high levels. The fungus produced attractive volatiles following ear inoculation of milk-stage field corn as well as on sterile, mature kernels in the laboratory.