Electrophysiological and Behavioral Responses of an Ambrosia Beetle to Volatiles of its Nutritional Fungal Symbiont.

Ambrosia beetles (Coleoptera: Scolytinae) cultivate their fungal symbiont within host substrates as the sole source of nutrition on which the larvae and adults must feed. To investigate a possible role for semiochemicals in this interaction, we characterized electrophysiological and behavioral responses of Xylosandrus germanus to volatiles associated with its fungal symbiont Ambrosiella grosmanniae. During still-air walking bioassays, X. germanus exhibited an arrestment response to volatiles of A. grosmanniae, but not antagonistic fungi Beauveria bassiana, Metarhizium brunneum, Trichoderma harzianum, the plant pathogen Fusarium proliferatum, or malt extract agar. Solid phase microextraction-gas chromatography-mass spectrometry identified 2-ethyl-1-hexanol, 2-phenylethanol, methyl benzoate and 3-methyl-1-butanol in emissions from A. grosmanniae; the latter two compounds were also detected in emissions from B. bassiana. Concentration-responses using electroantennography documented weak depolarizations to A. grosmanniae fungal volatiles, unlike the comparatively strong response to ethanol. When tested singly in walking bioassays, volatiles identified from A. grosmanniae elicited relatively weak arrestment responses, unlike the responses to ethanol. Xylosandrus germanus also exhibited weak or no long-range attraction to the fungal volatiles when tested singly during field trials in 2016-2018. None of the fungal volatiles enhanced attraction of X. germanus to ethanol when tested singly; in contrast, 2-phenylethanol and 3-methyl-1-butanol consistently reduced attraction to ethanol. Volatiles emitted by A. grosmanniae may represent short-range olfactory cues that could aid in distinguishing their nutritional fungal symbiont from other fungi, but these compounds are not likely to be useful as long-range attractants for improving detection or mass trapping tactics.