Stereochemistry of Host Plant Monoterpenes as Mate Location Cues for the Gall Wasp Antistrophus rufus

In spring, adult males of the gall wasp Antistrophus rufus L. emerge from inconspicuous galls in stems of their host plant Silphium laciniatum L. and search for sites on stems where females will later emerge. The behavior of males suggests that they use olfaction rather than visual or tactile cues in searching for mates. In an earlier publication, we reported that galls of A. rufus were associated with changes in enantiomeric ratios of alpha- and beta-pinene emitted by plant stems, and hypothesized that monoterpene stereochemistry served as a mate location cue for adult males. Here, we support this hypothesis with bioassays that demonstrate that males can discriminate between galled and ungalled stems, as well as between blends of synthetic monoterpenes with ratios of enantiomers representative of galled and ungalled stems.