Evidence for a sex pheromone in bark beetle parasitoid Roptrocerus xylophagorum

Male Roptrocerus xylophagorum (Ratzeburg) (Hymenoptera: Pteromalidae) exhibited courtship and mating behaviors including wing fanning, antennation, mounting, and copulation attempts when exposed to glass bulb decoys coated with a whole-body extract of females in hexane, acetone, or methanol. Activity of extract-treated decoys declined gradually over one week. Males responded much less strongly to freeze-killed female cadavers extracted with solvents than to unextracted cadavers; treatment of extracted cadavers with female extract restored male responses. The pheromone was found to be equally present over the surface of both the abdomen and head/thorax of females, and the origin of the pheromone could not be conclusively localized to any single body region. The activity of pheromone on females increased between day 1 and days 3–5 following eclosion; otherwise, pheromone activity was not significantly affected by either female age or mating. Males were arrested within the zone of a glass surface on which females had walked, suggesting that the pheromone might be substrate-borne. Recent exposure to females reduced male responsiveness, but responsiveness was fully restored after a few hours of male isolation from females. When hexane extracts of whole females were fractionated on silica gel, the pheromone's activity was largely recovered with the first, most nonpolar fraction. Female extracts and fractions were analyzed by coupled gas chromatography–mass spectrometry. Cuticular hydrocarbon alkanes were identified as the extract components whose concentrations correlated best with male responses. Evidence of the pheromone's long persistence, low volatility, low polarity, and presence over the insect's entire body surface further supported the hypothesis that the pheromone was composed of one or more cuticular hydrocarbons.