Lack of evidence for pheromone-mediated secondary attraction in the fir engraver, Scolytus ventralis (Coleoptera: Scolytidae)

Abstract

To test the hypothesis that host selection and mass attack by the fir engraver, Scolytus ventralis LeConte, is mediated in part by pheromones, an exhaustive series of experiments was conducted. Gas chromatographic (GC) analysis and GC-electroantennographic detection analysis was performed on Porapak Q-captured volatiles from virgin and mated beetles of both sexes, logs of grand fir, Abies grandis (Dougl.) Lindl., with males, females or both sexes boring in the bark, and trees undergoing attack in the field, and on extracts of abdominal tips from beetles topically-treated with methoprene, a juvenile hormone analogue, or beetles boring in methoprene-treated bark of grand fir. None of these analyses disclosed any sex-specific compounds or compounds that changed markedly in concentration following treatment. Extracts of the females' terminal abdominal glands with associated vaginal palpi contained exo-brevicomin, a common aggregation pheromone in the genera Dendroctonus and Dryocoetes, but laboratory and field experiments showed it to have no apparent role in long-range orientation. Extensive visual and videotaped observations revealed that females walking on grand fir bark displayed apparent "calling" and "marking" behavior. and during courtship males rubbed the females' abdominal declivity with their frons, placing their antennae in juxtaposition to the females' vaginal palpi. These results arc consistent with the alternative hypothesis that host selection and mass attack by S. ventralis are mediated solely by primary attractants to the host tree, but they do not rule out the possibility of short-range pheromone-mediated behavior.

Key words: Scolytus ventralis; Coleoptera; Scolytidae; chemical ecology; pheromones; host selection