Attraction of the bark beetle parasitoid Roptrocerus xylophagorum (Hymenoptera: Pteromalidae) to host-associated olfactory cues
Studies were conducted to identify host location cues used by Roptrocerus xylophagorum (Ratzeburg), a larval/pupal parasitoid of bark beetles. In Y-tube olfactometer bioassays, female R. xylophagorum were attracted to infested bark (i.e., phloem, cambium, and outer corky bark tissues) removed from bolts of loblolly pine, Pinus taeda L., colonized by the late instar larvae and pupae of the bark beetle Ips grandicollis Eichhoff (Coleoptera: Scolytidae). In contrast, bark taken from recently cut, uninfested bolts interrupted attraction to infested bark when these were presented together. Larval and pupal hosts isolated from infested bark were not attractive to parasitoids, whereas frass removed from the larval mines in infested bark was highly attractive. Bark from which hosts or both hosts and host frass were removed remained highly attractive. Bark sandwiches (fresh bark with the exposed surface pressed to glass microscope slides) infested with either third-instar or adult female I. grandicollis were attractive to female parasitoids, whereas bark sandwiches with only mechanical damage to the phloem tissue were unattractive. A steam distillate of bark infested with host larvae was attractive to female R. xylophagorum, whereas a distillate of fresh pine resin was not attractive. Volatiles from the experimental baits were collected on Porapak Q and analyzed by coupled gas chromatography-mass spectrometry. Several compounds were identified that distinguished baits with biological activity. These data show the importance of the complete host/plant complex for attraction of R. xylophagorum to its host's habitat and suggest a possible role for particular odors from uninfested host plant tissue in directing foraging parasitoids away from locations with few or no hosts.