The chemical ecology of host habitat location by larval parasitoids of the southern pine beetle, Dendroctonus frontalis Zimmermann: Olfactory cues and their possible sources

Studies were undertaken to isolate and identify olfactory cues responsible for attracting the hymenopterous larval parasitoids of the southern pine beetle (SPB), Dendroctonus frontalis Zimmermann, to infested pines. Field assays with sticky traps placed in active SPB infestations showed that two species of parasitoids, Roptrocerus xylophagorum (Ratzeburg), and Spathius pallidus (Ashmead), were attracted to the odors of whole bark infested with SPB larvae or steam extracts of such bark. These two parasitoid species differed in their responses to volatiles associated with particular host developmental stages. The hydrocarbon and oxygenated components of the steam extracts (fractionated by liquid chromatography) were found to act in synergy. Synthetic baits formulated using gas chromatography-mass spectrometry (GC-MS) analyses of bark extracts or bark odors failed to attract parasitoids.; The chemical basis for the timing of natural enemy arrival on SPB-infested trees was examined. Volatiles arising from loblolly pines infested with SPB were collected and analyzed (using GC-MS) periodically over the course of beetle attack and brood development. Characteristic changes in the composition of emitted volatiles were found to coincide with both the presence of distinct SPB life stages and the arrival of particular SPB natural enemy guilds on these trees.; The role of bark beetle-associated microbes in the production of volatiles from beetle-infested pine tissues was studied. Pine bolts were infested with pairs of the bark beetle I. grandicollis (Eichhoff) which had been either axenically reared or obtained in nature, and volatiles emitted from the beetles' gallery entrances were collected periodically during beetle brood development and analyzed using GC-MS. Elimination of symbionts produced minimal quantitative changes in the release of the beetles' pheromones. Some compounds previously found associated with parasitoid attraction to bark beetle-infested trees were enhanced by the presence of symbionts; others were reduced in concentration. Likewise, when SPB-associated fungi were inoculated into pine bolts or liquid cultures amended with pine oleoresin, some changes in the abundance of volatile compounds were observed that typically occur in bark beetle-infested trees. These data suggest that bark beetle-associated fungi may be important to the production of olfactory cues utilized by SPB parasitoids to locate trees infested with susceptible hosts.