Interspecific Variation in Host-Finding Cues of Parasitoids of the Southern Pine Beetle (Coleoptera: Scolytidae)
Experiments were performed with host-associated olfactory attractants of the larval parasitoids of the southern pine beetle, Dendroctonus frontalis Zimmermann, to elucidate both their biological origin and their chemical composition. Sticky-screen traps were erected in an active D. frontalis infestation and baited with parts of D. frontalis-infested loblolly pines (Pinus taeda L.) or their extracts. The diversity of parasitoid species landing on trees infested with larval D. frontalis was substantially greater than that attracted to traps baited with wood and bark taken from similar, infested trees. Females of four parasitoid species, Spathius pallidus (Ashmead), Roptrocerus xylophagorum (Ratzeburg), Dinotiscus dendroctoni (Ashmead), and Eurytoma tomici Ashmead, were attracted to bark infested with D. frontalis larvae. Two of these species, R. xylophagorum and S. pallidus, were attracted to debarked wood from host-infested trees although this tissue was free of hosts and host frass. Spathius pallidus were more attracted to the excised bark (containing D. frontalis larvae and frass) than the debarked wood from D. frontalis-infested pine bolts, while R. xylophagorum were attracted in similar numbers to both materials. When traps were baited with steam/water-distilled extracts of D. frontalis-infested bark, R. xylophagorum strongly preferred extracts from bark containing early in star larvae over extracts from bark infested with either younger (egg-stage) or older (late-instar larval and pupal) brood. In contrast, S. pallidus responded significantly only to extracts of late larval/pupal bark. Coupled gas chromatograph/mass spectrometer (GC-MS) analyses of the bark extracts revealed that the concentrations of numerous extract constituents correlated positively with trap catch of S. pallidus, but no such relationships were identified for R. xylophagorum. These data provide further evidence that members of the parasitoid complex associated with D. frontalis differ in their strategies for locating trees infested with susceptible hosts.