Olfactory Stimulants for Sirex nigricornis (Hymenoptera: Siricidae) and its Parasitoid, Ibalia leucospoides (Hymenoptera: Ibaliidae), in Odors of Stressed and Bark Beetle–Colonized Pines

Abstract We conducted studies with the native woodwasp Sirex nigricornis F. (Hymenoptera: Siricidae) in Louisiana to identify host-associated olfactory stimulants that may include attractive semiochemicals for this species as well as Sirex noctilio F., an invasive species that can attack healthy trees. Loblolly pines (Pinus taeda L.) treated with a stressing agent were felled and chipped 1–2 mo after treatment, and the chips were steam–water distilled in a Clevenger apparatus to extract volatile compounds. Using coupled gas chromatography–electroantennographic detection, we analyzed the distillates with antennae of both S. nigricornis and the Sirex parasitoid Ibalia leucospoides (Hochenwarth) (Hymenoptera: Ibaliidae) to identify olfactory stimulants. In addition, we assayed S. nigricornis antennae with synthetic mixtures containing 23 volatile compounds associated with stressed and dying pines, including pheromones of bark beetles (Coleoptera: Curculionidae: Scolytinae) that also use these hosts. Antennae of both male and female S. nigricornis responded to 29 identifiable volatiles in the distillates, whereas I. leucospoides responded to 23 volatiles. Eighteen compounds in the synthetic mixtures were olfactory stimulants for S. nigricornis. Olfactory stimulants in the woodchip distillates were predominantly hydrocarbon and oxygenated monoterpenes, and the strongest antennal stimulants among compounds in the synthetic mixtures were oxygenated monoterpenes associated with pine death and early decay (e.g., verbenone, fenchone, and terpinen-4-ol). Bark beetle pheromones (frontalin, endo-brevicomin, ipsenol, and ipsdienol) also stimulated S. nigricornis antennae. The very large number of host-associated olfactory stimulants for S. nigricornis implies that our data may offer limited assistance in targeting individual compounds for investigation as possible attractants and components for a Sirex lure.