Olfactory responses of the hemlock woolly adelgid predator, Laricobius nigrinus (Coleoptera: Derodontidae), to natural and synthetic conifer volatiles
Laricobius nigrinus Fender (Coleoptera: Derodontidae) is a specialist predator of the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), native to the Pacific Northwest. It has been introduced into the eastern United States for biological control of exotic hemlock woolly adelgid populations that threaten native hemlock. The possible role of olfactory cues in host finding by this predator has received little study. We used gas chromatography-electroantennographic detection (GC-EAD) to test adult L. nigrinus olfactory sensitivity to volatiles from foliage of both adelgid-infested and uninfested eastern hemlock [Tsuga canadensis (L.) Carrie` re] and western hemlock [Tsuga heterophylla (Raf.) Sarg.], and two hemlock woolly adelgid nonhost species (eastern white pine, Pinus strobus L., and western white pine, Pinus monticola Douglas ex D. Don). Adelgid infestation did not alter L. nigrinus EAD response profiles to volatiles from either hemlock species. In total, antennal preparations detected only two compounds in samples of foliage volatiles: myrcene in all four tree species and nonanal in eastern hemlock alone. However, in GC-EAD tests with synthetic blends of common conifer volatiles presented at higher concentrations than in our foliage samples, we additionally recorded responses to (-)-limonene, terpinolene, alpha-pdimethylstyrene, linalool, (-)-bornyl acetate, 4-allylanisole, and alpha-humulene. The apparent absence of olfactory stimulants specific to adelgid-infested foliage is consistent with published ambulatory olfactometer tests in which L. nigrinus adults were not more attracted to infested than uninfested foliage. Myrcene and nonanal should be further explored as compounds produced by hemlock woolly adelgid host trees that may influence L. nigrinus prey-finding efficiency.