Coexistence of three specialist predators of the hemlock woolly adelgid in the Pacific Northwest USA

The hemlock woolly adelgid (Hemiptera: Adelgidae:Adelges tsugaeAnnand) is an invasive insect, introduced from Japan to eastern North America, where it causes decline and death of hemlock trees. There is a closely related lineage ofA. tsugaenative to western North America. To inform classical biological control ofA. tsugaein the eastern USA, the density and phenology of three native western adelgid specialist predators,Leucopis argenticollis(Zetterstedt),Le. piniperda(Malloch) (Diptera: Chamaemyiidae), andLaricobius nigrinusFender (Coleoptera: Derodontidae), were quantified in the Pacific Northwest. Infested branches were collected from western hemlock (Pinaceae:Tsuga heterophylla(Raf.) Sarg.) at four sites around the Puget Sound, Washington and three sites in Oregon. ImmatureLeucopiswere identified to species using DNA barcodes.Leucopis argenticolliswas roughly twice as abundant asLe. piniperda.Laricobius nigrinuslarvae were more abundant than the two species ofLeucopisduring the egg stage of the first adelgid generation, butLeucopiswere present as feeding larvae during the second adelgid generation whenLa. nigrinuswas aestivating in the soil, resulting inLeucopisbeing more abundant thanLa. nigrinusacross the entire sampling period.Adelges tsugaeandLa. nigrinusdensities were not correlated, whileA. tsugaeandLeucopisspp. densities were positively correlated.Leucopisspp. andLa. nigrinusdensities were negatively correlated. These results support the complementary use ofLa. nigrinusand the twoLeucopisspecies for biological control ofA. tsugaein the eastern USA, and point to the need for further investigation of spatial and temporal niche partitioning among the three predator species.