Spring bird migration as a dispersal mechanism for the hemlock woolly adelgid

In eastern North America, the invasive hemlock woolly adelgid ( Adelges tsugae Annand), has expanded northward at a pace that exceeds predictions from mechanistic models, suggesting successful long-distance dispersal despite the only viable dispersive phase being a flightless nymph, or “crawler.” We hypothesize that migrating birds may contribute to long-distance dispersal of crawlers by passively transporting them in their plumage during northward migration. We collected hemlock woolly adelgid crawlers from the plumage of wild birds in Connecticut hemlock forests in spring and summer 2016–2017 and evaluated the factors that influence crawler loads on wild birds. Of 456 birds examined, 40 individuals of 22 species carried adelgid crawlers. Crawler loads varied strongly over time, showing a mid-spring peak that mirrored the phenological pattern in crawler abundance. However, crawler load was not affected by either local crawler abundance at capture sites or the degree of bird species association with hemlock forests. To test whether dispersed crawlers could start new invasions, we experimentally simulated avian-assisted dispersal of adelgids onto uninfested nursery hemlocks. Although rare, crawlers placed on birds did settle successfully on experimental branches during the adelgid’s summer generation. Our study confirms that birds carry hemlock woolly adelgid crawlers most often during the period of peak spring songbird migration, and that crawlers can move off bird plumage to settle on hemlock foliage. Bird-mediated, long-distance dispersal of crawlers likely has played a key role in hemlock woolly adelgid spread, and with warming temperatures, this mechanism may continue to be important for future range expansion.