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.
KeywordsAdelges tsugae Biotic interactions Phenology Invasive species Ectozoochory
Funding for this research came from the University of Connecticut IDEA and SURF Grant Programs, the Wilson Ornithological Society Jed Burtt Mentoring Grant and a Great Hollow Ecological Research Grant. We thank UConn Forest, Yale-Myers Forest, the Connecticut Agricultural Experiment Station, Great Hollow Nature Preserve, and Bent of the River Audubon Sanctuary for allowing us to conduct research on their property. We also thank Carole Cheah and Emmett Varricchio for aiding in the planting and watering of nursery hemlocks used in this study, and consultation during experiments. Thank you to Robert Bagchi and members of the UConn Bird Lab group for feedback on study design, statistical methods, and an early draft of this manuscript.
NJR and MWT conceived and designed the experiments. NJR performed the experiments with assistance of CSE and NPH. NJR and MWT analyzed the data. NJR wrote the first draft of the manuscript; all authors contributed to the final version.
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