Songbirds Helping Spread Hemlock Wooly Adelgid

Hemlock wooly adelgid, a tiny insect pest, was discovered in the western United States in the 1920s and was first observed in the East in the early 1950s near Richmond, Virginia, perhaps having traveled on nursery plants from Japan.  The hemlock wooly adelgid is a serious invasive I the East and it now can be found at least from Georgia to southern Nova Scotia.  The tiny aphid-like insect that threatens the health and sustainability of eastern and Carolina hemlock is smaller than a ladybug. Small, white, cottony sacs at the base of the hemlock needles usually indicate the insects’ presence.  But sap-sucking adults can assault entire hemlock stands.  As Hemlock health declines, tree mortality usually occurs within four to ten years.

            The main vectors aiding the spread of hemlock wooly adelgid are storm winds, nursery trees, and firewood.  Migratory birds, which assist hitchhiking adelgids are thought to be long distance transporters.  A recent article in Biological Invasionsby four Connecticut researchers strengthened the migrant-bird premise.

            Of 456 captured birds, examined with the aid of small jets of compressed air, 40 individuals of 22 species carried adelgid crawlers, the flightless nymph phase of the adelgid.  The bird species included hemlock-loving black-throated green warblers and blue-headed vireos.

            This confirmation raises the stakes. Avian-assisted travel could mean much faster spreading adelgids and, significantly, crawlers are at peak abundance during active spring migration.  Moreover, temperatures are warming and more adelgid eggs are surviving the winters.

            The migration pattern of songbirds, particularly those that favor hemlock habitat, may help to detect at least early-spreading populations of adelgids

This was originally published in a “Quick Takes” column by Paul J. Baicich in Bird Watcher’s DigestVol. 41, No. 6, July/August 2019.