Migrating Birds May Be Spreading an Invasive Pest That Decimates Forests

The hemlock woolly adelgid is moving north, and feathers could be providing the perfect ride, experiments show.

Across eastern North America, an insect the size of a grain of pepper is devastating ancient hemlocks with the help of some unlikely accomplices. 

Introduced in the 1950s to Richmond, Virginia, likely through nursery plants from Japan, the hemlock woolly adelgid spread rapidly in the 1980s and can now be found as far as southern Nova Scotia. The tiny critters, which suck up the sap from their host trees using straw-like mouth parts, have caused a massive hemlock die-off across the United States. “You have to go north of Massachusetts at this point to find really healthy stands,” says Aaron Ellison, senior scientist at the Harvard Forest. “The adelgids are moving very fast.”

As the invasive insect expands its horizons, scientists are racing to understand the extent of the damage. (Hemlocks are considered a foundation species, which means many plants and animals depend on their canopies for habitat and cooling.) But population models keep underestimating the rate of the invasion, likely due to the adelgid’s hitchhiking abilities: The pest is believed to travel long distances on nursery trees, firewood, and wind. Now, however, a new study says it may have a few avian helpers, too.

Unlike some of the adelgid's other transit options, migrating birds might be carrying the sap-sucking bugs directly to their hemlock hosts. In a paper published last month in Biological Invasions, University of Connecticut graduate Nick Russo, his advisor Morgan Tingley, and their colleagues tested the idea by taking a close look at forest species like Song Sparrows, Veerys, and Louisiana Waterthrushes in local adelgid-infested hemlocks. The researchers caught and sprayed 450 birds with jets of compressed air to remove any pests hiding in the feathers. Ultimately, they found that nine percent of the subjects, including hemlock-loving Black-throated Green Warblers and Blue-headed Vireos, carried adelgids or their eggs. 

But how do these insects catch a ride with birds in the first place? To discern that part of the relationship (which so far appears harmless to the birds), a previous experiment by Russo and Tingley's team placed stuffed migrant species in one of the same Connecticut hemlock forests and watched as the tiny adelgids made their way onto the decoys. They also placed a set of faux passerines, adelgids and all, on pest-free nursery trees and found that the insects were able to alight and colonize saplings.

While both these studies had a limited geographic focus, Russo and his peers think the avian-assisted invasion could be happening on a much greater scale, given that billions of birds travel north each spring to breed and make multiple stops along the way. What’s more, crawlers, the most mobile stage of the adelgid's life cycle, are at peak abundance during spring migration. The scientists also suspect that birds were the reason insects reached southern Nova Scotia in 2017—in a forest that’s separated by more than 60 miles of ocean from the nearest known location of the bug.

Still, pinpointing the exact cause of the Nova Scotia incident and others like it is next to impossible. Climate change further clouds the picture: Warming temperatures and increasing resistance to cold means more adelgid eggs are surviving the winter and digging into hemlock stores in spring. What is clear, however, is that the insect’s impact can have cascading effects on other northern forest dwellers. A 2018 study showed up to 30 percent declines in hemlock-dependent birds like Acadian Flycatchers, Blue-Headed Vireos, and Black-throated Green Warblers in adelgid-stricken stands in New Jersey and Pennsylvania. “If the hemlocks die, the birds don’t have the type of nesting habitat they need to thrive,” Russo says.

This makes any information that helps narrow possible new infestation sites all the more important, says Ron Neville, a plant health survey biologist for the Canadian government. Most of the habitat he helps manage is still clean—but he doesn’t expect that to last forever. “We want to assess the migration patterns of birds, particularly certain species that spend a lot of their lives in hemlock forest, and study how those migrate,” Neville says. “Hopefully that will help us detect populations of adelgids early.”

Birders can play their part, too, by reporting the easily identifiable egg masses to local forestry officials and making sure not to spread the pest to new areas. Neville and his team, for example, use lint rollers to debug after spending time among the hemlocks. Now if only the birds would catch on.