Tree-killing Hemlock Woolly Adelgids Hitch Rides on Birds
By Meredith Swett Walker
At first glance, you might not think these little bits of fluff could pose much of a threat. But, like Star Trek’s troublesome tribbles, hemlock woolly adelgids (Adelges tsugae) can quickly multiply and wreak havoc. When these tiny sap-sucking insects were introduced to the forests of the Northeastern United States, they lacked an effective predator and within a few decades they caused significant die-offs of eastern hemlocks (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana) — trees that are both economically and ecologically important to eastern forests. Woolly adelgids were clearly destructive, and they were spreading, but it wasn’t clear exactly how they were spreading.
Understanding how these fuzzy little hemlock killers get around is crucial to controlling them. In their native Japan, hemlock woolly adelgids produce a winged generation that can fly and disperse. But these winged adelgids feed only on spruce, not hemlock. In North America, there is no suitable spruce species for this generation to eat and the flying adelgids die.
Adelgids can be inadvertently dispersed by humans when they hitch a ride on nursery stock — this is likely how they were introduced to North America and how they initially spread. In response, eastern states instituted quarantines on hemlock nursery stock and hemlock products. People visiting infested hemlock stands were also advised to clean their clothing and gear to reduce the likelihood of adelgids being transported.
Humans are an important means of transportation for adelgids, which we can control to some degree. But the insects can also hitch a ride on other mammals like squirrels and deer, though these species are unlikely to transport them very far. Scientists suspected that adelgids could be dispersed long distances by birds, but to date only one small study has documented this. In the latest issue of Environmental Entomology, researchers Nicholas Russo, Carole Cheah, and Morgan Tingley of the University of Connecticut and the Connecticut Agricultural Experiment Station investigate exactly how and when birds pick up these tiny woolly hitchhikers.
When adelgid nymphs emerge from their woolly egg sacs, they are known as “crawlers.” These crawlers look like minute specks of pepper and they disperse short distances within the hemlock to find an open spot on the tree where they can feed. Crawlers may also find their way onto another animal and hitch a ride off the tree. Russo and his co-authors wanted to find out if the crawlers actively moved onto birds that perched in the infested hemlock, or if the miniscule nymphs were simply picked up when the birds brushed up against them. How did the degree of infestation affect the likelihood that a bird visiting the hemlock would pick up an adelgid hitchhiker? Timing was also an important question. Were the birds more likely to pick up adelgids during migration when they were traveling long distances, or during the breeding season when they were settled on territories?
Since live songbirds are not good at following directions, the researchers enlisted dead ones. They used taxidermy mounts of various warbler species known to frequent hemlocks as proxies for live birds. These mounts could be perched on hemlock branches with varying degrees of adelgid infestation, for varying lengths of time. Or the mounts could be brushed up against the branches as if the bird was hopping through the tree. Russo and his colleagues conducted trials from May-July 2015 to cover both the period when live warblers would be migrating up the East Coast and the breeding season when they would be settled on territories. To determine how many adelgids a bird had acquired during a trial, the mount was held over a white tray and blown with a stream of compressed air to dislodge crawlers and any egg sacs that might have stuck to it.
The researchers found that that birds picked up more crawlers when they brushed up against branches as opposed to perching on them. However, perched bird mounts picked up plenty of crawlers, suggesting that the adelgids may not be simply passive hitchhikers as previously thought. Russo suspects that crawlers may crawl onto any obstruction they encounter on their hemlock branch be it bird feet or a scientist’s flagging tape. Further explorations of crawler behavior are planned, and Russo is currently running an experiment to quantify in situ crawler wandering rate.
Severity of infestation was important as well. Birds perched on branches with more adelgid ovisacs picked up more crawlers. Crawler transfer to birds also varied over the course of the season. Birds picked up the most crawlers in May, which coincides with the end of bird migration in Connecticut. Russo et al.’s results provide mechanistic evidence supporting the idea that birds may spread adelgids northward during their spring migration. Indeed “the northern pattern of A. tsugae spread mirrors springtime terrestrial bird migration patterns in the eastern U.S.,” the researchers wrote.
The current distribution of hemlock woolly adelgids in eastern North America is limited by wintertime low temperatures. But one major climate change model predicts that by the end of this century, the entire northeastern U.S. will be climatically suitable for this species. Russo et al.’s research suggests that adelgids may already have a ride ready and waiting to facilitate a northward expansion of their range. A better understanding of how and when birds can disperse hemlock woolly adelgids will help scientists and forest managers better control the spread of this devastating invasive species.
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Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs http://picahudsonia.com and https://citizenbiologist.com or follow her on Twitter at @mswettwalker.