Cornell introduces silver flies to save hemlock forests

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Melissa Osgood
Mark Whitmore, Ky Barnett, Tracy Yardley
Robert Duckett
Mark Whitmore, left, Ky Barnett '18 and arborist Tracy Yardley apply a population of silver flies to a hemlock tree infested with woolly adelgids in Skaneateles, New York, June 25.

In an ongoing battle to save the ecologically important hemlock forests – found throughout New York state, especially in the Adirondacks – Cornell University researchers have high hopes for a new weapon against menacing woolly adelgids: silver flies from the Pacific Northwest.

“Countless numbers of hemlock trees have been affected in the eastern United States. To fight the woolly adelgids and to protect the hemlocks, we’re trying to build biocontrols in New York state before it is too late, as has happened farther south in the Appalachians,” said Mark Whitmore, Cornell Cooperative Extension forest entomologist in the Department of Natural Resources. “The Adirondacks,” he said, “are highly threatened.”

Whitmore released silver flies – with permits from the U.S. Department of Agriculture and New York’s Department of Environmental Conservation – along the shore of Skaneateles Lake June 25.

The silver flies likely will proliferate by laying their eggs among the adelgid eggs; the emerging fly larvae then will feed on adelgid eggs.

For more than a half-century, hemlock trees from Canada to the Carolinas have faced voracious attacks by the woolly adelgids, an insect the size of a poppy seed. Hemlocks are a key species in maintaining New York’s healthy forests and vibrant natural environment. The adelgids – which look like tiny, puffy cotton balls on hemlock branches – are all females and reproduce rapidly. Once the hemlocks are infested, the trees die after a few years, creating large dead spots in forests.

A member of the pine family, the evergreen eastern hemlock grows grandiose and graceful and plays a critical role in the ecology of New York’s forests. These trees grow near streams, inhabit slopes and reside in ravines, all the while protecting against sediment runoff and creating shady habitats for wildlife and plants that rely on cooler microclimates, including cold water brook trout.

Agricultural scientists have been using biocontrols on hemlocks for 25 years. In 2009, Whitmore and colleagues from the U.S. Department of Agriculture’s Forest Service and the University of Massachusetts-Amherst released 900 Laricobius nigrinus beetles into a stand of adelgid-infested hemlocks on Cornell Plantations land near Lansing and at two other sites on Seneca Lake.

Now, scientists believe the beetles and the silver flies (Leucopis piniperda and Leucopis argenticollis) are flanking the woolly adelgids for a more complete biocontrol

Whitmore says that last two winters, which were extremely harsh in upstate New York, has granted scientists a little breathing room to fight the adelgids. “The cold temperatures during the last two winters have given us a momentary reprieve, but the adelgids still have great potential at reproduction, and those that survived will build a more cold-tolerant population,” said Whitmore. In short, the winter-surviving adelgids will reproduce stronger offspring, and colder areas of the state like the Adirondacks will become more vulnerable.

The silver flies research team is being led by Whitmore, Kimberly Wallin of the University of Vermont and the USDA Forest Service, Darrell Ross of Oregon State University, Nathan Havill of the USDA Forest Service, and Bud Mayfield of the USDA Forest Service.


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Blaine Friedlander