Flies May Defend Hemlocks from Invasives

By Joshua Rapp Learn

Woolly adelgids The cotton-like egg sacks of the invasive hemlock woolly adelgid on an eastern hemlock in Tennessee. Researchers are testing tiny flies from the Pacific Northwest found to prey on adelgids.
Image Credit: Bud Mayfield, USFS

Ecosystem managers looking to stop the onslaught of an invasive aphid-like insect that has been laying waste to hemlocks up and down the East Coast could have another ally in their battle.

Researchers discovered that two species of tiny flies from the Pacific Northwest will attack and eat hemlock woolly adelgids (Adelges tsugae) on eastern (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana).

The adelgids — which are native to Japan and have few predators in eastern North America — kill these trees by attacking budding pine needle tips. Foresters say the invasive species are now killing hemlocks in at least 19 states.

But researches captured two species of silver flies, Leucopis piniperda and Leucopis argenticollis, in Washington State and released them on infested hemlocks near Grandview, Tennessee last month.

“This is the first time this has been done with these flies; it’s a brand-new idea. We’re hopeful,” said Kimberley Wallin of the University of Vermont and U.S. Forest Service, in a release. The flies were set free in what she calls “bug dorms” — bags secured around infested branches with differing numbers of flies.

Early results show that the silver flies are reproducing inside the bags, though researchers continue to observe the experiments. If the bugs can successfully reproduce in eastern climates, they could be used as a tool to fight the destruction of hemlocks.

“We don’t hope that the flies will eradicate all the adelgids,” Wallin said, “but if they could provide a check on the pest’s population size and territorial expansion, it could allow some hemlocks to persist and recover.”

The current trial is the result of 10 years of research by Wallin and other researchers, who had to first test the flies to make sure they would prey on the adelgids rather than other native insect species in order to get the necessary permits from the U.S. Department of Agriculture.

Other recent experiments involved sesame seed-sized Lari beetles (Laricobius nigrinus), another insect from the Pacific Northwest that can eat six to eight adelgids a day.

Wallin said that these flies are the “most promising lead in a long time” and that they are “a complement to the beetle work.”

“We need to be conservative when it comes to these kinds of releases of novel species,” she said, “but the adelgids are killing all the hemlock trees.”

Woolly adelgids were first detected in Virginia in 1951, and researchers compare the death they cause to that caused by the Dutch elm disease or chestnut blight.

“Once hemlock is removed, the soil type changes, the stream dynamics change, the forest type changes — and it’s hard to recover,” Wallin said. “We need to try to do something to protect these trees.”

Joshua LearnJoshua Rapp Learn is a science writer at The Wildlife Society.

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