Hemlock wooly adelgid vs. silver flies

The insect has pushed the Eastern hemlock to the brink of extinction, but a researchers are fighting back with a biocontrol combo.

Wooly adelgid egg sacs on hemlock

It’s a warm day in early June, and researcher Nicholas Dietschler is standing in front of an eastern hemlock sapling about a quarter of a mile up a steep ridge in New York’s Catskill Mountains. The evergreen is not looking good. Its lower branches are brittle and dead. Its upper limbs are balding. Dietschler visually scans the tree’s stubby needles. It doesn’t take long for him to find what he’s looking for. Tiny, woolly white bumps the size of sesame seeds coat the sapling’s spindly branches. Dietschler runs his thumb along the bumps. “Blood,” he says, holding up a finger streaked with orange. “They’re alive.” 

The tiny bumps are the egg sacs of a destructive insect called the woolly adelgid, which caught a ride with Japanese goods bound for America in the early 20th century and has been wreaking havoc on the nation’s Eastern forests ever since. The aphid-like insects suck the sap out of hemlock twigs, killing the trees where they stand.

A blue cooler lies open on the ground, filled to the brim with neatly stacked plastic vials. The survival of a species hangs by a thread, and the outcome could depend on the contents of the blue cooler.

The individual hemlock that Dietschler, the researcher, is standing in front of is likely doomed. But at the very top of the ridge, a grove of pristine, old-growth hemlocks grow straight and healthy out of the forest floor.

“The reason we’re releasing here is because of that stand up there,” Dietschler says, looking uphill. What he’s releasing rests inside that blue cooler: a little more than 1,000 live silver flies. If all goes according to plan, the flies will feast on the woolly adelgid in the infested trees, arresting the spread of the invasive insect and protecting the healthy hemlocks at the top of the ridge. The battle between adelgid and fly is a preview of future fights to curtail invasive species. Non-native pests are difficult to contain in normal climatic conditions. Can scientists save the nation’s trees from invasives supercharged by climate change? The Northeast stands to lose a great deal if they can’t.

Read the full article by Zoya Teirstein here, in WIRED.


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