Milder winters mean more of this insect invading CT’s hemlock trees
Cold winters are key to curbing an invasive pest that’s depleted Eastern hemlock trees for decades in Connecticut. Warm winters, fueled in part by climate change, allow these insects to spread — but a deep cold snap in 2023 slowed their advance.
Carole Cheah, a research scientist with the Connecticut Agricultural Experiment Station, personally visited over 35 sampling sites around the state after the cold snap and saw the impact. The cold snap was so impactful on the hemlock woolly adelgid in part because of a lack of snow to insulate the population.
“Most insects are overwintering; hiding in crevices. They're not out there, exposed to this extreme cold, whereas the adelgid is,” Cheah said.
Winter kill temperatures for adelgid vary based on the area of the state, Cheah said. This winter, temperatures are projected to be “leaning above'' normal in the region, federal weather officials said, but powerful and cold nor’easters are still possible. 2023 brought record-high global temperatures, driven by climate change and El Niño.
Eastern hemlocks have many uses, from sheltering wildlife, improving stream water quality, and creating lumber, paper and mulch. The insect, native to Japan, attaches itself to the stem of the hemlock trees and feeds on its sap.
“It drains the tree’s resources, and the tree is unable to put out new growth. And unfortunately, the health of the trees depend on having this new foliage to photosynthesize and replenish all their reserves,” Cheah said.
Drought also puts pressure on the hemlocks. Historic drought conditions have affected New England as recently as 2020 and 2022, while 2023 brought record rainfall to the region.
Cheah has studied the hemlock woolly adelgid for 30 years and has observed its expansion to new locations in Connecticut, like higher elevation areas. Her team currently uses ladybeetles, a native predator of the adelgid, as a way to control infestations.
The adelgids wiped out in February 2023 were one of two generations that develop every year. Cheah said that for now, this is good news for the state’s Eastern hemlock trees: She’s had to search closely to find more pockets of the pest.
According to new U.S. Forest Service insect research, the adelgid caused damage across almost 6,000 acres of Eastern hemlock trees in 2022.