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Cold Weather Provides Welcome Relief For Connecticut’s Troubled Hemlock Trees

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Scott M Salom, Virginia Tech / U.S. Department Of Agriculture
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Hemlock woolly adelgid on an eastern hemlock tree.

Hemlock trees in Connecticut have been having a tough go of it thanks, in part, to a small sap-sucking insect: the hemlock woolly adelgid.

First identified in Connecticut in the 1980s, this invasive Japanese insect eats through conifer trees and has contributed to die-offs of native conifers like the Eastern hemlock.

But Carole Cheah with the Connecticut Agricultural Experiment Station said something might finally be causing adelgids to die off: all this cold weather.

“I have been looking for adelgids since the summer,” Cheah said. “I have been hardly able to find any adelgids at all. Even in places where I used to be able to collect adelgids.”

For years, Cheah’s been going out to look for adelgids all over Connecticut.

Woolly adelgids are active during more mild parts of the season, when temperatures are in the 30-to-40-degree Fahrenheit range.

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Credit Nathan Havill / USDA Forest Service
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USDA Forest Service
Slide-mounted HWA adult (left) and developing HWA (right).

But through years of sampling, Cheah said she’s found extreme winter temperature drops (at least -11 F in the northwest corner, -8 F in central Connecticut, or -6 F along the shore) are killing massive numbers (more than 90 percent) of adelgid populations.

So are successive days of sub-zero temperatures.

“My feeling is that, at least with regard to the adelgid, we no longer have such a serious threat as we had just a decade ago,” Cheah said. “I feel very optimistic about the future for our eastern hemlocks.”

But Cheah cautioned it’s a tempered optimism.

Hemlocks are susceptible to drought, which notably impacted the state in 2016 and 2017.

Cheah also said increased snowfalls could actually insulate surviving adelgids, helping them to ride out extreme winter weather.

Meanwhile, northwestern adelgid populations appear to be developing some cold-resistance, and urban “heat islands” could warm up adelgids in more developed portions of the state.

Then there’s another, more cold-tolerant, insect, which may be taking the adlegid’s place: the elongate hemlock scale.

“We should not take our eye off the [elongate hemlock] scale, because that is not going to be as impacted by the winters,” Cheah said. “Even though the hemlock woolly adelgid may have lessened its impact on the hemlocks, I believe the scale needs to be further researched and is definitely impacting them now.”

Patrick Skahill is a reporter and digital editor at Connecticut Public. Prior to becoming a reporter, he was the founding producer of Connecticut Public Radio's The Colin McEnroe Show, which began in 2009. Patrick's reporting has appeared on NPR's Morning Edition, Here & Now, and All Things Considered. He has also reported for the Marketplace Morning Report. He can be reached by phone at 860-275-7297 or by email: pskahill@ctpublic.org.

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