Contagion does not mean extinction. Even for trees.

Which is why lovers of hemlock — the evergreen, not the Socratic suicide sipper — can be happy in Connecticut. A combination of cold snaps, welcome rain and human intervention may have saved this essential tree from being lost.

Take Steep Rock Preserve in Washington. For many years, a non-native insect, the hemlock woolly adelgid devastated the hemlocks there. Another insect, the hemlock scale, was established there as well.

But now, said Rory Larson, Steep Rock’s conservation and program leaders, the loss has been checked.

“They’re looking really good,” he said of the healthy trees now thriving there.

The same story holds true at Mine Hill, the preserve owned by the Roxbury Land Trust.

Like the staff at Steep Rock, the trust worked with Carole Cheah of the Connecticut Agricultural Experiment Station to release a biological control — a lady beetle — to combat the adelgid infestation.

The Mine Hill hemlocks now are healthy as well.

“So far, so good,” said Ann Astarita, the trust’s executive director. “We’re really happy with the results.”

Cheah also released beetles at Webb Mountain Park in Monroe. Dave Solek, the park’s ranger and Monroe’s tree warden, said the bio-control has done its work.

“We’re seeing beautiful green hemlocks,” Solek said.

“I couldn’t believe it,” Cheah said of the health of Webb Mountain’s hemlocks.

But the fight continues. Cheah said she’s found new pockets of adelgid infestation in northeastern Connecticut. Unlike past winters, the winter just past had no cold snap — almost no cold — to kill the insects.

“It’s been a non-winter,” she said.

On the other hand, Cheah said, she’s having trouble finding adelgids at most of her field sites.

“We have a totally different story,” she said of the hemlock comeback.

Hemlocks are native evergreens that like the state’s acid soils. They are also shade tolerant and grow in places that other trees can’t manage.

Their dense foliage provides shelter for both wintering birds and spring migrants. They shade streams and rivers, cooling the water for fish and aquatic insects. They’re what’s called a foundation species — one that’s essential for an ecosystem.

“With climate change, they’re critical,” said Chris Martin, chief of forestry for the state Department of Energy and Environmental Protection.

The existential threat to hemlocks probably came on plants imported to the U.S. from Asia in the 1940s. Those plants carried adelgids — tiny insects that build a white cottony swab of a nest under the hemlock needle They feed on a tree’s sap, robbing it of nutrients, and killing it within a few years.

Researchers found it in Virginia in 1951. It spread to Connecticut in 1985.

The DEEP’s Martin said adelgid damage was most noticeable on hemlocks growing in thin soil on steep hills.

“You saw a lot of trees dying along the Housatonic River,” he said.

Forester Matt Bartlelme, owner of Bart’s Tree Service in Danbury, lives on Hemlock Shores on Candlewood Lake.. When he moved there in the early 1990s, he said, there were groves of healthy hemlocks. Most of them died.

“You’d see hemlocks that looked they had Christmas decorations on them,” he said of the strings of white nests under the branches.

The rescue began when the Agricultural Experiment Station found a biological control — an Asian lady beetle, Sasajiscymnus tsugae, that feeds on the adelgid. Over the years, the station released tens of thousands of these beetles throughout the state.

Cheah no longer gets state funding for the project. But a Pennsylvania company, Tree Savers, grows the beetles and shares its surplus with her.

“I’ve got 700 right now,” she said.

Next, Cheah said, there were a string of years — 2014 through 2017 — when winter temperatures plunged dramatically. Those zero-degree days iced the adelgids, greatly reducing their numbers throughout the state.

The drought years of 2015 through 2017 posed a new threat to the hemlocks. Stressed by the dry weather, they became more vulnerable to hemlock scale damage.

But the rains returned in 2018 and the hemlocks revived. Healthy again, they were able to better resist the scale.

“The rain really saved the tree,” Cheah said.

The threats remain. Hemlock scale is established in the state. If drought years return, it can damage the trees anew. So could a resurgence of adelgids.

But Cheah — a 25-year veteran of the hemlock wars — sees healthy trees that had been dying.

“I am quite optimistic,” she said.

Contact Robert Miller at earthmattersrgm@gmail.com