In southwestern Nova Scotia, one morning in late July, forester Mary Jane Rodger picked up a hemlock branch clipped from the towering canopy overhead and leaned in close to examine a fleck of white on its delicate dark-green needles. She was on the lookout for signs of a killer—one that now puts all of the province’s hemlocks at risk.

Eastern hemlocks, with their narrow trunks and scaly bark, don’t have the obvious majesty of Western red cedars, but stands like this one create their own magic: a permanent cool twilight, a moss-carpeted microclimate that shelters everything from migrating birds to brook trout. In Nova Scotia, where much of the province has been logged since the arrival of Europeans, long-lived hemlocks make up a significant part of the sliver of old-growth forest that remains, spared by their low commercial value and ability to grow in hard-to-reach areas like the banks of rivers. Yet, having made it to the twenty-first century, these survivors are now threatened by a tiny menace lurking on their branches.

In search of this threat, Rodger held the branch close to her face. “It’s just sap,” she concluded after a moment’s inspection. Rodger was looking for the egg sacs of a tiny sap-sucking insect known as the hemlock woolly adelgid. The sacs, which are the most visible sign of the adelgid’s presence, look like many things: cotton wool, spider eggs, bird poop. “It’s this weird dichotomy because it’s like a treasure hunt,” Rodger says—except, in this case, “you don’t really want to find what you’re looking for.”

What the adelgid’s egg sacs most resemble for eastern hemlocks, though, is trouble: since arriving in the US state of Virginia from southern Japan in the early twentieth century, the adelgid has carved a swath of dead and dying trees along the eastern seaboard. It is now coming for Ontario, Quebec, and the Maritimes, likely hitching rides on birds and other animals and migrating north thanks to steadily warming winters. In Ontario, the insects were first detected in Etobicoke, in 2012, then along the Niagara River the next year. They were then thought to be eradicated in the area until they turned up again in 2019. In southwest Nova Scotia, people first reported the adelgid in 2017—although the state of some hemlocks suggested the insects had arrived long before.

Wherever it is found on eastern hemlocks, the adelgid, unchecked by the predators that keep it under control in its native ranges of East Asia and the Pacific Northwest, drains the cells that store water and nutrients at the base of the hemlock’s delicate needles, causing the needles to turn a reddish-yellow and eventually drop off. The tree essentially dies from starvation in as little as three years. Whole stands of hemlocks are often affected at once, leaving grey gashes on the landscape. Without a solution, nearly all of the eastern hemlocks could die this way in the coming decades. That threat—along with an increasing onslaught of other invasive pests—has prompted scientists to reexamine a controversial remedy from the past.

If native flora or fauna are being threatened by invasive species, the thinking goes, maybe the solution is to introduce new predatory species that could kill off the invaders. The strategy, called biological control, has a checkered history, but when it works, it can rebalance the scales. For trees like the eastern hemlock and the ecosystems they create, biocontrol may just be their best chance for long-term survival.