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.

The list of invasive insects moving unchecked through Canadian forests reads like a long and increasingly unruly testament to humanity’s reshuffling of the biological deck: emerald ash borer, brown spruce longhorn beetle, gypsy moth. Together, these threats are reconstituting the country’s woodlands, with consequences for both ecology and industry. As of 2015, roughly 400,000 hectares of forests were destroyed by invasive pests annually, nearly half of what is harvested by foresters. But, as the list of invasive species has grown, the list of potential solutions has not, leading scientists to return to biocontrol, a practice that had gone out of favour decades ago.

The popular narrative around biocontrol can be seen in a 1998 episode of The Simpsons: after Springfield is overrun with an invasive population of “Bolivian tree lizards,” Principal Skinner suggests that introducing “Chinese needle snakes” could take care of the problem. When Lisa questions whether that would just lead to the town being overrun with snakes, Skinner has his answer ready: no, that population will in turn be controlled by snake-eating gorillas, which will “simply freeze to death” when winter arrives.

If biocontrol has a reputation for being cavalier, it’s not without cause. Take the case of the cane toad, released in Queensland in a 1935 attempt to control the beetles that were attacking sugar cane crops. The toxic toads quickly spread from their release point, with devastating—and ongoing—consequences for the populations of native predators that eat them. Even one of biocontrol’s greatest Australian success stories, the use of Argentine cactus moths to beat back prickly pear cacti, which were spreading aggressively in the early twentieth century, had unintended consequences when attempts were made to replicate that success in other areas. In the Caribbean, prickly pear cacti were exploding as a result of deforestation; but, when locals tried releasing cactus moths, the insects spread to the southern United States and Mexico, where they continue to damage those countries’ native cacti.

Despite these high-profile failures, biocontrol was often the only option for eradicating pests prior to the development of insecticides. In Canada, where government efforts of biocontrol started in the late nineteenth century, the importation, rearing, and release of new predators was successfully used against invasive species like the European spruce sawfly, which was threatening spruce stands in Quebec and northern New Brunswick in the 1930s. But this success also had a twist: a virus that had hitched a ride on one of the introduced predators was actually responsible for much of the sawfly’s decline. In other words, the government got lucky. Canadian scientists weren’t exactly exercising an overabundance of caution, says Chris MacQuarrie, a research scientist with the Canadian Forest Service. “It was a much different time, and they didn’t have some of the same rules about, you know, testing.” Out of a federal lab in Belleville, Ontario, scientists imported and reared over 200 potential biocontrol agents for the spruce sawfly alone, and many were released with little sense of whether they would actually go after the sawfly or attack something else. “So they would just bring stuff in, see if they could grow it in the lab, and sort of throw it out in the woods and see what happens.”

Later, some researchers, drawing on new insights about the complexity of ecosystems, started raising questions about the potential for unintended consequences—so-called non-target effects. But the shift away from biocontrol had as much to do with the chemical revolution brought about by the Second World War as it did with concerns about the unintended consequences of introducing new invasive species. The war made predators harder to source from Europe, where most invasive species at that time were from. And, in the decades after the conflict, pilots, planes, and chemicals were suddenly ubiquitous. “You have sort of the change in the paradigm of pest management because you have things like DDT,” says MacQuarrie. Biocontrol projects continued, with hits and misses, but on a smaller scale—the introduction of the masked shrew to Newfoundland, in 1958, to control larch sawfly, was one example. (The shrew didn’t much care for the sawfly but loved Newfoundland, thank you very much.) By 1972, the federal lab in Belleville had closed.

At the same time, the consequences of insecticides were becoming better understood. In 1962, Rachel Carson’s Silent Spring drew attention to the devastating consequences of DDT, particularly in the forests of New Brunswick, where converted bombers had started spraying for spruce budworm in 1952. Scientists documented extremely high mortality among Atlantic salmon and aquatic insects in areas sprayed with DDT, and by highlighting this and other effects of pesticides, including human illnesses, Silent Spring helped spark the environmental movement, which in turn led to Canada and the US banning DDT in 1972. Aerial spraying of other pesticides still continued, but in a more targeted way. Meanwhile, the same concerns that Carson wrote about also galvanized a generation of scientists, such as Mark Whitmore, to rethink how we approach our environments. “I just remember the big planes spraying thousands and thousands and thousands of acres. And I was thinking about all the collateral damage,” he says. Whitmore started considering the potential of biocontrol—but, he stresses, of doing it right. Whitmore is now a forest entomologist at Cornell University, in New York state, where he runs a lab dedicated to the biocontrol of the hemlock woolly adelgid.

Unlike the occasionally slapdash plans of the past, more recent research by scientists like Whitmore shows how complex and careful modern biocontrol can be. In 2003, researchers in Virginia started releasing a small black beetle called Laricobius nigrinus, which had been found in a hemlock seed orchard on Vancouver Island, to control the hemlock woolly adelgid—and, unlike in earlier attempts at biocontrol, the beetles were released only after lab tests showed they attacked solely their targets. Nearly twenty years later, those releases have begun to take a bite out of adelgid populations, but it remains unclear if this will effectively control the problem. And another issue remains: the adelgids have two generations each year, and according to Whitmore, the beetles have been targeting only the first. Whitmore is now investigating additional predators: two species of silver fly, painstakingly collected off western hemlocks in the Pacific Northwest, to attack the second generation. “If you want to talk about a frustrating profession, this is it,” Whitmore says. “How many adelgids do you think are up in those trees? Does the word bazillion come to mind? And then how many of these flies do I release? Well, you know, we might be lucky to have 500.”

Once the flies are released, finding them again to track their progress is difficult. Whitmore has begun investigating the use of eDNA, where genetic material taken from hemlock foliage or rainwater collected in traps can provide evidence of a species’ presence. Determining whether the flies have synced up with their prey and survived the cold winter is another question. Whitmore’s lab has done experiments to show that silver fly pupae can withstand the winter in containers outside but has yet to verify this in the wild. “We have released thousands and we’ve yet to find any that we’re certain have overwintered here in New York,” he says. But showing that a biocontrol agent attacks only the target species is essential—all of which is to say, modern biocontrol is a slow process. As Whitmore explains, “Populations don’t magically build up and reproduce to the point that they can control the adelgid.”

In Canada, the establishment of large-scale biocontrol in forestry is far behind the US: since the 1970s, programs here have been dwindling. But, in recent years, faced with a new generation of invasive insects—the emerald ash borer and the hemlock woolly adelgid in particular—scientists are now looking at these alternatives. To expedite the process, Canadian researchers are drawing on the testing done in United States: that leaves only plucking the predators from their native ranges, quarantining them, mass rearing new offspring, and conducting field trials. It’s a process that, if it were started today, could take at least ten years to begin seeing results for the adelgid—if it worked at all. Biocontrol on the emerald ash borer—which can kill up to 99 percent of ash trees in an affected area and has already wiped out millions from Manitoba to Nova Scotia since arriving in the 1990s—is further ahead: researchers started releasing parasitic wasps in 2012 and were planning to continue assessing the impacts last year, before COVID-19 threw a wrench in those plans. “It takes a long time,” says MacQuarrie, who’s part of the ash borer team. “It’s not quite as quick and easy as, you know, spraying something or sticking the tree and then you see the insect die.”

The rosiest projections for biocontrol programs would see populations of these insects reduced, but scientists do not anticipate eliminating the intruders altogether: in the twenty-first century, it seems there is no avoiding change. In the best-case scenarios, some trees would die, but others would survive. And, yes, adding even more predators into the mix would, on some level, change the forest; but, without biocontrol against the adelgid, nearly all eastern hemlocks could be wiped out. It appears that, sometimes, a glut of intruders is better than one. “Hopefully we’ll have an impact and will be able to save those [hemlocks], save the species for future generations,” says Whitmore. “If you don’t have hope, what do you have?”

More than hope, what biocontrol offers is a tantalizing vision of balance: of a planet that functions without—or in spite of—human intervention. The loss of eastern hemlocks, which have until now withstood centuries of environmental degradation, is a particularly painful example of how far we’ve strayed from that path, but it’s hardly the only one. For all the cautious work done in labs and all the tests that go into modern biocontrol, we may be left only with the impression that, at best, biocontrol will yield occasional glimpses of equilibrium, like dioramas of an earlier age. If everything goes as planned, the hushed dark of a hemlock stand or the yellow glow of a September street lined with white ash will only be reminders of how hard we have to work to recover even a fraction of what we’ve lost—and of how quickly, and precipitously, our ecosystems can change.

Back in the forest in southwestern Nova Scotia, the first leg of Mary Jane Rodger’s detection survey ended on a hopeful note, with no adelgid found.

But, a couple of hours later, Rodger sent a text. “Found on our second site in the understory.”

Above it, a photo of a young hemlock, dark green and delicate, and—thick along its branches—the white egg sacs of the hemlock woolly adelgid. Against the needles, they look like flakes of snow: signs of the gathering storm.

Moira Donovan
Moira Donovan is a journalist based in Halifax. Her work has appeared in the Christian Science Monitor, Vice, and on CBC Radio.