Journeys in ancient and old growth forests

Hemlock Woolly Adelgid moves north

Understanding our options as Hemlock Woolly Adelgid moves north into the Adirondacks and Canada 

Hemlocks killed by HWA in Catalochee, Great Smoky Mountain National Park
Hemlocks killed by HWA in Cataloochee, Great Smoky Mountain National Park

Roads into Great Smoky Mountains National Park wind in S-bends, with a steep slope on one side and a sharp drop into a river valley on the other. The difficult access to this area historically kept loggers out, and impressive old-growth forests remain here; but today the rugged terrain offers little protection against a millimetre-long introduced insect that can kill a 500-year-old hemlock tree in a few years, called the Hemlock Woolly Adelgid (HWA). When I visited the Smokies in the spring of 2014 and saw the old-growth hemlock forests there reduced to skeletons by the Adelgid, it was a warning of what’s coming soon to my home province of Ontario.

The initiation for hiking the trails in the Cataloochee Valley was wading knee-deep across an ice-cold river. The trail then wound alongside a creek through forests that were dominated in many places by hemlocks, commonly aged between 300 and 500 years old – now dead. I saw only a handful of living hemlocks, mostly small trees. Otherwise, where shady hemlock groves had once grown there were forests of dead trees, the fine branches already mostly decayed.

The culprit, Hemlock Woolly Adelgid, may have been imported from Japan as early as 1911, clinging to evergreen shrubs planted in the Japanese garden of a woman named Sallie Dooley in Richmond, Virginia. The insect was first detected infesting a native Eastern Hemlock in Richmond in 1951; but as there were few hemlocks nearby it didn’t really spread. By the late 1980’s, however, the adelgid had spread to an old hemlock forest 40 miles from the city, and had killed 90 per cent of the trees. Today, it is affecting about half the range of hemlock, with mortality as high as 98 per cent.

Hemlocks logs such as this one used as bridge have been aged to over 500 years
Hemlocks logs such as this one used as bridge have been aged to over 500 years

It was a warm spring day in Cataloochee, and I’d stripped down to a T-shirt by the time I reached the first hill-top. Leaves were still tightly wrapped up in buds, there were few signs of life; I later found out that this is the most active time of year for the adelgids that killed so much of this forest, and had I looked I could likely have seen them on the few remaining live hemlocks, appearing as small white woolly masses attached to the twigs near the base of the needles. Enclosed inside each woolly mass is a tiny sucking insects that has attached permanently to the branch, and as the insect develops it lays a cluster of up to 100 eggs which will hatch into a temporarily mobile crawler stage. All of the Hemlock Woolly Adelgids in North America are female and reproduce asexually, because the alternate tree host for the sexual stage of the insect are Japanese spruces -adelgids can’t survive on North American spruces. This limitation may have slowed down their spread slightly as the sexual stage is winged, whereas the stage we know here is flightless;

hemlock Woolly Adelgid
hemlock Woolly Adelgid

but it’s a minor point at best, as both crawlers and the woolly cases full of eggs may be carried between trees by birds or humans (including on nursery stock) which can move adelgids many kilometers – or by wind which commonly moves them tree to tree.

Once the adelgids get onto a tree it may take a few years for numbers to build to the point where they’re noticed. “The problem with this insect is it likes to get into the middle-upper crown more often than not,” says Jeff Fidgen, a Canadian Forest Service scientist. “That population can sit up there and spread for a while before it is evident at ground level.” The chronology of hemlock decline following arrival of the adelgids is muddied by a few factors, not least of which is the uncertainty of how long woolly adelgid was present on many sites before being detected. Climate can also play a big role in severity of HWA outbreaks – both broad climatic gradients, in which outbreaks spread and kill trees more rapidly at the southern end of hemlock’s range than the north; and seasonal variation that affects mortality year-to-year on a site.

2012 map
Distribution of HWA in the United States

Two things are clear at least: once a hemlock forest is infested, tree mortality only trends up over time; and sometimes adelgids kill whole forests extremely quickly. The insect was discovered in Great Smoky Mountains National Park in 2002 and within a decade an estimated 80% of the hemlocks in the park were dead. Significant drought in 2007-08 accelerated the process. In Shenandoah National Park, Virginia, it was found in 1988; five years later many of the hemlocks were dead and today eastern hemlock has been virtually extirpated from the park. On the other hand research in the Delaware Water Gap National Recreation Area found that after ten years of monitoring hemlock survival was 73%.

At first it was thought that cold winters would limit the spread of the insect in the north of hemlock’s range, but as it moves north it has – amazingly, for an insect with no sexual reproduction – steadily adapted to colder climates. Mark Whitmore, at Cornell University, has been studying cold tolerance of the adelgid in upper NY state. His research over the last two winters has shown that at the north end of the range, even during the past two unusually cold winters, survival has remained high enough to sustain adelgid populations. “Last winter everyone talked about the ‘polar vortex’, and it was a pretty cold winter; I did get around 90% mortality,” says Whitmore, “but by the beginning of this fall I had densities that were remarkably similar to those before the cold mortality occurred.” Two generations per year, each with a high reproductive potential, allows HWA to bounce back almost immediately from even high levels of mortality. And Whitmore adds that “the scary part is that research has indicated cold tolerance is a genetically linked trait, so that populations developing from the survivors of the past couple cold winters may make for even hardier HWA in the future.”

Whitmore worries that the cold winters may have made forest managers complacent; with a few warm seasons mortality of hemlocks might climb significantly. In any case there’s little indication that cold will inhibit the spread of the adelgid. “I have no question it will go through zone 5,” says Whitmore. “Maybe when you get up into the Algonquin area, maybe you’ll get a change, but down lower I don’t think there’s any question about its capacity to spread.”

Hemlock Woolly Adelgid was found in Ontario's Niagara Gorge
Hemlock Woolly Adelgid was found in Ontario’s Niagara Gorge

In 2012 Hemlock Woolly Adelgid was detected in Ontario for the first time. It was found at two locations: Etobicoke, where it likely came in on some infected nursery stock; and in 2013 in the Niagara Gorge, an important old-growth forest known to have 435 year-old hemlock trees. The Etobicoke find, in a residential area, was easily dealt with; the infected trees were cut and burned, the neighbourhood was surveyed for further signs of adelgid. It probably stopped there. The Niagara gorge is trickier, because it’s nearly impossible to fully survey the tall hemlocks to determine the extent of an infestation. Adelgid was once again found in the gorge in 2014, and it would be optimistic to think that taking out a couple of trees removed the insect. “There’s a good probability that the insect is somewhere in that gorge, hidden in the tops of trees,” says Taylor Scarr, Ontario’s provincial forest entomologist. The best way forward may be to try to eliminate the population with systemic insecticide application, an approach that Scarr says is being considered by the province.

It’s also an approach that Mark Whitmore advocates, and that he used in the Zoar Valley old-growth forest near Buffalo. “I was giving a lecture nearby and I contacted them and got the preserve steward to give me a tour. And within a half hour I found one tree that was infested,” says Whitmore. “We spent the rest of the day looking and we didn’t find any others. I still can’t believe we were fortunate enough to find just this one infested tree. We treated not just that tree but 600 trees all around it thinking we needed a buffer because HWA wasn’t likely to be on just that one tree. It was a very rapid response, we got in there and hopefully we took down that population, buying time for biological control to develop. I think that kind of strategy could be used effectively in Ontario with adequate early detection.”

Laricobius nigrinus feeding on HWA
Laricobius nigrinus feeding on HWA. Photo: Mark Whitmore

The biological control Whitmore talks about are specialized predators of the adelgid, especially a beetle called Laricobius nigrinus, sometimes affectionately called Lari. The beetle is imported, but not from Japan – actually from the west coast of North America. For our area, beetles are often brought from Idaho because that’s where the most cold-tolerant L. nigrinus beetles are found, but also from Washington State. How, you may wonder, did a predator specialized in eating a Japanese adelgid end up living in Idaho? It took scientists a while to realize it, but Hemlock Woolly Adelgid is native to the west coast of North America, where it feeds on Western Hemlock, which has evolved to live with it. A whole suite of predators have also evolved to predate it, effectively keeping its numbers in check. So much so that Eastern Hemlock trees planted in arboretums in the west survive long term, protected by the predators and relatively unaffected by HWA.

So far the predators are not working so well here. They establish and grow in numbers, eating thousands of adelgids and even having a marked effect on HWA populations in some places; but the trees still die. Well, most of them. There are some places where predators have thrived and tree decline seems to have slowed, even reversed, but the hard evidence isn’t in yet. If one, or several of the predators working in concert can control the adelgid there’s hope for hemlock – if not, it is likely to become functionally extinct, meaning it no longer plays a meaningful role in the ecology of North American forests. Similar to chestnut or elm trees.

Hemlock forest in Ontario's Algonquin Park
Hemlock forest in Ontario’s Algonquin Provincial Park

Emotionally, this is a big deal to me, having grown up sitting under the protection of overhanging hemlock boughs watching rainstorms sweeping across our lake in cottage country, and spending time as an adult canoeing in Algonquin Park, camping in old-growth hemlock forests. I’m not the only one who will miss it – brook trout often depend on cool, shady hemlock forest to moderate stream temperature; deer profit from the thin snow under the sheltering hemlock boughs in winter, which creates deer yards. Hemlock are considered a foundation species, because they exert such a strong influence on their environment that they create a unique habitat. When hemlocks are gone, no other tree species is a surrogate. Many people will miss it too, maybe more than they realize. I know someone who can’t identify a hemlock tree, but grew up in an area where hemlock is a dominant tree – when it’s gone, she will miss it. But are we doomed to lose them?

Probably a lot of our hemlocks will die. When it gets into New York’s Adirondack Mountains, where hemlock is one of the most abundant trees, and when it really gets into Ontario, and Quebec, people will feel it. When it swept through the Smoky Mountains a decade ago, wiping out nearly all the old-growth hemlock stands there, a lot of people felt heart-broken. As it keeps moving north early detection will be important, because the predators take so long to establish and grow a population. Likely the only way to keep hemlock on the landscape will be limited use of insecticides to protect some of our best examples of hemlock forests (which can act as genetic reservoirs), combined with an aggressive biocontrol program. “That’s what we’re doing now in many of the state parks around here,” says Mark Whitmore, “we’re treating the most valuable aesthetic and genetic resources with insecticides, and then leaving other trees untreated and releasing predators. The predator population can build on the untreated trees and we still have the treated trees for the future.” Long-term prospects depend on improved biocontrol, but spotting an HWA infestation early may mean the difference between losing most of the hemlocks in an area, and saving many of the them.

Julie Holmes describes how to spot a HWA infestation
Julie Holmes describes how to spot a HWA infestation

On a warm spring day in April 2015, in a forest just north of Toronto, Julie Holmes of the Canadian Food Inspection Agency (CFIA) had us hunting for imitation adelgids – bits of Q-tip cotton glued to the underside of hemlock branches. This turns out to be the closest approximation to Hemlock Woolly Adelgid for developing a “search image.” CFIA uses a modified version of the sampling methodology developed in the US that is relatively simple to use, and surveys low branches of hemlocks, as well as watching for twigs blown off in windstorms (which can give an indication of what’s happening in the canopy). Sampling takes place mostly from March-May. By the summer months the adelgid are dormant, and in a life-stage that is very hard to spot.

Since the 2012 find in Etobicoke, CFIA has increased survey efforts for HWA. The agency conducts intensive annual surveys at the two sites in Ontario where HWA was detected, and also continues to do annual detection surveys
in Ontario and Eastern Canada. Holmes led two early detection workshops geared to forest stakeholders this spring, because she says “the more people who are out there looking for this, the better.” Also she says it would be helpful to know where people are doing surveys and not finding adelgids.

HWA at Willow point 4 small
HWA-infested hemlock in upper NY State. Photo: Mark Whitmore

Given the role that chance plays, it’s hard not to agree with Holmes about public involvement. The first detection in Ontario was reported by the property owner. Taylor Scarr relates that “an arborist was brought in to do some tree pruning, saw the woolly stage on the underside of the branches and said I think I’ve heard of this thing.” The second find, in the Niagara gorge, was the result of the CFIA’s surveys; but was it luck that they found the only infestation in the area, or might there be others? The agency surveys new areas every year, according to Holmes, but the potential for amateur naturalists and forest managers to contribute to the surveys is great.

The surveys are the first step in establishing chemical or biological control. “There’s no point doing much in terms of biocontrol until you have a population to grow your predators in,” says Taylor Scarr. “Biocontrol only works when you have a viable Hemlock Woolly Adelgid population.” The predators are released into field insectaries, where they multiply and can be collected and redistributed. According to Mark Whitmore, the best way to do this is probably to “find hemlock hedges that are infested with adelgid and release the predators on those hedges. Which makes them easy to collect when we go back.” Whitmore has found that hemlock hedges, usually found in peoples’ yards, work much better than individual small trees for raising and collecting the predators. “We should be planting more hemlock hedges,” I half-jokingly say. “I would recommend it, before you get the adelgid.” answers Whitmore.

At a regional scale, HWA infestations are often exacerbated by the tendency of landowners to want to salvage all the hemlock once the pest is detected in or near their forest. “We’ve lost more hemlock to salvage logging than we have to the adelgid, and it was the same with chestnut” says Aaron Ellison, a researcher at Harvard Forest. Cutting the trees preemptively may preclude finding any resistant individuals in the population (it is unclear if eastern hemlocks have any resistance), and if they are near a predator release site it may also reduce populations of HWA where predators can grow. Most forest management practices are not effective at controlling adelgid. Fertilizing trees makes infestations worse, and cutting them (other than the first few trees in an area) has little effect.

If you think you’ve found Hemlock Woolly Adelgid

If you’ve found something that looks like HWA, you can compare it to its look-alikes. Then if you think you’ve found Hemlock Woolly Adelgid, in Canada contact the CFIA Plant Health Surveillance Unit, call 1-800-442-2342, or contact your regional CFIA office. In NY State, you may contact your local Cornell Cooperative Extension office or the New York State Department of Environmental Conservation at: 1-866-640-0652, e-mail In Vermont call 802-879-5687, in New Hampshire 603-464-3016, and in Maine 207-287-2431.

Take photos of suspected infestations and detailed notes, including GPS coordinates, and the date. Don’t go from a suspected HWA-infected site to any other hemlock forest without first showering and washing your clothes.


Hemlocks killed by HWA

HWA in Ontario webinar

CFIA fact sheet

US Forest Service HWA Resources

HWA sampling methodology

NYISRI fact sheet

A death in the forest (New Yorker)


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2 Responses to Hemlock Woolly Adelgid moves north

  1. It’s so distressing seeing all the beautiful old ash trees around Toronto being cut down. Can’t imagine losing the hemlock (and apparently beech has its own invasive that’s arrived in Ontario) as well. We’ve truly made a mess of things.

  2. Great writeup Michael. Will Blozan showed me the devastation in spring of 2013 when I too had to wade that frigid stream to get into the Cataloochee valley! I went to see the giant tulip trees but got a lot more than I came for with the HWA/ancient Hemlock-loss story. I was comforted at his suggestion that our cold winters should keep the HWA at bay, but I guess that just isn’t the case. I’ll keep my eyes peeled for the nasty critters. I don’t know if I can take losing another tree species in Ontario.
    Keep up the good work!

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