Scientists Take the Long View on Saving Eastern Hemlock Stands from Woolly Adelgid

Courtesy of Steven Katovich,


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The hemlock woolly adelgid, an insect that has decimated the eastern hemlock population over the last 40 years, is not a parasite — it’s an herbivore. 

“Herbivores eat plants,” said Dr. Evan Pressier, an associate professor of biological sciences at University of Rhode Island. “It can be a bit of a dicey distinction, but most people would talk of the hemlock woolly adelgid as an herbivore because it does eat plant tissue. It’s not a parasite in a traditional sense.” 

Pressier has spent 15 years researching the hemlock woolly adelgid, known as HWA, and experimented with genetically-resistant hemlocks as well as bio control, which involves introducing the natural predators of the insect. 

HWA, or Adelges tsugae, is a sap-sucking insect, similar in appearance to tiny cotton balls, that came to the U.S. from Japan where it has natural predators that keep its populations manageable. In North America, it has no natural enemies, said Preisser. 

“No one knows for sure how it got here,” he said. “It is likely that it came on ornamental hemlocks from Asia brought into U.S. gardens.”

Courtesy of  Carole Cheah at CAES

As with many invasive species, there was a lag from when the HWA arrived to when it became a real pest to the eastern hemlock, a shade-tolerant conifer often used in landscaping. 

“The adelgid seemed to have arrived in the early 20th century, around 1910. It did not seem to spread until the 1970s,” said Preisser. “Once it started, it came up north first and then went south.” 

The bug “currently infests nearly one-half of the native range of hemlock in the East,” according to the United States Department of Agriculture Forestry Service. The HWA population is entirely female and breeds twice a year. Each insect lays about 100 eggs in the winter and about 25 eggs in the summer. 

The affected trees are important because they create a specific type of habitat where other species can live, said Preisser. 

“Hemlocks shade streams and create cool, shaded pools, which is good trout habitat during hot weather. The trees create a cool moist microclimate on steep banks that allows species that aren’t tolerant of heat to live in those environments. ” Preisser said. “If you lose the  hemlock you don’t get anything else that comes in and does the same thing. There’s no ecological equivalent.” 

Bulletproof trees

There are 13 species of hemlocks and 11 are resistant or tolerant to adelgid, explained Preisser, who studied a “bulletproof” stand of hemlocks in New Jersey near the Delaware Water Gap. These trees were surrounded by hemlocks that had died from adelgid. The surviving trees were free of adelgid as well as elongated hemlock scale, another pest that infests hemlocks. 

“We took cuttings from those trees and we propagated them, and when they got big enough we planted those plants alongside other hemlocks of similar age and size that we knew were vulnerable to adelgid in eight different forests from Virginia through Massachusetts and we left them for 4 years so they could be attacked by adelgids,” he said. “What we found was that those trees that were cuttings from the resistant trees survived at a very high rate and about half of the susceptible trees died.” 

Preisser said the experiment suggested there is a potential for rare individual trees that are resistant to adelgid to survive, reproduce and create offspring that will eventually repopulate the landscape, even without bio control. But that the process would be exceedingly slow. 

“The resistant tree work was essentially trying to speed that process by several thousand years because if we can propagate individuals that are resistant to adelgid and out-plant them then we can recover hemlocks much more quickly,” he said. “What we’re talking about is accelerating the pace of evolution.”

Bio control

Preisser became interested in the adelgid when he moved to east from California and started his postdoctoral research with Dr. Joseph Elkinton of University of Massachusetts Amherst, whose focus is on bio control of the adelgid. 

“Biological control is really the only option we have for solving this problem,” said Elkinton. “It’s an old idea and has been applied against any number of insects around the world. Sometimes with spectacular success and sometimes with less success — it depends, each system is unique.”

Elkinton said the focus of his work has been on the population ecology of the adelgid, or  what causes the “boom and bust in populations” and on biological control of those populations. 

“All of these natural systems have a suite of natural enemies. Insect predators and parasitoids — insects that lay their own egg inside the insect host — those are widely used agents of biological control,” he said. “We have thousands of herbivorous insects of all kinds. The vast majority of are at low density don’t have significant impact and don’t have any significant impact on their host plants and the reason is they are well controlled by a suite of natural enemies that evolve with them.” 

The practice of biological control involves going to the country of origin, finding natural enemies of the pest and studying them carefully to make sure they are specialists, meaning they only attack the target insect and not other related insects.

Two specific predators of the adelgid are a beetle called Laricobius nigrinus from British Columbia, which have been shown to damage about one third of the winter-laid eggs of the adelgid, and two species of silver flies that are predators of western hemlock adelgids. 

Elkinton said that his research has focused on why the adelgid has not caused significant damage to the western hemlocks in the Pacific northwest. His experiment at the Washington Arboretum in Seattle includes both western and eastern hemlocks growing side by side. 

“We have natural enemies controlling the adelgid in the Pacific northwest,” he said. “So it makes sense to try to introduce those natural enemies to the east coast. If we can get them established in the east we can solve the problem.”

Elkinton said that the as-yet unpublished work of his graduate student, Ryan Crandall, has shown that while Laricobius is causing significant mortality to the winter generation of adelgid eggs, the beetle is not keeping up with fecundity of the adelgid. 

“For the population to remain stable one of those eggs needs to survive to maturity. If two of the 100 survive, it means the population will double in size,” he said. “It just takes a little bit of survival and the population comes rushing back. The Laricobius consumes a lot of eggs, but it has essentially zero impact on the adelgid population.”

In other words, Laricobius is important but it can’t do the job by itself. That’s why Elkinton has turned his attention to two species of silver flies that prey on the summer generation of adelgid.

“The hope is if we can get the silver flies established, then maybe it will solve the problem — maybe it will, maybe it won’t,” he laughed. “We’ll find out. It’s a multi-decade effort.” 

This kind of work is about the long view, he said.

“We’re confident that bio control does work in Pacific Northwest. We’re not sure we understand all the agents involved — there may be a whole suite of agents with the silver flies and Laricobius. It takes a lot of research to figure this out and it takes decades to actually do it,” he said.

Elkinton said he’s also working on another idea — as yet unpublished — the natural mortality of the adelgid. 

“In summer of 2018 had we had a very rainy August and the adelgid populations collapsed due to a fungal disease. We’re working on this now to document this. We’re trying to identify this fungus species. It gave the trees a big respite,” he said. 

Two years later, the adelgid is recovering and in many places it cannot be found, but it is coming back, Elkinton said. 

“The adelgid is subject to various naturally occurring mortalities that explain why we still have hemlocks,” he said. “The other thing is every now and then we get a polar vortex in the winter and temperatures will drop to minus 20 fahrenheit here and that will kill the adelgid too. You get 99 percent of the mortality of the adelgid so the trees get a respite.”

Ironically, killing the adelgid also kills the Laricobius larvae, he said. 

“If you blow away the adelgid you blow away the food supply of the Laricobius larvae and they have nothing to feed on,” he said. 

Laricobius don’t survive well in New England winters but are abundant in warmer research sites in New Jersey and Virginia, Elkinton said. 

“Work like this takes decades to pull off. I’ve been working on this for 20 years and we’re not even halfway there in terms of solving the problem. We may solve it eventually, but it’s never quick,” he said. “It will take another decade to solve the problem, if we do — there’s no guarantee.” 

“We’ll know we’ve solved the problem when you find adelgid densities very low like they are right now in many places and they stay that way over a long period of time,” he said. 

Connecticut-based research

Dr. Carole Cheah, a researcher at the Connecticut Agricultural Experiment Station, has been researching the use of an Asian lady beetle, Sasajiscymnus tsugae, for the biological control of adelgid since 1994. The beetle, tiny and ladybug-like, devours the adelgid in all stages of its life cycle, making it a superior predator of the adelgid, said Cheah. 

Sasajiscymnus tsugae, Courtesy of Dr. Carole Cheah

The beetle was discovered by the now-retired Mark McClure, who was “a giant in HWA studies,” said Cheah. When adelgid was first reported to CAES in 1985, McClure traveled to Japan in search of natural predators and found a promising mite that led him to Sasajiscymnus tsugae. 

Cheah started working with McClure in 1994 and has focused exclusively on adelgid populations in Connecticut ever since. 

“I was brought on to study the beetle’s characteristics and to see if it had potential for biological control,” she said. 

Between 1995 and 2001, Cheah released about 175,000 beetles at 16 sites around the state. Today, many of those trees are still alive, she said. 

One problem for her research is documenting the survival of the beetles because they’re tiny — about 2 millimeters in length — and completely black. 

“They are cryptic, they cling and hide in the foliage. They’re not easily dislodged and they fly upwards so it’s hard to recover them after a release,” Cheah said. “The method that is used to recover depends on you tapping tree branches into sheets and looking for the predator, but hemlock forests are 50 to 80 feet tall.” 

At one site, after sampling lower branches unsuccessfully, Cheah went up in a bucket truck and found beetles 50 to 60 feet up in the canopy. But that kind of access isn’t possible in the inner forest sites where she does research, so she has changed some of her criteria for evaluating the trees. 

“You can not use a bucket truck in 99 percent of sites — it’s a real challenge,” she said. “What I have done is to look at the health of the hemlock tree. It doesn’t matter if I can find a beetle or not. The ultimate goal in my case is to save the hemlock trees. If I don’t see improvement in the hemlock trees then I tease out other factors like precipitation levels and drought.” 

She said there is a close correlation between the foliage of a hemlock and the amount of HWA on the tree. If the tree has significant adelgid infestation then it would not be able to put out new growth. 

Cheah said she has studied the Laricobius nigrinus and various Chinese species of beetles that were brought in to be evaluated. She has not studied silver flies, but she remains convinced that the lady beetle is the superior predator of the adelgid. 

“Frankly Sasajiscymnus tsugae holds my interest because I’ve seen its efficacy and its ability to be reared in a lab situation,” she said. “It’s phenomenal what they eat and they are so adapted to HWA. All stages of the lady beetle — larvae and adults — will attack all stages of HWA.” 

Most significantly, the beetle attacks the dormant stage of adelgid during the summer in its second generation of eggs, which no other predator does, said Cheah. 

“The adelgid remain dormant until mid-October and then they start feeding again, start spinning wool and that’s the generation that you see going through the winter, culminating in these large woolly masses with eggs,” she said. “Imagine the impact of significant predation by beetles during the summer when the adelgid is not doing any damage. If you could release sufficient lady beetles to predate on this you could have a remarkable decrease of adelgid before they start breaking dormancy and affecting your trees through the winter.” 

She has submitted a new research proposal in conjunction with the Connecticut Department of Forestry to quantify the impact of the beetle on the adelgid during the dormant phase while factoring in environmental and climate conditions as a means of “augmentative biological control.” 

She has faced skepticism about her work with Sasajiscymnus tsugae, but she said the survival of hemlocks in Connecticut proves her method has merit. 

“I can’t see the hemlocks die, not after all this work, that’s what’s driving me,” she said. “It’s a strategy that’s available to us and clearly it wasn’t a complete failure or we’d have no hemlocks to look at now.” 

Too early to tell

Anthony Irving, a board member of the Lyme Land Trust, observed the advent of adelgid in southeastern Connecticut about 15 years years ago. 

“It had a very strong impact on the existing hemlock stands. We probably lost 30 to 50 percent of our hemlocks outright,” he said. “The adelgid itself attacks the bottom of the tree by sucking on the needles and doesn’t really make it to the top of the tree so a lot of those hemlocks that you see today have these lollipop tops on them. The question is whether there is enough foliage remaining that those trees can continue to survive or have they been compromised to such a degree that eventually they will weaken and also die.”

The mortality of a portion of the hemlocks opened up space in the overstory for other trees to grow — perhaps a positive in the overall health of the forest, said Irving, who is a graduate of the Yale School of Forestry and Environmental Management and co-founder of Ecological and Environmental Consulting Services, which specializes in forest ecology and land use management. 

“The understory hemlock trees seem to be in relatively good shape which is obviously good for the future if it indeed remains that way in that status,” he said. “The double whammy is that we have a number of other tree pathogens or pests that are attacking the trees.”

The Emerald Ash Borer are decimating ash trees across the region. American beech trees are dying from Beech Bark Disease that results from an interaction between the tree, a non-native insect and three fungal species. Gypsy moths killed 30 percent of the region’s white oak trees.

Irving said there is no overall prescription to get rid of tree pests. Bio control is one piece but other weather factors, including drought, rain and polar vortexes, play a significant role. 

HWA was knocked back by polar vortexes and the severe winters of 2013 and 2014 because it is very sensitive to cold, but it’s too soon to make predictions about future adelgid infestations, Irving said. 

“Is it going to be adaptive over time as it continues to spread northward? I think the jury is simply out on that,” he said. “Have the adelgids run their tidal wave, allowing younger understory hemlocks to be able to subsist and become the next generation of hemlock? It’s too early to tell but they’re out there.”