Invasive insect threatens carbon storage in Great Lakes forests



Hemlock Woolly Adelgid (Adelges tsugae) on Tsuga canadensis. Image: E.P. Mallory

By Kari Eickholdt

An invasive insect increasingly threatens one of the Great Lakes region’s most important trees for storing the carbon that causes global warming.

Researchers have found that eastern hemlock trees felled by the invasive woolly adelgid could emit 4.5 tons of carbon across almost two and a half football fields.

Eastern hemlock trees take their sweet time to grow – between 250 and 300 years to reach maturity, according to the Natural Resources Conservation Services. They thrive in the shade and live for 800 years or more, which is why the hemlock woolly adelgid insect infestation is worrisome.

Once an eastern hemlock tree is killed by the hemlock woolly adelgid, it stops removing carbon dioxide from the atmosphere, said Peter Quinby, the founding executive director of Ancient Forest Exploration and Research, a group that brings these forests back to their original character. As the tree decomposes, the dead wood releases that carbon back into the atmosphere.

“Not only are these big trees no longer absorbing the gas, but also they are breaking down partly into (carbon dioxide,) which adds (carbon dioxide) to the atmosphere,” he said. “So, it’s sort of a double whammy in that sense.”

The hemlock woolly adelgid (Adelges tsugae), is a small, cotton-like insect from Asia that attacks the eastern hemlock trees by feeding on the nutrients and sap from the base of the needles on the underside of branches.

Hemlock Woolly Adelgid. Image: Shimat Joseph, University of Georgia

The insect was first reported in Richmond, Virginia, in the 1950s. It has since migrated from Georgia up the East Coast into multiple Canadian provinces, and as far westward as Michigan, aided by climate warming, wildlife migration, logging and by people who move firewood.

“The egg sacks of the hemlock woolly adelgid are very, very tiny,” said Nicole Mielewczyk, biologist for the Canadian Food Inspection Agency. “But they’re very fluffy and really easily adhere to bird feathers. So, as birds are coming up from the South to migrate back into parts of Ontario or parts of Michigan or parts of New York, they are carrying hemlock woolly adelgid and then dropping them off as they come into new hemlock trees.”

A study published in August in The Forestry Chronicle found that hemlock has very little natural resistance to the invasive bug. Across eastern North America, 90% of trees can die within four to 15 years of infestation, regardless of their age and size, the study said.

The Canadian Food Inspection Agency tries to prevent their spread by surveying them and encouraging the public to report potential infestations.

Canadian residents can contact their local Canadian Food Inspection Agency office to request written authorization to remove logs, branches, or firewood from a regulated area. Authorities also encourage people to report sightings of the insect at the inspection agency.

U.S. residents can report any findings of the insect at the Early Detection Distribution Map System website.

Forests at risk

To understand the long-term effects of the loss of eastern hemlock trees, Danielle Ignace and Jesse Bellemare, professors of plant science at Smith College in Massachusetts, studied birch and hemlock-dominated plots at the college’s Ada & Archibald MacLeish Field Station.

A study published in the 2018 journal Ecosphere showed that the loss of eastern hemlock trees is expected to have long-lasting effects on ecological processes like decomposition, carbon storage, and nitrogen cycling in Eastern U.S. forests.

This impact is evident from the decline in soil carbon pools and the increase in soil respiration, the study said. Although it may take decades, the substantial loss of hemlocks has long-term consequential impacts on forest structure, function and carbon pools.

Their findings suggest that the decomposition of the fallen hemlock trees could deliver 4.5 tons of carbon per hectare into the atmosphere. One hectare is 2.47 acres of land.

Eastern hemlocks are also important to many other ecosystem services such as nitrogen cycling, an essential process that keeps plant growth stable, Mielewczyk said.

“They have different microclimate environments, and physical environments that can impact the forest ecosystem,” Mielewczyk said. “The death of hemlock trees would also make forests vulnerable to other invasive species that thrive in disturbed areas and therefore freely change the landscape of many forests within southern Ontario.”

Trapping the bug

The Canadian Food Inspection Agency and the Canadian Forest Service work with their U.S. counterparts to control the spread of the invasive insect.

“There are a few different insecticides that are effective at knocking down major populations of hemlock woolly adelgid, controlling and keeping that low level,” Mielewczyk said. “In Canada, we don’t have many of those approved yet, because there are different regulations for insecticides between Canada and the United States. We are working actively on more insecticide regulation, but it’s a really slow process, unfortunately.”

Other insects from Asia, such as the silver fly and the laricobius beetle feed on the hemlock woolly adelgid and can serve as biological control.  And Canadian officials recently started the Hemlock Woolly Adelgid Monitoring Network.

Grand Valley State University in Michigan developed a trap to monitor the insect. It is the size of a dinner plate and holds microscope slides. Vaseline is rubbed on the trap and placed under hemlock trees to catch insects that drop from them.

If there are signs of the insect, the Canadian Food Inspection Agency analyzes its environmental DNA. Once the insect is detected, the location is sent to inspection agents to conduct a visual survey.

“What other members of the public can do is just keep an eye on the trees in their area,” Mielewczyk said. “They can take a look to see if they begin exhibiting any of the signs of the hemlock woolly adelgid infestation, which can include grayish green needles or defoliation.”

Other signs include discoloration of foliage, premature needle loss and premature bud and shoot dieback, which is the death from the tip inward.

In similar efforts, Quinby’s organization created a committee called the Catchacoma Forest Stewardship Committee with the help of the advocacy group, Wilderness Committee Ontario.

Local naturalists, ecologists and citizens who care about the forest help the committee and offer educational opportunities.

“The committee works together to try to pursue permanent protection. We also think, develop, and run educational and recreational activities to raise awareness about the value and importance of the forest,” Quinby said.

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