Destructive zigzag sawfly found in St. Lawrence Co.
By Mike Lynch
An insect that leaves a distinct mark on elm leaves is one of the newest terrestrial invasive species to be discovered in New York, but scientists are still determining what its impact will be on the trees.
The state Department of Environmental Conservation spotted the elm zigzag sawfly this summer while doing surveys in St. Lawrence County, just outside of the Adirondack Park.
The forest pest can cause defoliation of trees, making them vulnerable to disease and other pests, or it could outright kill them, if the sawflies remain for several years. Defoliated trees can often recover if the outbreak isn’t sustained.
DEC found the bug in Wilson Hill Wildlife Management Area, Lost Nation State Forest and Brasher Falls State Forest.
What concerns invasive species experts is the fly could potentially spread quickly and widely, attacking a tree already decimated by Dutch elm disease.
Caused by a fungus that is transported by elm bark beetles, Dutch elm disease was discovered in the U.S. in the 1930s. Since then, it has killed more than 40 million American elm trees.
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The DEC doesn’t have an inventory of trees in the Adirondack Park, but says the region contains “very few elm” compared to other forest species. However, the highest concentration is found in the Cranberry Lake area, in southeastern St. Lawrence County.
Elm are scattered around New York, but the zigzag sawfly seems to be focused in rare areas where the trees are surviving.
“Most of our elm is along the St. Lawrence Seaway along that border,” said DEC research scientist Jessica Cancelliere. “Unfortunately, the two are related in terms of the presence of elm zigzag and the proximity of the most abundant and healthiest elm that we have in New York State.”
Elm is a hardwood that can be used for firewood or lumber, but the trees are rarely used that way anymore.
Elms are raised and sold by nurseries, particularly for street plantings, said Thom Allgaier, invasive species coordinator for the plants division of the state Department of Agriculture and Markets.
Forests that now have elm stands could suffer from the loss of diversity and tree canopy, he said.
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The elm zigzag sawfly, native to China and Japan, is fairly new to North America, being first discovered in Quebec in 2020, Virginia in 2021 and North Carolina in 2022. Europe has had it since 2003.
How the bug traveled to North America is unknown.
But DEC warned the sawfly is capable of flying up to 56 miles in a year — further when assisted by wind currents. They can also be transported accidentally on infested nursery stock.
The extent of the damage these pests will do in northern New York is unknown now because they are new to the region. Their impacts have varied in different places. So far, the damage observed to the St. Lawrence County trees has been minimal.
The invasive species does its damage in the larval state when it feeds on leaves, leaving a zigzag pattern as it moves along. Older larvae can consume the whole leaf.
“The feeding pattern is pretty unique and can easily be spotted solely in the lower parts of the canopy or in dropped leaves.” Allgaier said.
Larvae live for 15 to 18 days, then spin cocoons. Adult flies emerge days later and lay eggs, according to the Canadian Department of Natural Resources (DNR). Adults are small, black flies.
The bug is asexual, so only females are needed to reproduce. They lay about 25 eggs on average, Allgaier said. Researchers have found they can lay up to 60 eggs at a time.
“The populations can explode quite rapidly,” he said. “These numbers can build very,very quickly, which is common for most invasive species.”
But reproduction differs in each area. In Virginia, researchers observed two generations in 2021 and one the following year, according to North Carolina Cooperative Extension.
In Europe, four to six generations per year have been witnessed, said Canada’s DNR. In Canada, the sawfly has been found in southern Quebec, including Montreal.
Canadian research scientist Veronique Martel said scientists are exploring whether there have been multiple introductions of the zigzag sawfly in North American or whether its spread from Quebec to the southern U.S. unnoticed.
There hasn’t been a high level of defoliation in Canada, but North Carolina has seen a lot of damage, said Martel, who works with the Canadian DNR.
She said there may be more impact on elms in southern climates that are warmer because the reproduction season is longer.
Canada researchers have also noticed birds and other insects feeding on the zigzag sawfly.
“There are natural enemies, and we’re ( thinking) that might be one of the reasons we don’t see high levels of defoliation,” Martel said.
The Saint Lawrence and Eastern Lake Ontario Partnership in Regional Invasive Species Management (SLELO PRISM) also has opportunities for the public to get involved in their volunteer surveillance network.
For more information, visit the Invasive Species Centre website or SLELO’s website.
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