If you see an elm tree leaf with an odd, zigzag-shaped missing section, Maine forestry officials want to hear about it. It could be evidence of a new and potentially destructive insect pest in the state called the elm zigzag sawfly.
The elm zigzag sawfly was observed in Quebec a year ago and officials there this summer reached out to their counterparts in adjacent states in the United States and Canadian provinces asking for help determining if the insect is spreading — and, if so, how far.
Native to Asia, the elm zigzag sawfly was observed in Eastern Europe in 2003. Since then it has spread to more than 15 other European countries where it is causing significant damage to elm trees.
So far, it has not been seen outside of Quebec in North America.
“This is a new and unfolding story in North America,” said Michael Parisio, forest entomologist with the Maine Forest Service. “It’s difficult to say at this point how bad of a problem it could be here.”
Two weeks ago, at the request of his counterpart in Quebec, Parisio set out traps — squares of yellow plastic coated with a sticky material — on elm trees in Augusta and Hermon. Because the traps have only been out for a couple of weeks, Parisio said it’s too soon to give any updated information on whether the insect has made it to Maine.
“Our first step is to determine if it’s even a problem here,” Parisio said. “Then figure out the extent of the problem and what to do about it.”
The larval stage of the insect feeds exclusively on elm tree leaves. As the name implies, the elm zigzag sawfly creates a distinct zigzag pattern as it nibbles its way through the leaves. According to Vernonique Martel of the Canadian Forest Service at the Laurentian Forestry Center in Quebec, while enough of the sawflies feeding on elms can defoliate a tree, the real threat to the trees is from a combination of the defoliation and disease.
“We have been looking at Europe and what we have seen is that the insect does not seem to be killing the trees,” Martel said. “But the defoliation is enough of a stress for the trees that you have an increased risk from dutch elm disease.”
Dutch elm disease is caused by a fungal pathogen spread by the elm bark beetle. It has decimated elm populations around the United States, including stands in Maine. An otherwise healthy elm tree weakened by defoliation is susceptible to the fungus.
“We are finding [the elm zigzag sawfly] more and more in Quebec,” Martel said. “We thought it would be a good idea to look at all of our neighbors in the United States and in all of the Canadian provinces.”
Parisio said his agency was more than happy to cooperate with the Quebec officials.
“At this point the insect is considered to be an exotic because it has not reached the threshold for economic damage to be considered invasive,” Parisio said. “That’s why it is so important to monitor for them.”
No one can say how the elm zigzag sawfly got to Quebec from Europe, but the best guess, according to Parisio, is that eggs of the sawfly hitched a ride in soil around plants being imported to the province from Europe.
Once eggs hatch, Martel said the insect can spread over a great distance on its own, traveling up to 50 miles from where it hatched.
If any of the insects are found in Maine, Parisio said monitoring efforts would ramp up to determine the extent of the population. Any information on where and how many are found will be shared with Martel.
In Canada, Martel said her agency will continue tracking the insect and try to determine which species of elm are under attack. They will also study how fast the sawfly can reproduce in a northern environment and whether or not it has any natural predators in North America.
Both foresters encourage anyone who has observed the telltale zigzag pattern in an elm tree leaf to contact their municipal, state or provincial forestry agency to report it.
Best case, according to Parisio, is the sawtooth is just another nuisance bug that does not create the kind of damage here it does in Europe.
“There are a lot of exotics here in North America that simply exist in the background without being very noticeable at all and without becoming invasive,” Parisio said. “Which category this particular insect will fall into remains to be seen.”