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Tracking the zigzags: Sawfly spreading in north country could pose threat to elm trees

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A small, winged insect with a shiny black body and appetite for elm is now making its way across New York, after being detected in St. Lawrence County last year.

The invasive species, called the elm zigzag sawfly (Aproceros leucopoda), was found in southern Quebec in 2020, in Virginia in 2021, and then in North Carolina, Maryland, Pennsylvania and New York last year.

The state Department of Environmental Conservation confirmed last September that the elm zigzag sawfly was found for the first time in New York at three locations in St. Lawrence County — Wilson Hill Wildlife Management Area in the town of Louisville, Brasher State Forest and Lost Nation State Forest near Stockholm.

The insect poses a serious threat to already diminishing populations of elm trees in the United States.

When young elm zigzag sawflies feed on elm leaves, they cut out a distinguishable snake-like pattern, hence the zigzag in their name. After they go through their larval and pupal stages, they reach their adult form, which is when they can do the most damage — sometimes completely defoliating trees.

Elm zigzag sawflies are not actually flies, even though their common name might suggest they are. They are wasps, belonging to the order Hymenoptera, and family Argidae.

Melissa K. Fierke, a forest entomology professor at SUNY College of Environmental Science and Forestry in Syracuse, is working with the DEC to conduct research on the species.

Fierke frequently makes a six-hour round trip from Syracuse to check her insect traps at several observation sites in St. Lawrence County. Last Thursday was her most recent trip, which included stops at Brasher State Forest and Wilson Hill Wildlife Management Area. As usual, many of the traps — sticky yellow pads twist-tied to elm trees — were speckled black with sawflies.

The goal of Fierke’s research is to determine how many generations the elm zigzag sawflies are producing per year.

“With insects, you can have multiple generations per year, and we don’t know how many they have. So that’s kind of the larger overarching goal of the folks from different states that are studying it — to try to see how many different generations it has in the different states,” Fierke said.

Knowing the number of generations will determine how rapidly the species can multiply, which will help to determine the amount of long-term harm it is capable of.

“What we are seeing now is that there are some places with pretty heavy defoliation, and there’s other places where there’s just a little bit, and we don’t know how long the insect has been there, and if the populations are going to build,” Fierke said.

If populations build, elm trees will suffer, because they have already been made vulnerable by disease.

“This is in conjunction with Dutch elm disease, that has pretty much decimated elms across North America. To have one more thing that’s introduced and is going to give them problems, is what we are worried about,” Fierke said.

Dutch elm disease is caused by invasive fungal pathogens — most commonly by the species Ophiostoma novo-ulmi — and is spread by native elm bark beetles when they feed, or through the root systems of adjacent trees.

When populations of a tree species suffer, a larger group of organisms that rely on that tree, also suffer.

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