DELAWARE, Ohio (WCMH) – An invasive insect known for leaving unique patterns on the leaves of elm trees has been spreading this summer.

In July, Ohio became the latest state to confirm a population of elm zigzag sawflies had been found. A researcher at the U.S. Department of Agriculture’s Forest Service Northern Research Station lab in Delaware, Ohio was the first to make the discovery. The research station has been growing elm trees for decades, which are the pest’s host of choice.

The insect was also confirmed in northern Franklin County in August. 

Forest Health Program Manager Tom Macy said the invasive bug, native to Asia, has been on the Ohio Division of Natural Resources radar since it was first detected in North America — in Quebec, Canada in 2020, then the U.S. in 2021 and 2022.

“We may find that this insect is more widespread than we realize, and now that people are kind of on the lookout for it, we might find it in more places,” Macy said. 

Massachusetts and Vermont confirmed their first populations of the elm zigzag sawfly this year as well. Other affected states include North Carolina, Virginia, Maryland, Pennsylvania, and New York.

The insect larva is up to half an inch long, caterpillar-like, and light green in color. In the fall, they form cocoons on the ground in leaf litter or soil. Adults are about a quarter of an inch long, shiny, black, and winged.

Its telltale trait, though, is its feeding pattern. As seen in the image above, the elm zigzag sawfly is known to eat the leaves of an elm tree in a zigzag path.

“I think they’re going to be the only insect that causes this interesting sort of zigzag pattern that it chews through the leaves,” Macy said. “When the larvae hatches out of the egg they start to feed on the leaves and create that pattern.” 

The bug does not sting and is harmless to people and animals, according to Brian Heath, a North Carolina Forest Service forest health specialist. It is currently considered a quarantine pest in the U.S., meaning the USDA has to confirm each new reported detection in individual counties, Dr. Kelly Oten, an assistant professor and extension specialist with North Carolina State University’s forest health resource, told Nexstar’s WGHP.

Besides the obvious impact to the leaves it eats, the extent of the damage these insects cause or how they may affect the U.S. is still an active area of research.

“We’re trying to determine just how much damage they might do,” Macy said. “I know last year [in North Carolina], they had almost totally defoliated a large elm tree in someone’s yard. So, they are capable of doing pretty heavy defoliation but they don’t do that everywhere, because there are places in Canada and Europe where they’re known to occur that they don’t seem to cause much of a problem.” 

Macy said it is likely the elm zigzag sawflies will spread quickly, due to their ability to rapidly reproduce. The bugs are all female, so they reproduce asexually, only needing a single bug to start a new population.

“There’s research evidence to show that they can have multiple generations per year,” Macy said. “They can go from egg to adult and then lay eggs again within like three weeks.”

How the insects got to the U.S. is a mystery. Macy said he can only speculate, but it’s possible that they came to the U.S. from the shipment of goods. 

“Once it was introduced to North America we don’t know either if it’s being moved around by people accidentally or … the adults are just flying on their own,” Macy said. “There is some research that shows the adult sawflies are pretty strong flyers. They can move potentially 28 to 66 miles in a year. So they are capable of dispersing pretty well on their own.” 

If you believe you’ve found an elm zigzag sawfly, Macy recommends taking a clear photo of it and sharing it with your local wildlife officials. If possible, you can try to collect the bug or even the leaf it is on into a bag or jar. This can help officials identify the insect.