OHIO—A sharp eye at a Forest Service Northern Research Station lab in Delaware, Ohio, made a surprising discovery in July 2023. On an American elm leaf, biological technician Josh Wigal noticed a strange zigzag pattern. Luckily, this individual recognized it from a forest health training and immediately suspected the culprit—the elm zigzag sawfly.

Native to Asia, this invasive insect was first discovered in North America in 2020, with initial discoveries in Canada. In subsequent years, this sawfly was detected in states across the Midwest and eastern United States. However, Ohio had yet to experience the pest. Within days after the initial detection, hundreds more elm leaves at the nursery displayed the telltale zigzag patterns.

At its Delaware lab, the Forest Service has been collaborating with state and private partners to counter Dutch elm disease, a fungal infection first discovered in the United States in the 1930s, which has devastated elm populations. Forest Service scientists have been breeding and testing American elm with resistance to Dutch elm disease. Needless to say, the appearance of a new elm threat at this site came as unwelcome news.

“The discovery of elm zigzag sawfly at our lab initially left us uncertain about what to expect next,” recalls Kathleen Knight, a research ecologist with the Forest Service Northern Research Station. “With thousands of well-cared-for elms serving as a potential sawfly buffet, my mind immediately went to the worst-case scenario of a rapid population explosion of this new pest.”

Fortunately, the first summer season witnessed minimal destruction from the sawfly. However, since this sawfly can lay 50 eggs with the potential of up to six generations per year, Knight and her colleagues set out to address the unknowns of the insect through data collection efforts, close monitoring of elm zigzag sawflies in the experimental plantings and nursery, and consultation of scientific literature from locations with a longer history of dealing with this invasive insect.

“This spring, as temperatures warmed, we were ready to monitor and study EZS,” explained Knight. “Our initial finding was a positive one: The first larvae emerged from the ground before the American elms had developed leaves.” This mismatch in timing may be due to the insect’s non-native origin, which is bad news for the sawfly but great news for the American elm.

Further investigation in the surroundings of the Delaware lab suggest that the elm zigzag sawfly might have been present for a few years already. “There are a lot of planted elms including Asian species in the surrounding community and American elms growing in nearby forests. Commercially sold elm trees and even bags of soil with EZS eggs could easily have facilitated long-distance movement of the new pest,” explains Knight. “It was only our technician’s training that allowed for the quick identification once it reached our trees.”

With the presence of the sawfly confirmed, researchers at the lab are seizing the challenge as an opportunity to learn more about the new insect neighbor.  Working collaboratively with interns from the Ohio State University, researchers are collecting data on the sawfly lifecycle and how it fits into its new Ohio environment.

“Initial research on EZS is proving to be quite informative,” says Knight. “Europe experienced this invasive before North America, and while isolated instances of severe elm defoliation were observed, EZS has not been a driver of widespread tree mortality in that region. Unlike the emerald ash borer, which methodically kills ash trees, EZS does not appear to be as destructive.”

“The elm zigzag sawfly is here to stay,” observes Knight, “so we at the Forest Service Northern Research Station and our partners are seeing this as an opportunity to be on the forefront of understanding and managing this new threat to American elms.”