Once thought to feed only on elm trees, the elm zigzag sawfly is showing it can survive on other species, indicating it could threaten a wider range of North America’s urban trees.

Recent research from North Carolina State University reveals that the invasive pest, which is native to Asia, is now feeding on Japanese zelkova, an ornamental tree commonly planted along urban streets and in yards.

First detected in North America in 2020, the elm zigzag sawfly feeds on the leaves of elm trees in distinctive zigzag patterns. Heavy infestations can strip trees nearly bare, weakening them and making them more vulnerable to other stresses.

“Our discovery shows that the elm zigzag sawfly is more adaptable than we thought,” said Kelly Oten, an assistant professor in the Department of Forestry and Environmental Resources and Extension specialist at NC State, who led the study.

Earlier studies suggested that the elm zigzag sawfly would not lay eggs on Japanese zelkova, but Oten and her collaborators observed sawflies laying eggs, feeding on leaves and even completing part of their life cycle on the trees.

The researchers monitored a group of Japanese zelkova trees planted alongside elm trees in Ohio, observing leaves and using sticky traps weekly from April 2025 to September 2025 to track sawfly activity and potential host use.

Oten explained that the sawflies consumed zelkova leaves only early in the season, before nearby elm trees had fully leafed out, after which they shifted to feeding on the elms. “We suspect it was a simple case of eating what was available.”

She added that the preliminary observations are meaningful in understanding the ability of the elm zigzag sawfly to spread and thrive. While Japanese zelkova may not be a favored host, it allows sawfly populations to reproduce and establish populations.

This early-season feeding increases the overall threat to elms. By the time elm leaves emerge, early-emerging sawfly populations are better sustained, allowing them to persist and potentially leading to heavier defoliation and additional stress on already vulnerable trees.

Although disease–resistant elms are now being planted, elms remain vulnerable. Defoliation and other stress can increase attraction to bark beetles that spread Dutch elm disease, and elm zigzag sawfly feeding may heighten this pressure. It is not yet known whether resistant trees can withstand these added stresses.

Since it was first detected in Quebec, Canada, the elm zigzag sawfly has spread to three other Canadian provinces and 15 U.S. states, including North Carolina, where it was found in 2022, affecting elms in Surry and Stokes counties near Pilot Mountain.

The rapid spread of the elm zigzag sawfly suggests it may be moving as a “hitchhiker,” Oten said. Researchers have found cocoons on twigs, leaves and even vehicles. And since sawflies overwinter in soil, they could also be transported with plants.

Homeowners, municipalities and nurseries can take practical steps to help limit the spread of the elm zigzag sawfly. Inspecting vehicles, outdoor equipment and plants for cocoons and removing them before moving them to new areas can prevent new populations from establishing. 

Although the elm zigzag sawfly is unlikely to become a catastrophic threat, it shows how quickly non-native insects can adapt, highlighting the need for policymakers to invest in early detection, rapid response, and proactive urban forest management.

Oten and collaborators are currently testing insecticides to help manage feeding impacts and hope to share results soon. They are also studying how the elm zigzag sawfly affects different varieties of zelkova to better understand its potential impacts on these trees.

This post was originally published in College of Natural Resources News.