The elm zigzag sawfly (Aproceros leucopoda) might be the most stylish defoliator out there. Early instars create the insect’s namesake zigzag feeding pattern, while later instars skeletonize leaves almost completely. (Photo by Kelly Oten, Ph.D.)

By Kelly Oten, Ph.D.

Kelly Oten, Ph.D.

Hot take: The zigzag pattern is out. And I don’t mean in terms of what’s trending. (Though, honestly, who can keep up when “6-7” is the latest craze?) I’m talking about a new pattern showing up on elm leaves across North America—tiny zigzags that mean trouble. The culprit? The elm zigzag sawfly, a new invasive insect spreading fast and leaving its mark wherever it goes.

Six years into its North American invasion, the elm zigzag sawfly (Aproceros leucopoda) is making waves—or, more appropriately, zigzags—across the continent. It gets its common name from the distinctive zigzag feeding pattern its larvae carve into elm leaves when young. While this might be the most adorable feeding pattern ever, don’t let the cute factor distract you. As they grow, they can strip foliage down to the veins, defoliating entire trees when populations are high enough. Repeated heavy feeding over several years can weaken trees, causing branch dieback and reduced growth, likely worsened when combined with other stressors like drought or disease.

In our new article published this month in the Journal of Integrated Pest Management, colleagues from North Carolina State University, the U.S. Forest Service, and I document the elm zigzag sawfly’s rapid spread across North America, discuss long-range dispersal pathways, and share a surprising discovery: a new host tree, Japanese zelkova.

Since its first North American detection in Québec in 2020 and the United States in 2021, this small but prolific defoliator has now been confirmed in 14 U.S. states and four Canadian provinces, stretching from New Hampshire to Minnesota and south to North Carolina.

In the first six years since its detection in North America, the elm zigzag sawfly (Aproceros leucopoda) has been found in 14 U.S. states and four Canadian provinces. (Map by Kelly Oten, Ph.D.)

That kind of travel is impressive for an insect barely half an inch long. But, like most invasive species, the elm zigzag sawfly probably isn’t doing it on its own. While adult sawflies are estimated to spread 28-56 miles (45-90 kilometers) a year on their own, human activity likely speeds things along. In our paper, we document sawfly cocoons attached to vehicles and wood products, suggesting that long-distance hitchhiking may be a major driver of its spread. Because they overwinter in soil, the transport of potted nursery stock may also play a role. And here’s the kicker—elm zigzag sawfly populations are entirely female and reproduce without mating, a process called “thelytokous parthenogenesis.” That means even a single insect can start an entirely new infestation. They’re the ultimate “Boss Babes” of the insect world.

Until recently, the elm zigzag sawfly was thought to feed only on elms (Ulmus spp.), but our research revealed a surprising twist: It’s also capable of feeding on Japanese zelkova (Zelkova serrata), a host previously tested but not thought to be a viable one. This ornamental tree, closely related to elm and prized for its toughness and resistance to Dutch elm disease, is a popular choice along streets in North America. At the U.S. Forest Service Northern Research Station in Ohio, we observed evidence of the elm zigzag sawfly larval feeding, pupation, and adult emergence on zelkova saplings planted near infested elms.

Like most invasive species, we’re helping elm zigzag sawflies (Aproceros leucopoda) spread more quickly than they would on their own. Here, sawfly cocoons are shown attached to the side mirror (A) and wheel well (B) of a truck. (Photos by Jared Beach)

But timing is everything, and it seems to have played a major role here. Early in the season, Japanese zelkova leafed out before nearby elms, giving the sawflies an early food source upon emergence. Once elm leaves fully expanded, however, the sawflies shifted back to elm, suggesting that Japanese zelkova may serve as an occasional alternate host when elm foliage isn’t available.

So, it’s spreading fast and now it’s got an eye on your zelkova trees—what can you do? Keep your eyes open for that telltale zigzag pattern on elm or zelkova leaves, especially in spring and early summer, and report sightings to your local extension office or state forestry agency. The elm zigzag sawfly may have style, but this is one trend we don’t want catching on.

Kelly Oten, Ph.D., is an assistant professor and extension specialist in the Department of Forestry and Environmental Resources and North Carolina State University. Email: [email protected].