All-female wasp spreads rapidly in North American elms

It measures less than a centimeter, produces only daughters and conquers the continent without a single male. Meet the elm zigzag sawfly, named for the delicate zigzag patterns it carves into the leaves of the elm tree.

Despite its name, the elm zigzag sawfly (Aproceros leucopod) is not a fly; it is a type of wasp first discovered in North America in 2020. Native to East Asia, it has expanded its range at an “alarming” ratereport the researchers in the Journal of Integrated Pest Management. For towns still recovering from the destruction of elm canopies to fungal disease, this is an unwelcome second wave of onslaught. And new evidence suggests it won’t stop with elms.

In just over five years, the pest has spread to 15 U.S. states, from New Hampshire to Minnesota and south to North Carolina, says Kelly Oten, an entomologist at North Carolina State University in Raleigh. “We just had to add Indiana,” says Oten, who maintains a map of elm zigzag sawfly sightings.

Newly hatched elm zigzag sawflies cut zigzag lines in the leaves. “The feeding model is strangely cute,” Oten says. But as the larvae mature, they can strip enough foliage from a tree to leave it almost bare.

To see if the wasp would attack trees beyond the elms, Oten’s team planted Japanese zelkova (Zelkova serrata) near an infestation site in Ohio. Many American cities have begun planting zelkovas, a relative of the elm, because they appear to be resistant to Dutch elm disease, a fungal disease. which killed tens of millions of American elms in the last century.

Researchers observed the wasp ovipositing, feeding, pupating and emerging as adults on young zelkova trees, which produce new leaves earlier than elms in spring. As the season progressed, the wasp returned to elm trees, suggesting that zelkova could serve as an alternative host when elm foliage is not available, researchers say.

This discovery surprised Véronique Martel, a forest entomologist at Natural Resources Canada, who reported the first detection of the zigzag elm sawfly in North America in the province of Quebec in 2020. “It is rare that insects can change hosts,” she said. Martel, who was not involved in the study, suspects that being able to feed on zelkovas could make the wasps even more efficient, allowing them to start multiplying earlier in the year. “They can form many generations in one summer,” she says.

A crucial factor behind the rapid spread of the zigzag sawfly is an unusual reproductive strategy called thelytoque parthenogenesis, in which females lay unfertilized eggs that only produce more females. This means that even a single egg clinging to firewood or a car can start a new infestation. No males have ever been found.

For reasons that researchers do not yet understand, zigzag sawfly larvae cut only minor zigzag patterns in some elms, while severely defoliating elms in other areas. “At this point, we don’t know if it’s going to kill the tree or just put a lot of stress on it,” Oten says.

Unwitting human accomplices likely contributed to the rapid spread of the sawfly. Oten’s team documented cocoons with zigzag elm sawfly pupae clinging to truck mirrors and wheel wells, which can carry the insect well beyond its natural dispersal zone of 45 to 90 kilometers per year. Researchers believe the pest may have arrived in North America hidden in the soil of a houseplant.

With elm trees native to most of the eastern United States and Canada, zigzag elm sawflies have a large territory. “I think we’ll have a lot more reporting from other counties and probably more states” in 2026, Oten says. Extreme temperatures could potentially limit the insect’s range, she notes. “But right now it’s North Carolina all the way to Canada. It’s pretty wide.”

Oten tests pesticides to help homeowners protect their trees. Initial trials of two soil-applied insecticides are promising and she hopes to publish the full results within a few months. For now, she recommends checking vehicles for cocoons before leaving infested areas and reporting zigzags to local extension offices.