She’s lower than a centimeter lengthy, produces solely daughters and is conquering the continent with out a single male. Meet the elm zigzag sawfly, named for the fragile zigzag patterns it carves into elm tree leaves.
Regardless of its identify, the elm zigzag sawfly (Aproceros leucopoda) just isn’t a fly; it’s a kind of wasp first found in North America in 2020. Initially from East Asia, it has expanded its vary at an “alarming” charge, researchers report within the Journal of Built-in Pest Administration. For cities nonetheless recovering from elm canopies misplaced to a fungal illness, it’s an unwelcome second wave of assault. And newly rising proof suggests it received’t cease at elms.
In simply over 5 years, the pest has unfold to fifteen U.S. states, from New Hampshire to Minnesota and south to North Carolina, says Kelly Oten, an entomologist at North Carolina State College in Raleigh. “We simply had so as to add Indiana,” says Oten, who maintains a map of elm zigzag sawfly sightings.
Freshly hatched elm zigzag sawflies reduce zigzagging traces into leaves. “The feeding sample is oddly cute,” Oten says. However because the larvae mature, they’ll strip sufficient foliage from a tree to depart it nearly naked.
To see if the wasp would assault timber past elms, Oten’s crew planted Japanese zelkova (Zelkova serrata) close to an infestation website in Ohio. Many U.S. cities have began planting zelkovas, an elm relative, since they seem proof against Dutch elm illness, the fungal affliction that has killed tens of thousands and thousands of American elms within the final century.
The researchers noticed the wasp laying eggs, feeding, pupating and rising as adults on zelkova saplings, which produce new leaves sooner than elms within the spring. Because the season progressed, the wasp switched again to elms, suggesting that zelkova might function an alternate host when elm foliage isn’t out there, the researchers say.
The discovering shocked Véronique Martel, a forest entomologist with Pure Sources Canada who reported North America’s first elm zigzag sawfly detection within the province of Quebec in 2020. “It’s uncommon that bugs can change hosts,” she says. Martel, who wasn’t concerned within the examine, suspects that having the ability to feed on zelkovas may make the wasps much more profitable, by permitting them to start out multiplying earlier within the 12 months. “They’ll make quite a lot of generations inside a summer time,” she says.
An important driver behind the zigzag sawfly’s speedy unfold is an uncommon reproductive technique known as thelytokous parthenogenesis, by which females lay unfertilized eggs that produce solely extra females. Which means even a single egg hitching a trip on firewood or a automotive can begin a brand new infestation. No males have ever been discovered.
For causes researchers don’t but perceive, the zigzag sawfly larvae reduce solely minor zigzag patterns in some elms whereas severely defoliating elms in different areas. “At this level, we have no idea if it’ll kill the tree or simply stress it drastically,” Oten says.
Unwitting human accomplices have most likely aided the sawfly’s speedy unfold. Oten’s crew documented cocoons with elm zigzag sawfly pupae clinging to truck mirrors and wheel wells, which may carry the insect far past its pure dispersal vary of 45 to 90 kilometers yearly. Researchers suppose the pest might have arrived in North America hidden within the soil of a houseplant.
With elms native throughout many of the japanese United States and Canada, elm zigzag sawflies have an enormous territory out there. “I do suppose we can have many extra studies in extra counties and certain extra states” in 2026, Oten says. Temperature extremes might ultimately restrict the insect’s vary, she notes. “However as of proper now, it’s North Carolina to Canada. That’s fairly huge.”
Oten is testing pesticides to assist householders shield their timber. Early trials of two soil-applied pesticides present promise, and she or he expects to publish the complete outcomes inside months. For now, she recommends checking autos for cocoons earlier than leaving infested areas and reporting zigzag patterns to native extension workplaces.
