Unwelcome Guests: The Rapid Spread of Elm Zigzag Sawfly across North America
The elm zigzag sawfly, a tiny wasp no longer than a centimeter, is making headlines in North America for its alarming and unprecedented spread without the presence of males. Native to East Asia, this species, scientifically named Aproceros leucopoda, was first discovered on the continent in 2020, and in just over five years, it has successfully established itself in 15 states from New Hampshire to North Carolina, threatening urban forests as cities recover from previous ecological challenges like Dutch elm disease.
Characteristics and Behavior
Not to be mistaken for a common fly, the elm zigzag sawfly is noteworthy for the intricate zigzagging patterns it carves into the leaves of elm trees. Kelly Oten, an entomologist at North Carolina State University, describes the larval feeding patterns as “oddly cute,” yet the damage they inflict can lead to severe defoliation, leaving trees nearly bare. The sawfly's rapid expansion can be linked to a unique reproductive strategy known as thelytokous parthenogenesis, where females produce unfertilized eggs that only develop into more females. This means that a single female—or even a single egg transported inadvertently by humans—has the potential to trigger a new infestation.
Oten’s ongoing monitoring has revealed that the sawfly's infestation has recently expanded to Indiana, and she maintains a detailed map documenting sightings across the region. The situation remains worrisome, especially for cities still nursing the wounds inflicted by the past prevalence of elm diseases.
Diversifying Targets: Zelkova Trees
Researchers have recently discovered that the elm zigzag sawfly may not limit its feedings solely to elms. Oten's team experimented with planting Japanese zelkova (Zelkova serrata)—a close relative of the elm, favored for its resistance to Dutch elm disease—near an infestation site in Ohio. To their surprise, the sawflies were observed laying eggs, feeding, and pupating on zelkova saplings. This behavior indicates that when elm foliage is scarce, the elm zigzag sawfly can exploit zelkova trees as a secondary food source, potentially maximizing its reproductive success by starting to multiply earlier in the spring.
Véronique Martel, a forest entomologist with Natural Resources Canada, noted that such a host switch is rare among insects, suggesting that the sawfly's ability to adapt could make it even more formidable.
Human Contributions to the Spread
Human activities may be compounding the issue as the sawfly hitchhikes on vehicles and equipment. Oten and her colleagues have collected sawfly cocoons from unexpected spots, including truck mirrors and wheel wells, allowing these pests to travel far beyond their natural habitat. Unsurprisingly, researchers theorize that the sawfly initially arrived in North America through soil from a houseplant, illustrating the fragility of ecosystems in an age of global trade and transportation.
With the extensive distribution of native elms across eastern North America, the potential for further infestations is significant. Oten predicts more reports of the pest’s presence in additional counties and states in the coming years. While extremes in temperature may eventually restrict the wasp's range, as of now, it poses a serious threat from Canada down to North Carolina.
What Can Be Done?
In response to the growing threat, Oten is testing different pesticides to protect trees from the host-plant switchers. Initial trials involving two soil-applied insecticides yield promising results, and she plans to present detailed findings soon. In the meantime, Oten advises homeowners in infested areas to actively inspect their vehicles for cocoons and to report any zigzag patterns observed on elm leaves to local extension offices.
To safeguard these iconic trees and the ecosystems that rely on them, it will take collective effort and vigilance—both from scientists and the public. The situation remains fluid, but the message is clear: awareness and action are key in combatting the invasive elm zigzag sawfly.
For those interested in learning more about this species or seeking advice on management practices, additional resources and ongoing updates can be found through local extension offices or by visiting Science News.