An unusual insect is rapidly expanding its range across North America: the elm zigzag sawfly, a wasp species capable of reproducing without males. First detected in 2020, this pest is already established in 15 U.S. states, from New Hampshire to North Carolina, and is showing no signs of slowing down. The species’ unique reproductive strategy and adaptability pose a growing threat to elm trees—and now, potentially other species as well.
An All-Female Invasion
The elm zigzag sawfly (Aproceros leucopoda ) isn’t a fly, but a type of wasp native to East Asia. What sets it apart is its ability to reproduce through thelytokous parthenogenesis : females lay unfertilized eggs that develop into only more females. This means a single wasp can establish a new population without needing a mate, making it exceptionally efficient at spreading. No males have ever been observed in North American populations.
This reproductive method explains the alarming speed of its expansion. The sawfly has spread across a vast geographical area in just over five years, hitching rides on vehicles and potentially arriving in North America hidden within plant shipments.
Beyond Elms: A Changing Host Range
Originally targeting elm trees, the sawfly’s feeding habits involve carving distinctive zigzag patterns into leaves. While initially considered merely aesthetic damage, the larvae can defoliate entire trees, leaving them weakened and vulnerable.
Recent research shows the sawfly is expanding its diet to include Japanese zelkova trees, a close relative of elms often planted as replacements for trees killed by Dutch elm disease. This is significant because zelkovas leaf out earlier in the spring, providing the sawfly with an alternate food source when elms are still dormant. This expanded host range could accelerate its spread and make eradication more difficult.
Human-Assisted Spread
The sawfly’s rapid dispersal isn’t solely due to its reproductive strategy. Researchers have found cocoons attached to truck mirrors and wheel wells, indicating that human transportation is a major factor in its spread. This means that even routine travel can carry the pest hundreds of kilometers beyond its natural range.
Uncertainty and Response
The severity of the sawfly’s impact on elm trees remains unclear. In some areas, it causes only minor damage, while in others, it leads to near-complete defoliation. Researchers don’t yet know if this variation is tied to tree species, environmental factors, or the sawfly’s own genetics.
Entomologists are currently testing pesticide treatments to mitigate the damage. Preliminary results show promise, but further research is needed. In the meantime, experts recommend checking vehicles for cocoons before leaving infested areas and reporting any zigzag patterns in elm leaves to local extension offices.
The elm zigzag sawfly’s aggressive spread underscores the challenges of invasive species management. Its unique reproductive strategy, combined with human-assisted dispersal, makes it a formidable opponent for North American ecosystems. As the pest continues to expand its range, understanding its behavior and developing effective control measures will be crucial to protecting vulnerable tree populations.
