For much of the 19th and early 20th centuries, American elms were more than just trees. They were defining features of the nation’s streetscapes, with tall, arching branches creating shaded tunnels that lined city streets and small-town avenues.

Today, however, these iconic trees are far less common. Dutch elm disease decimated much of the population, and a new threat has emerged: the elm zigzag sawfly. This small, invasive pest is spreading across North America, feeding on elm leaves and adding a new layer of stress to already vulnerable trees.

Efforts to restore elm populations have led to the development of disease-resistant varieties designed to withstand Dutch elm disease. These new elms offer hope for the species, but the arrival of the elm zigzag sawfly complicates restoration efforts, affecting not only elms but also other species.

To help the public better understand this pest and protect their trees, we’ve compiled an FAQ addressing the most common questions, drawing on the expertise of Kelly Oten, an assistant professor in the Department of Forestry and Environmental Resources at North Carolina State University.

Oten, who also serves as an Extension specialist, conducts research on understanding and mitigating the impacts of invasive insects and diseases, including the elm zigzag sawfly, which she is actively tracking across the United States and Canada.

Common Questions

• What is the elm zigzag sawfly? 
• What does it look like? 
• What trees are impacted? 
• What damage can it cause to trees? 
• How does it spread? 
• Can it be transported on plants?
• How does it impact North American elm trees?
• Where has it been found in North America?
• When did it appear in the U.S.?
• How can I prevent its spread?
• How can I help research and control efforts?
• What other pests threaten elm trees?

What is the elm zigzag sawfly?

It is an invasive insect that primarily feeds on the leaves of elm trees. Its larvae create a distinctive zigzag pattern on the leaves as they feed. Native to Asia, it has been invasive in Europe for over 20 years and was first detected in North America in 2020.

What does it look like?

The adult is a small, wasp-like insect that is black with pale yellow legs but does not sting. The larvae are caterpillar-like creatures that feed on the edges of elm leaves, leaving the characteristic zigzag feeding pattern.

What trees are impacted?

It primarily affects elm trees, including the American elm, winged elm, and Chinese elm. A recent study conducted by Oten and her collaborators also found that the invasive species is feeding on Japanese zelkova trees.

What damage can it cause to trees?

The damage can range from minimal to severe. Minimal damage consists of a few zigzag feeding patterns on leaves, which may not significantly impact the tree’s health. Severe damage can occur when large infestations strip the tree of its leaves, weakening it and making it more vulnerable to pests, diseases and environmental stress.

How does it spread?

The insect is capable of flying, but human activity likely helps its spread. Larvae attach cocoons to objects like twigs, leaves, and vehicles, which can transport the pest over long distances. Trucks, particularly wheel wells and side mirrors, have been found carrying sawfly cocoons.

Can it be transported on plants?

It is possible, though not confirmed, that the sawfly could spread through the transport of plants. The sawfly spends its winter in the soil and could easily be moved in potted plants, though whether this is a significant pathway remains uncertain.

How does it impact North American elm trees?

North American elms are already under stress from threats like Dutch elm disease. The elm zigzag sawfly can further weaken the trees by causing defoliation, making them more susceptible to additional diseases and pests. Researchers are closely monitoring its interaction with Dutch elm disease to understand the combined effects.

Where has it been found in North America?

Since its discovery in Quebec, Canada, in 2020, the sawfly has spread to 15 U.S. states and three other Canadian provinces, including North Carolina in 2022. It is primarily spreading across the eastern and midwestern U.S.

When did it appear in the U.S.?

The first confirmed U.S. detection occurred in May 2021 in Virginia, near Winchester in Frederick County. Larval specimens were submitted to the Insect Identification Laboratory at Virginia Tech and were confirmed by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) Plant Protection and Quarantine (PPQ) program.

How can I prevent its spread?

Inspect vehicles, especially wheel wells and mirrors, for cocoons before traveling. Monitor elm trees for zigzag patterns on leaves, particularly in areas where the sawfly has been detected. Dispose of larvae or cocoons properly.

How can I manage infestations?

To manage an infestation, remove heavily infested branches by pruning and disposing of them appropriately. You can also use insecticides that are effective against sawflies, following local guidelines. Encouraging natural predators, such as birds and parasitoid wasps, may help control the sawfly population.

How can I help research and control efforts?

You can help by reporting sightings to local agricultural or environmental authorities. Additionally, staying informed about ongoing research and supporting efforts to control invasive species can contribute to long-term management solutions.

What other pests threaten elm trees?

North American elms face Dutch elm disease, an invasive fungal disease spread by bark beetles. Elm bark beetles also target weakened trees, carrying pathogens that make trees more vulnerable to further damage.

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