Column, The Outside Story

The Outside Story: A new invasive is Zigzagging across North America

There’s a new invasive insect zigzagging its way across North America. First reported by citizen scientists in Quebec in 2020, the elm zigzag sawfly (Aproceros leucopoda) has now spread to North Carolina, Virginia, Maryland, Ohio, Pennsylvania, New York, Massachusetts, and Vermont. This new pest, which is native to Asia, has the potential to cause major devastation to one of our native tree species.

As its name suggests, this sawfly infests elms (Ulmus species), feeding on the tree’s leaves in a zigzag pattern. The larvae, which feed spring through summer, resemble tiny green caterpillars but with six or more pairs of prolegs (fleshy abdominal limbs); caterpillars have five or fewer pairs of prolegs. These larvae eventually consume entire leaves and may completely defoliate trees. Branch dieback and tree mortality can occur if trees are severely defoliated for several years in a row. Once done feeding, the larva spins into a cocoon — attached to a leaf during summer, or overwintering in leaf litter — where it pupates into an adult.

The adult sawflies are all female and are parthenogenic, meaning they can reproduce asexually without fertilization from a male. They emerge in the spring and lay up to 50 eggs on leaf tips. The eggs hatch in about a week. In our region, elm zigzag sawflies typically produce two generations per year. In their native Asia, there may be as many as four generations per year. The number of generations produced each year determines the amount of defoliation.

You may be asking, “Didn’t Dutch elm disease already kill off all the elm trees?” While the disease has decimated elms, it did not eliminate them, and resistant trees exist. Researchers are working to breed these survivors and restore the species to critical habitats including floodplains. In river floodplains such as those found along the Connecticut River, the American elm is a foundation species – one that forms the base of an ecological community and influences biodiversity. Restoring elm is even more important now, with the loss of ash tree species from emerald ash borer (another invasive species) and the increase of flooding events due to climate change.

The Nature Conservancy (TNC) is working with the U.S. Forest Service on a large elm restoration project in the Northeast. The project, initiated by former TNC floodplain forest ecologist Christian Marks, began a decade ago with the crossbreeding of survivor trees from the Northeast with trees resistant to Dutch elm disease. Since 2014, researchers have planted more than 11,000 of these hybrid elms at 72 sites across New York, Connecticut, Massachusetts, Vermont, and New Hampshire.

Although these trees face pressure from deer browsing, flooding, ice jams, and other insect infestations, survival rates are as high as 88% at some research sites. In 2026, researchers plan to inoculate surviving trees with the fungus that causes Dutch elm disease to determine which trees are truly resistant. The final goal of the project is to create seed orchards of elms that are disease resistant and climate adapted.

TNC senior conservation planner Gus Goodwin manages the project and is hopeful it will be a success despite this new threat. While elm zigzag sawfly has not been detected in the elm restoration plantings in the Northeast, researchers have detected the pest in Ohio, where they have replicated the work done by TNC.

According to Kathleen Knight, a Forest Service research ecologist, sawfly defoliation in the summer of 2023 at the Ohio site was less than 1% of the surface area of the leaves, which is much less than the damage typically seen by other pests such as Japanese beetles. Sawfly populations observed at the sites in Ohio had only two generations in 2023, with the second generation comprising fewer individuals. This, along with the fact that elms are known to support a diverse population of native insects, suggests that native predators may control sawfly populations.

Researchers are still working to determine whether elm zigzag sawfly could be as devastating as other pest introductions, including Dutch elm disease, and will continue to work on elm restoration despite this new challenge. You can help track the spread of elm zigzag sawfly by reporting sightings to your state forestry agency or cooperative extension office.

Jen Weimer is a forest health expert, photographer, and writer living in the forests of New Hampshire. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited byNorthern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation:

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