The zigzag sawfly (Aproceros leucopoda) is an invasive insect from East Asia that feeds exclusively on elm trees. It gets its common name from the distinctive zigzag pattern its larvae chew into elm leaves, a calling card that makes it one of the easier tree pests to identify. Native to Japan, eastern Russia, eastern China, and the Korean peninsula, this small wasp-like insect has spread rapidly across Europe since 2003 and reached North America in 2020, raising concerns about the health of elm populations already weakened by decades of Dutch elm disease.
How to Identify Zigzag Sawfly Damage
The most recognizable sign of a zigzag sawfly infestation isn’t the insect itself but the feeding pattern left behind. Young larvae eat from the edge of an elm leaf in a back-and-forth zigzag path, leaving a characteristic pattern that looks like someone cut the leaf margin with pinking shears. As the larvae grow larger, they consume entire sections of the leaf, eventually leaving only the midrib and major veins. On a heavily infested tree, this progresses from scattered zigzag notches in spring to near-total leaf loss by midsummer.
The adult sawfly is small, roughly 6 to 8 millimeters long, black-bodied, and easy to overlook. The larvae are pale green caterpillar-like creatures, though sawflies are actually related to wasps and bees rather than butterflies or moths. One distinguishing feature: sawfly larvae have more pairs of fleshy leg-like appendages along their abdomen than true caterpillars do.
Which Trees Are at Risk
Zigzag sawflies feed on elms and, as far as researchers have documented, only elms. Multiple species are vulnerable. In Europe, heavy defoliation has been recorded on wych elm, field elm, and Siberian elm. Any elm species in a region where the sawfly is present could potentially serve as a host, which is particularly concerning in areas where elms have been replanted after Dutch elm disease losses or where disease-resistant elm cultivars line streets and parks.
Rapid Spread Across Two Continents
The zigzag sawfly was first detected outside its native range in Europe in 2003. From that initial foothold, it spread across much of the continent within two decades, reaching countries from Poland and Hungary to the United Kingdom.
In North America, the timeline has been even faster. The first detection came in Québec, Canada, in 2020. By 2021 it had been confirmed in Virginia. The following year brought reports from North Carolina, Maryland, Pennsylvania, and New York. In 2023, Ohio, Vermont, and Massachusetts were added to the list. By 2025, Michigan confirmed its first detection in St. Clair County. The pattern suggests the insect is moving steadily through the eastern and midwestern United States, and additional states will likely report detections in coming years.
Part of what drives this rapid expansion is the sawfly’s reproductive biology. Zigzag sawflies reproduce through parthenogenesis, meaning females can produce offspring without mating. Males are extremely rare or absent in invasive populations. This means a single female arriving in a new area can establish a population on her own, and multiple overlapping generations per year allow numbers to build quickly.
How Much Damage Can They Do
When populations boom, the defoliation can be dramatic. At study sites in Romania, individual wych elm trees lost between 74 and 98 percent of their leaves. In Hungary, some field elm and Siberian elm trees were completely stripped. Urban amenity trees in Budapest suffered around 70 percent leaf loss, visibly degrading the trees’ appearance in parks and along streets.
The good news is that zigzag sawfly defoliation, even when severe, does not typically kill trees outright. Infested elms generally push out new leaves either later in the same growing season or the following spring. No tree mortality directly caused by the sawfly has been documented so far. The damage is largely aesthetic, though it comes with real physiological costs. Repeated heavy defoliation over consecutive years forces trees to tap energy reserves to regrow foliage, and this stress has led to branch dieback in some cases. A tree that has to regrow its canopy year after year grows more slowly, stores less energy, and becomes more vulnerable to drought, other pests, and disease.
Managing an Infestation
For homeowners with a few affected elm trees, the simplest approaches are often the most practical. Hand-picking larvae off leaves works for light infestations, especially on smaller or newly planted trees. A strong blast of water from a garden hose can dislodge larvae from branches within reach. Insecticidal soaps or horticultural oils applied when larvae are still small can also suppress populations without broad environmental impact.
For larger-scale infestations, several chemical treatments have shown effectiveness in European field trials. Foliar-applied insecticides have successfully reduced first-generation larval numbers, and bioassays with certain systemic compounds killed high percentages of larvae. A biological approach using an insect-killing fungus (Beauveria bassiana) has also reduced larval populations in field trials, offering a more targeted option that avoids harming beneficial insects.
Monitoring is another important piece. Yellow sticky traps, particularly fluorescent yellow versions designed as “cloak” traps, have proven effective at capturing adult sawflies. These can help you detect an infestation early, before defoliation becomes severe, and give you a head start on treatment timing.
What to Watch For on Your Elms
If you have elm trees in the eastern or midwestern United States, or in southern Canada, check leaves starting in late spring. The telltale zigzag feeding pattern along leaf edges is the earliest and most reliable sign. Look on the undersides of leaves for small, pale green larvae. Because multiple generations can occur in a single season, damage may appear in waves from spring through fall, with each generation producing heavier defoliation than the last.
If you spot the characteristic damage, your state’s cooperative extension service or department of agriculture typically wants to know. Reporting new detections helps researchers track the sawfly’s spread and can trigger monitoring efforts that benefit the broader community. Many states have online reporting tools or hotlines specifically for invasive pest sightings.