New Pest Alert — Elm Zigzag Sawfly

The USDA Forest Service’s Eastern Region Forest Health staff released a pest alert on elm zigzag sawfly, an invasive insect that defoliates elm trees. Check out the pest alert to learn where the insect is found, how to identify it and the threat it poses to elm trees.

Elm zigzag sawfly has the potential to spread quickly. Since its first North American detection in 2020 in Quebec, it has been found in numerous states in the Northeast, mid-Atlantic, South and Midwest. The elm zigzag sawfly is a strong flyer with the ability to disperse up to 55 miles a year. People may inadvertently bring it to new locations when they move trees, leaf litter or even inanimate objects that have its cocoons attached.

The adult is a shiny black insect, about 8 millimeters long, with dark wings. The yellowish-green larvae are about 11 millimeters long with distinctive markings. Another telltale sign of an infestation is the zigzag pattern the larvae create when they feed on elm leaves. See the pest alert for more detailed descriptions of the elm zigzag sawfly’s appearance at different stages, as well as the damage they cause.

Elm zigzag sawfly infestations can lead to severe or almost total defoliation of elm trees. Currently, none of the pesticides available for general sawfly management are specifically labeled to control this insect.

An elm leaf exhibiting elm zigzag sawfly damage. USDA Forest Service photo by Craig Larcenaire.