In the 1930s, America saw a catastrophic decimation of neighborhood elms. These beautiful and graceful trees were attacked by the invasive Dutch Elm disease.

While work was being done on breeding elm stock that is resistant to attacking pathogens, an incredible discovery was made in Ohio — another invasive insect. It is a new(er) invasive called the Elm Zigzag Sawfly. In 2020, the sawfly hit the shores of our country, starting in Virginia in 2021 including the Midwest and the eastern U.S. by 2023.

They were discovered in Delaware, Ohio, by an observant researcher at a research station of the Forest Service Research Lab. This researcher saw the characteristic zigzag patterns on the leaves of these research stands of trees and knew immediately what it was. Now the word is out.

This fly overwinters in a pupal stage, emerging in the spring to coincide with the leaf emergence of elms. These caterpillars go through six instar stages, reaching adulthood to complete the 24- to 29-day life cycle.

These sawflies reproduce by parthenogenesis, or the act of reproducing without fertilization from a male. No male sawfly has been found. The adult is a quarterinch, with a black body and white to yellow legs.

It resembles a wasp with a wider waist. Each lays up to 60 eggs at a time and can have up to six generations per growing season in Europe. (So far, only two generations have been recorded in the United States).

The emerging caterpillars are small pale green with a black stripe on the head and markings on the legs. They live as adults for four to six days.

Eggs are laid on the edges of elm leaves, where they hatch in four to eight days and begin to feed for 15 to 18 days. This defoliation stresses the trees, which weakens them. When ready to pupate, the caterpillar spins a cocoon like a sheet on the underside of a leaf. This protects the pupa as it matures. And the cycle begins again.

It is vital if you see the characteristic zigzag pattern on elm tree leaves that you take a picture and report it to the Ohio Department of Agriculture at this link – https://go.osu.edu/zigzagreport. As well, you can download good pictures to the Great Lakes Early Detection Network Mobile App and possibly collect a sample. We can each be vigilant to help with this new invasion.

For additional photos, details and to learn more about this insect, visit https://go.osu.edu/zigzag.

Hughes is an Ohio State University Extension Master Gardener Volunteer in Mahoning County.