The elm zigzag sawfly (Aproceros leucopoda) is now established in several places in North America. This non-native forest pest can be a threat to elm species and hybrids and is capable of large defoliation events. Will it turn into a full-blown invasive species, or simply co-exist with native herbivores in natural and urban forests? At this point, we don’t know. A new review highlights elm zigzag sawfly biology, ecology, and management strategies in the U.S. (Image by Matthew Bertone, Ph.D., originally published in Oten et al 2023, Journal of Integrated Pest Management)

By David Coyle, Ph.D.

David Coyle, Ph.D.

Sometimes I feel like a broken record writing (and reading) articles for Entomology Today about invasive forest pests. This time, it’s the elm zigzag sawfly (Aproceros leucopoda), which was first detected in Quebec, Canada, in summer 2020.

Of note, this detection was made from an observation on the public crowdsourced citizen science website iNaturalist further supporting the usefulness of such online platforms. Native to East Asia, the elm zigzag sawfly, or EZS, is now established in Europe, Canada, and five eastern U.S. states, and these recent occurrences are detailed in a new article published last week in the Journal of Integrated Pest Management. Kelly Oten, Ph.D., assistant professor and extension specialist in forest health at North Carolina State University, is lead author on the article, along with colleagues at Virginia Tech, NC State, and the Maryland, Pennsylvania, and New York state departments of agriculture.

EZS feeds on elm (Ulmus) species and hybrids, which in and of itself isn’t all that interesting. However, what is interesting is the pattern that early instars make as they feed from the edge of the leaf toward the midrib. Larvae feed in a zigzag pattern, creating unmistakable feeding damage that can be relatively easily identified. Another species of sawfly (Sterictiphora) also make this zigzag feeding pattern, but these feed on Prunus hosts, not Ulmus.

As EZS larvae age, feeding intensifies, and entire trees can be defoliated. Thus far tree mortality has not been attributed to EZS feeding, but, as with any herbivore, complete defoliation events can be detrimental to tree health and vigor.

The elm zigzag sawfly (Aproceros leucopoda) is now established in several places in North America. This non-native forest pest can be a threat to elm species and hybrids and is capable of large defoliation events. Early instar larvae make a zigzag pattern as they feed from the edge of the leaf toward the midrib, creating unmistakable feeding damage that can be relatively easily identified. A new review highlights elm zigzag sawfly biology, ecology, and management strategies in the U.S. (Image by Matthew Bertone, Ph.D. [left], and Eric Day, Ph.D. [right], originally published in Oten et al 2023, Journal of Integrated Pest Management)

As with any invasive species, one of the first questions asked is “how did it get here, and from where did it come?” Well, it’s very likely that EZS in the U.S. came from Canada, and genetic analyses indicate that the Canadian population is probably from Europe. Other researchers speculate it arrived as cocoons, as this is the only life stage that would be capable of surviving the trip, and cocoons can be found in the soil. Put this all together, and the possibility of cocoon-contaminated soil seems like a solid guess as to how EZS arrived in North America.

Few management recommendations have yet been developed for this pest. In natural settings, little can be done. For trees in managed landscapes, any tactic that works for other defoliators would likely work for EZS, such as physically removing them from foliage or applying insecticides (following all label directions, of course). More work is ongoing in this area (e.g., how many generations does it have, and what are some management strategies in managed and natural landscapes?), and I for one am excited to see what we can learn about this relatively new forest pest in North America.

David Coyle, Ph.D., is an assistant professor in the Department of Forestry and Environmental Conservation at Clemson University. Twitter/Instagram/TikTok: @drdavecoyle. Email: dcoyle@clemson.edu.