Here We Go Again: Meet the Elm Zigzag Sawfly, Another Non-Native Forest Pest
By 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.Little work has been done on EZS in North America other than documenting where it is, what it does, and what it looks like. EZS are small sawflies, green as larvae with black T-shaped markings on the head. They have been known to attach their pupae to inanimate objects, giving them potential to spread long-distances via human-mediated movement—which sounds a lot like other known invasive species such as the spotted lanternfly (Lycorma delicatula) and spongy moth (Lymantria dispar). Whether they ascend to this level of invasiveness is yet to be seen.
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.
Journal of Integrated Pest Management