Image: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org An elm zigzag sawfly (Aproceros leucopoda) larvae feeding on a leaf There’s a new invader in town! Elm zigzag sawfly, so called for its unique feeding pattern, has made its way down to North Carolina. The invasive forest pest was observed in Surry and Stokes county in August 2022. While it might look like a harmless little caterpillar when it’s young, the insect can be quite harmful to infested elm trees. Elm zigzag sawfly, or EZS for short, is a defoliating insect when in its larval stage, meaning it feeds on the leaves of trees. It infects various types of elm trees including American elm, winged elm, Chinese elm, and more. There is a lot that remains unknown about how EZS will behave in North Carolina right now, but researchers at NC State University and other institutions are currently working to find the answers we need. The effect of a warmer climate and how that could impact population growth is of particular concern. Elm Zigzag Sawfly's Journey Around the GlobeElm zigzag sawfly is native to eastern Asia, where it is a minor pest. However, in Europe, where it has been found since 2003, EZS can be quite damaging to host trees. EZS was first observed outside of its native range in Hungary and Poland, but has since been confirmed in at least 13 more European countries. In 2020, elm zigzag was documented to have made its way to North America for the first time. The small wasp-looking fly was actually first identified through the citizen science app iNaturalist in Quebec, Canada. The observation was then confirmed by professionals from the Canadian Food Inspection Agency in August 2020. Once in North America, EZS quickly began moving south. In 2021, it was confirmed in Virginia. In 2022, it was confirmed in Maryland, Pennsylvania, and North Carolina. As of this article being published, the infestation in North Carolina seems to be localized, meaning it is contained to one area that spans the border of Surry and Stokes county. Adult elm zigzag sawflies are strong fliers, which allows them to spread to new regions relatively quickly. In addition to flying, they can also spread via transport accidentally facilitated by human activity. EZS builds cocoons that can be attached to objects that people might relocate to areas not yet infested; they can also hide in soil or on plants being moved by people, all of which contributes to their spread. What's So Bad About a Little Green Caterpillar?When in the larval stage (the green caterpillar looking insect pictured above), elm zigzag sawfly is a prolific defoliator. Defoliation doesn’t always threaten the health of the tree; sometimes, it is simply an aesthetic issue that is typically remedied during a late-season period of growth or during the next growing season. However, after multiple seasons of intense defoliation from dense populations of pests, the health of the host tree can be severely diminished. Extended intense defoliation can lead to branch dieback and eventually the death of the tree. As of 2021, no tree deaths have been directly linked to EZS. However, researchers are concerned that the warm climate of the Southeastern United States will allow EZS populations to skyrocket, reaching dangerous levels for the elm trees they infect. Additionally, elm trees across the United States are already being threatened by Dutch elm disease, a fungal infection that can be quite damaging. Elm zigzag sawfly is wildly successful at reproducing, which is another factor that makes this insect so good at invading new regions. No male EZS have been observed, meaning this species is likely entirely made up of female sawflies. Female EZS reproduce without males, so when they travel to new areas they don’t need to find a mate to lay fertilized eggs. EZS have also been recorded to have up to six generations during a single year in the wild. The number of generations could be higher in areas where temperatures get warmer earlier in the year and colder later in the year. A longer period of warm weather gives EZS a longer time to reproduce. Not needing a mate and the potential for many generations in a single year are indicators that this invasive pest could be quite a problem for North Carolina. How Can We Stop It?One way you can help prevent the spread of elm zigzag sawfly is by checking for cocoons (pictured below) when leaving areas where their presence has been confirmed. The cocoons can be attached to objects that are transported, which can lead to the accidental introduction of EZS to a new region. Unfortunately, the cocoons can be quite difficult to spot. The most important thing you can do to prevent the spread is report any sightings of EZS or indicators that they might be nearby. Images: Kelly Oten, NC State University EZS Cocoons attached to a fence post and a leaf respectively How to Identify Elm ZIgzag SawflyLarval elm zigzag sawflies emerge from the egg a pale gray-green color but turn a bright green color as they grow. The larvae are quite small, only growing to be slightly more than 1 cm long. They have 6 true legs on the upper part of their body. The second and third pair of legs have a dark T-shape marking on them. The larvae also have a dark band on their head. Image: Matt Bertone, NC State University An elm zigzag sawfly larvae Adult elm zigzag sawflies are also quite small at about 1 cm long. Their white-green legs and white patch underneath the thorax (the middle body segment of insects) can be used to identify them. Image: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org A mature elm zigzag sawfly Adult EZS and larval EZS can be quite difficult to identify without a trained eye. The feeding pattern of EZS larvae, on the other hand, can be quite easy to spot. When the larvae are young, they feed in a zigzag pattern between leaf veins. As they age, they begin to eat the entire leaf tissue, while still leaving behind the mid-vein, meaning they no longer create that characteristic zigzag. Image: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org The characteristic zigzag feeding pattern of young EZS larvae So You Think You've Identified Elm Zigzag Sawfly... Now What?If you think you’ve found elm zigzag sawfly or evidence of its presence, it's important to report it to the right people. Reporting potential sightings is crucial to slowing, and hopefully stopping, the spread of invasive pests. You can report your findings to your local NC Forest Service county ranger, your local extension office, or you can email Dr. Kelly Oten ([email protected]) with NC State University. ReferencesElm zigzag sawfly (Aproceros leucopoda). 2023 Feb 14. Forest Research. [accessed 2023 Feb 21]. https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/elm-zigzag-sawfly-aproceros-leucopoda/#:~:text=Origins%20and%20background. Martel, V., O. Morin, S. Monckton, C. Eiseman, C. Béliveau, M. Cusson, and S. Blank. 2021. Elm zigzag sawfly, Aproceros leucopoda (Hymenoptera: Argidae), recorded for the first time in North America through community science. Can. Entomol. 154(1): E1. Oten K, Bertone M. 2022 Aug 31. Elm Zigzag Sawfly | NC State Extension Publications. NC State Extension Publications. [accessed 2023 Feb 21]. https://conte nt.ces.ncsu.edu/elm-zigzap-sawfly#:~:text=The%20elm%20zigzag%20sawfly%20. AuthorsDelaney Serpan, Courtney (Smith) Johnson
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March 2023
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