Given the hundreds of thousands of elms already lost in the last 100 years to Dutch elm disease, it seems unfair that the Ulmus genus should face a new enemy so soon. But the world of invasives isn’t fair, and a new threat has reared its head: elm zigzag sawfly (Aproceros leucopoda).
This insect is a member of the family Argidae, or argid sawflies. Ten species in the Aproceros genus have been identified worldwide, and most of these – like the elm zigzag sawfly – are native to Asia.
The adult is a wasp-like fly about
6-7 mm in length. The larvae begin as ashy white and tiny, less than 2 mm in length, but become green with black spots and develop a black triangle on their back as they mature.
Even if this has yet to be confirmed in your area, it’s one to stash in your collection of “Wanted” posters, especially if any of your clientele make use of elms – native or non-native. Fortunately, the feeding pattern of the larvae – and the source of its common name – makes it easy to distinguish from other elm-feeding insects, especially if you see it early on.
The elm zigzag sawfly was detected in Poland and Hungary in 2003, presumably having been transported from Japan or China via elm seedlings or other materials (although nobody knows for sure, as is often the case with these new arrivals). It then spread to Belgium and the Netherlands and, in 2017, made its first appearance in Great Britain, where arborists, scientists and volunteers continue to speculate on its eventual impacts.
It remained a European concern for nearly two decades, but then citizen scientists armed with the iNaturalist app identified it in Sainte-Martine, Quebec, in the summer of 2020, marking the first time it had been confirmed in North America. Then, in 2021, it was found in Winchester, Virginia.
One may reasonably presume that the elm zigzag sawfly is surviving undetected in locations between Virginia and Quebec, all along the Interstate 81 corridor. Evidence in Europe suggests that its spread depends on “hitchhiking” on vehicles, as well as via the movement of elm plant materials. However, it’s also true that the pests are strong fliers and can spread more than 50 miles annually by their own means.
The elm zigzag sawfly will complete a full life cycle in 24 to 29 days. One characteristic that makes this pest particularly prolific is that it’s parthenogenetic – it doesn’t need a male to reproduce – and it can produce up to six generations per year.
The adult female will lay between seven and 49 eggs along the margins of an elm leaf, and the new larvae will emerge in about a week. Then, over the course of a couple of weeks, they’ll move through six larval instars before pupation.
The insect will pupate in a silken and lattice-like cocoon attached to the underside of an elm leaf and emerge as an adult a week later. These cocoons can be a good means of confirming the sawfly’s presence, in addition to the zigzag feeding pattern of the larvae.
The sawfly doesn’t seem to prefer one species of elm over another, so both native and ornamental species are potential targets. Its common name stems from the meandering feeding pattern on elm leaves, and, while this can be a giveaway of the pest’s presence, heavy infestation in a single tree and feeding by more mature larvae can conceal the initial zigzagging, making it blend with feeding patterns caused by more commonplace caterpillars.
Because up to six generations can occur each year, feeding by caterpillars will happen consistently from late spring to fall.
Defoliation in excess of 90% has been noted in some of the European countries where the sawfly is active. Obviously, this level of defoliation repeating year after year will weaken the affected elms and eventually lead to their mortality.
One of the greatest negatives of the elm zigzag sawfly from an ecosystem perspective is competition with native Ulmus-dependent species. In Europe, for example, the white-letter hairstreak butterfly and white-spotted pinion moth feed on the elms that survive in fencerows in the wake of Dutch elm disease.
In North America, an estimated 500 insect species breed, feed, deposit eggs or hibernate in elms, and a handful of moth and butterfly species depend primarily or exclusively on elms for their life cycles, including the ochre dagger moth, the ruddy dagger moth, the four-horned sphinx, the double-toothed prominent and the question mark butterfly.
In the case of the double-toothed prominent moth, the body of the caterpillar has evolved to mimic the toothed edge of an elm leaf, providing effective camouflage against predators. So any measurable loss of elms would certainly extend itself to negative impacts for this particular moth species.
Attempts to curtail the spread of Aproceros leucopoda in Europe have focused on inspecting and limiting the movement of elm plants and encouraging purchase of such materials from as local a nursery as possible. Cleaning and disinfection of equipment has been part of the overall management as well.
The English tree care industry has worked to develop a strong relationship with citizen scientists, through such groups as Observatree, to help detect elm zigzag sawfly and other earlier insect and disease infestations.
Suspect elms that are pruned or felled should be left on site, if possible. Because cocoons can overwinter in fallen leaves, even the duff under elms should be handled cautiously.
In the long run, and through all the necessary labeling hoops, systemic pesticides may have value in a broader management strategy, but the parthenogenetic reproduction and long-range flight of the adults will mean that reinfestation can happen quickly.
Climate and elevation limitations to the expansion of the pest are not fully understood at this point, but it’s believed these could be important. For instance, while defoliation rates of 90% or more have been recorded in other European countries, they’ve averaged 1-2% in Bulgaria. Whether that’s attributed to a warmer and drier climate, higher elevations or other factors is yet to be determined.
Cold winters aren’t likely to curb this pest in the eastern United States, as it can withstand temperatures as low as minus 22 degrees F.
It’s much too early to know what the elm zigzag sawfly will mean to urban and suburban forests and landscapes in the U.S. The best thing tree care professionals can do now is to stay vigilant and encourage their clients to pay attention also. Subscribe to any pest-alert bulletins maintained by the nearest land-grant university, if you haven’t already, and if you do notice the zigzag feeding pattern in an elm leaf, please report it to your local cooperative extension office.
Phillip Meeks is a forester and an educator in the fields of agriculture and natural resources in southwest Virginia.