The elm zigzag sawfly Aproceros leucopoda [EZS] is native to East Asia. All sources report it is native to eastern China and portions of Japan; some add eastern Russia and the Korean peninsula [NCSU extension]. It has been introduced to both Europe and North America. In both regions the outbreak either originated much earlier than the detection or the insect spread quickly.
The European outbreak was first detected in 2003 in Hungary and Poland. Within a few years outbreaks were reported over an area of 1,700 km, from eastern Ukraine to Austria. Within 10 years of the first detection in Europe, it was found to be present in Belgium, The Netherlands, and Germany – apparently the result of separate instances of human-assisted transport [https://jhr.pensoft.net/articles.php?id=4395]. By 2018 the insect had reached the United Kingdom. [https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/elm-zigzag-sawfly/]. EZS now occupies areas from the United Kingdom and France in the west, to Russia and Kazakhstan in the east (Ashikbayev et al. 2018, cited by Kelly Oten blog).
The first confirmed detection of the elm zigzag sawfly in North America occurred in August 2020 in Québec, Canada (Martel et al. 2021; https://www.invasivespeciescentre.ca/first-confirmed-sighting-of-a-new-invasive-in-north-america-elm-zigzag-sawfly/). Canadian authorities were unable to determine the introductory pathway or amount of time the insect had been present [Invasive Species Centre].
The Canadian population was not the only outbreak on the continent, however. Earlier the same year, the tell-tale feeding pattern – zigzags on the tree’s leaves — was observed in Frederick County, Virginia (northwestern corner of the state). Since no specimens were recovered, identification was not confirmed. A year later, specimens collected from the same confirmed the presence of EZS [David Gianino, State Plant Regulatory Official (SPRO) of Virginia, pers. comm.]. Additional surveys that summer led to detections in eight additional Virginia counties [K. Oten blog].
Also in the summer of 2021, symptoms were observed in Lehigh County, Pennsylvania. Again, however, the absence of a specimen prevented formal identification [K. Oten blog]. In 2022, official detections were made in Pennsylvania, North Carolina, Maryland, and New York [NCSU extension]. The New York infestation might have spread naturally across the St. Lawrence River from the Canadian population [https://www.wwnytv.com/2022/10/03/new-pest-affecting-elm-trees-st-lawrence-county/].
In 2023, outbreaks were detected in Ohio, Vermont, and Massachusetts [NCSU extension map]. In 2024, Wisconsin found EZS in several counties [University of Wisconsin extension]. Neither the Canadian Food Inspection Agency nor the [Canadian] Invasive Species Centre reports on EZS’ more recent status in Canada.
Biology
Like all hymenopterans, EZS goes through four life stages: egg, larva, pupa, and adult. Eggs are laid along leaf margins; after hatching, larvae feed on leaf foliage in a zigzag pattern towards the mid-vein. Older larvae consume the leaf more entirely, leaving behind the mid-vein and thicker lateral veins only. The larvae then spin a cocoon in which they pupate. Summer pupae are net-like and attached to leaves or twigs. Overwintering pupae are solid-walled and found in leaf litter or soil. (See the North Carolina State University extension website for photographs showing these stages.) Even early in the summer, some larvae might spin overwintering pupae [K. Oten blog]. Feeding larvae are usually present from May to September [CFIA factsheet]. Adults are able to begin oviposition immediately after emerging. Since the elm zigzag sawfly is parthenogenetic, it reproduces without mating. In fact, male EZS have never been recorded [NCSU extension]. Each female lays between 7 and 49 eggs [University of Massachusetts extension].
The elm zigzag sawfly can complete this life cycle in just 20-36 days when not overwintering (Blank et al. 2010). The number of generations produced per year is uncertain and might depend on environmental conditions. In Europe, two to six generations have been recorded (Blank et al. 2010; Mol and Vonk, 2015). In Virginia, two generations were recorded in 2021, but only one in 2022. In North Carolina, at least four generations were observed in 2023. Lab-reared elm zigzag sawfly have up to seven generations per year [NCSU extension].
EZS tolerates a wide range of climates. Blank et al. (2010) thought it probable that this species could spread into northern and south-western Europe wherever elms grow. In North America, the Canadian Food Inspection Agency expressed concern that EZS would be able to withstand temperatures as low as –30°C, which includes much of Canada [https://www.invasivespeciescentre.ca/first-confirmed-sighting-of-a-new-invasive-in-north-america-elm-zigzag-sawfly/].
EZS’ ability to multiply rapidly and have multiple generations per year suggest populations can build up quickly [https://www.cabi.org/isc/datasheet/118020#tosummaryOfInvasiveness]. Such explosive growth has been observed in North Carolina in 2022 and in Europe several times before [K. Oten blog]. Large populations are capable of severe defoliation and may cause long-term impacts on tree health.
Spread
The adults are strong fliers, capable of spreading 45-90 km (~27-55 mi) per year (Blank et al. 2014). Other characteristics that facilitate spread include
- parthenogenesis: females can reproduce without mating, allowing relatively small numbers to disperse and begin new populations [University of Wisconsin extension].
- all stages are small and inconspicuous.
- potential for multiple generations per year, so populations can build up quickly
- overwintering cocoons have hard sides so can probably with stand inhospitable conditions. Their presence in leaf litter or duff makes it likely that they might be transported with plants in commerce [https://www.cabi.org/isc/datasheet/118020].
- at least when infestations are heavy (e.g., in North Carolina), cocoons were found not just attached to leaves and stems, but also non-living objects, e.g., fence posts, vehicles, firewood. This suggests that pupae could be attached to vehicles or other objects transported long distances [K. Oten blog; University of Wisconsin extension; University of Massachusetts extension].
Probable Hosts
The elm zigzag sawfly has been recorded feeding on many elm species and several hybrids. In Japan, scientists collected sawfly larvae on Ulmus japonica and U. pumila (Blank et al. 2010). In Europe, feeding has been recorded on U. minor, U. pumila and U. pumila var. arborea, U. glabra, and possibly U. laevis (Blank et al. 2010). In the United Kingdom it has been recorded as feeding on several of these species as well as English elm (Ulmus procera) [https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/elm-zigzag-sawfly/].
Documented hosts in North America include Siberian elm (U. pumila); Chinese or lacebark elm (U. parvifolia), Ulmus ‘Cathedral’ Japanese X Siberian hybrid, English elm (U. procera), as well as American elm (U. americana), winged elm (U. alata) [NCSU extension] and slippery elm (Ulmus rubra) [University of Massachusetts extension]. It appears that all species of elm trees native to North America are at risk. EZS outbreaks might also threaten native elm-browsing insects which might be out-competed by elm zigzag sawfly [https://www.growertalks.com/Newsletters/View/?article=3500].
Impact
Defoliation levels vary considerably (all sources). While individual larvae cause little harm, populations can grow quickly and then defoliate large elm trees [NCSU extension; photographs]. In Europe, defoliation has reached 74% or even 100% in some countries. However, in other countries, defoliation rates appear to be much lower (1-2%) [https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/elm-zigzag-sawfly/]. In both North Carolina [NCSU extension] and Massachusetts [UMASS extension] significant/severe defoliation occurred in at least some years.
The impact on tree health is uncertain. In general, defoliation by insects causes little long-term harm to tree health. In Europe, trees severely defoliated by EZS are typically able to re-leaf. Some might suffer branch dieback and/or reduced growth (Blank et al. 2010; Zandigiacomo et al. 2011). However, if trees are heavily defoliated year after year, they might become weakened or stressed, which predisposes them to other pests or results in tree death [NCSU extension; UMA extension]. Tree mortality has not been observed in either Europe (where major outbreaks occur periodically) or the US. Death of individual branches has been observed [University of Massachusetts extension].
Still, state phytosanitary officials expressed concern about the sawfly’s spread at their annual meeting in 2024 [FTC observation]. EZS is one of several pests that are of concern but are not regulated. (They also mentioned beech leaf disease. The general conclusion was that officials should expand education and outreach to both affected industries (e.g., nursery industry) and the public. They should also inform other states of which pests are present on their territory, so the non-affected states would be able to take precautionary actions to try to prevent introduction. Funding to study these organism might be available through the Plant Pest and Disease Management and Disaster Prevention Program (§7721 of the Plant Protection Act).
In both Europe and North America, conservationists fear that elms already severely depleted by Dutch elm disease will be unable to sustain any decline in vigor caused by defoliation (Blank et al. 2010; https://www.cabi.org/isc/datasheet/118020#tosummaryOfInvasiveness https://www.invasivespeciescentre.ca/first-confirmed-sighting-of-a-new-invasive-in-north-america-elm-zigzag-sawfly/).
Management
Since most trees can recover from sporadic or minor defoliation events, the state extension programs say control efforts are often not warranted. Furthermore, since EZS is a new pest species, little is known about effective management. Results of experimental treatments using various pesticides or biopesticides are reported from Hungary (Blank et al. 2010), Romania (Fătu et al. 2020), and Virginia [NCSU extension]. The University of Wisconsin and University of Massachusetts extension service mention several predators present in Europe. The University of Massachusetts extension service notes that the multicolored Asian lady beetle, Harmonia axyridis, which preys on EZS in Serbia, is also present in Massachusetts (actually, it is widespread in the United States); its impact on elm zigzag sawfly in Massachusetts is uncertain.
The extension websites discuss several general-purpose pesticides that are labeled for use on trees and shrubs. The University of Wisconsin website suggests several factors that should be considered in deciding whether to treat EZS infestation and, if so, which strategy to pursue. Several recommend consulting a certified arborist with regard to treating large trees – which are difficult to treat with sprays. The University of Massachusetts reports that chemical management has proved unsatisfactory in Europe because of the rapid buildup of multiple subsequent generations of the sawfly after trees are treated.
Sources
Ashikbayev, N. Z., N. S. Mukhamadiyev, G. Z. Mengdibayeva, M. B. Temirzhanov, and N. K. Kuanyshbaev. 2018. Development of forest entomology in Kazakhstan, pp. 42–47. In T. I. Espolov, K. M. Tireuov, E. I. Islamov, S. B. Baizakov, K. T. Abayeva, E. Z. Kentbaev, and B. A. Kentbaeva (eds.), Actual problems of sustainable development in forestry complex, vol. 2. Aitumar Publishing, Almaty, Kazakhstan.
Blank, S.M., H. Hara, J. Mikulas, G. Csoka, C. Ciornei, R. Constantineanu, I. Constantineanu, L. Roller, E. Altemhofer, T. Huflejt, and G. Vetek. 2010. Aproceros leucopoda (Hymenoptera: Argidae): An East Asian pest of elms (Ulmus spp.) invading Europe. European Journal of Entomology, DOI: 10.14411/eje.2010.045
Blank, S. M., T. Köhler, T. Pfannenstill, N. Neuenfeldt, B. Zimmer, E. Jansen, A. Taeger, and A.D. Liston. 2014. Zig-zagging across Central Europe: recent range extension, dispersal seed and larval hosts of Aproceros leucopoda (Hymenoptera, Argidae) in Germany. J. Hymenopt. Res., 41: 57-74. https://jhr.pensoft.net/articles.php?id=4395
iNaturalist. Available from https://www.inaturalist.org. Accessed August 2022.
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: E1.
Martynoz, V. V., and T. V. Nikulina. 2017. Population surge of zigzag elm sawfly (Aproceros leucopoda (Takeuchi, 1939): Hymenoptera: Argidae) in the Northern Ciz-Azov Region. Russ. J. Biol. Invasions, 8: 25-34.
Mol, A. W. M., and D. H. Vonk. 2015. De iepenzigzagbladwesp Aproceros leucopoda (Hymenoptera: Argidae), een invasieve exoot in Nederland. Entomol. Ber. 75: 50-63.
Papp, V., M. Ladányi, and G. Vétek. 2018. Temperature-dependent development of Aproceros leucopoda (Hymenoptera: Argidae), an invasive pest of elms in Europe. J. Appl. Entomol., 142: 589-597.
Sinon, S. First confirmed sighting of a new invasive in North America: elm zigzag sawfly – Invasive Species Centre. https://www.invasivespeciescentre.ca/first-confirmed-sighting-of-a-new-invasive-in-north-america-elm-zigzag-sawfly/
(United Kingdom) Forest Research Elm zigzag sawfly (Aproceros leucopoda) https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/elm-zigzag-sawfly/
Wu, X. Y. 2006. Studies on the biology and control of Aproceros leucopoda. Plant Prot., 32: 98-100.
Zandigiacomo, P., E. Cargnus, and A. Villani. 2011. First record of the invasive sawfly Aproceros leucopoda infesting elms in Italy. Bull. Insectology, 64: 145-149.
Online sources
Canadian Food Inspection Service Aproceros leucopoda Takeuchi – (Elm zigzag sawfly) – Fact sheet – inspection.canada.ca accessed November 2024
(Canadian) Invasive Species Centre Elm Zigzag Sawfly (Aproceros leucopoda). https://www.invasivespeciescentre.ca/invasive-species/meet-the-species/invasive-insects/elm-zigzag-sawfly/ accessed November 2024
North Carolina State Extension https://content.ces.ncsu.edu/elm-zigzag-sawfly Elm Zigzag Sawfly Invasive Forest Pests accessed November 2024; excellent photographs of both sawfly (eggs, larva, cocoons, adult) and defoliation.
University of Massachusetts extension https://ag.umass.edu/landscape/fact-sheets/elm-zigzag-sawfly accessed November 2024. Lists some of the active ingredients that may be used to manage sawfly caterpillars. Reminds readers always to confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide.
University of Wisconsin-Madison Extension Pest Alert Elm Zigzag Sawfly Authors: PJ Liesch, UW Insect Diagnostic Lab. Last Revised: 8/10/2024 https://hort.extension.wisc.edu/articles/elm-zigzag-sawfly/ accessed November 2024