Elm zigzag sawfly
- French common name: Tenthrède en zigzag de l’orme
- Other common names: Elm sawfly
- Scientific name: Aproceros leucopoda Takeuchi
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Argidae
General information and importance
The elm zigzag sawfly is native to east Asia. It was discovered in 2003 in Europe, and in 2020 in North America. It was first discovered by a citizen in Quebec, Canada. Molecular analyses suggest this Canadian population came via Europe but the pathway for introduction is unknown. It attacks healthy elm (Ulmus) trees, apparently irrespective of elm species, size, age or habitat. Urban trees seem to be particularly susceptible. Elm zigzag sawfly can reduce tree growth, but no tree mortality has been reported.
Distribution and hosts
The confirmed distribution of elm zigzag sawfly in 2023 in North America was from southern Quebec and Ontario in Canada and in Vermont, New York, Pennsylvania, Ohio, Virginia, North Carolina, and Maryland in the northeastern United States. It is also widely distributed throughout much of Europe. It attacks most species of elm throughout its native and introduced ranges, including elm cultivars resistant to Dutch elm disease.
Tree parts affected
Leaves
Symptoms and signs
The defoliation caused by young larvae is easily recognized. The insect creates zigzag patterns in the leaf that start at the leaf margin and continue between two veins inwards towards the midrib. As the larvae get larger, they eat almost the entire leaf except for the veins, making it more difficult to recognize the telltale zigzag signs later in the season or during severe infestations. Skeletonized leaves are also considered diagnostic.
Adult elm zigzag sawflies are dark brown or black, wasp-like insects about 5 millimetres long, with yellow to white legs. Larvae resemble small caterpillars. Young larvae are green with prolegs and a globular light green head capsule, as well as a pair of appendages on their tail-end. Older larvae have T-shaped brown marks on their second and third pairs of legs and a lateral dark stripe on both sides of their head capsule. During pupation there are distinct cocoons: those of summer, and those used for overwintering. Cocoons formed in summer are located on the underside of elm leaves or on nearby above-ground structures such as fences. These cocoons are thin-walled and lattice-like with the pupa visible inside. Cocoons formed later in the season for overwintering are found on the ground and are solid-walled and opaque to protect pupae during winter.
Life cycle
The elm zigzag sawfly can have several generations per year. Four to six generations have been reported in Europe. First-generation females lay 7 to 49 eggs on the edges of elm leaves in the spring. Larvae hatch in 4 to 8 days and feed, resulting in the characteristic zigzag pattern on leaves. Older larvae can consume the entire leaf. The feeding period lasts 15 to 18 days, after which, mature larvae spin their light-coloured cocoons on the lower surface of leaves to pupate. Adult females emerge 4 to 7 days later and lay eggs of the next generation. No males have ever been observed, so the species reproduces asexually (parthenogenesis) with all eggs being unfertilized and developing into females.
Damage
Larvae of the elm zigzag sawfly can entirely defoliate elm trees. Although this can affect the growth and aesthetics of the host tree, no mortality has been reported, likely because severely defoliated elms can produce a second flush of leaves later in the season. However, repeated stress from defoliation throughout the growing season will weaken trees and possibly increase their susceptibility to other insects and pathogens. To date in Canada, the extent and severity of damage by this insect has been generally low.
Prevention and management
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
There are currently no management strategies deployed in Canada against this sawfly. In Europe, although insecticides can control larvae, they are typically not deployed because of the ability of the insect to quickly reinvade treated areas. Generalist predators, such as birds and true bugs, have been observed in Canada feeding on this insect.
Photos
Summer cocoon
Sébastien Bélanger
Larva
Olivier Morin
Larva
Olivier Morin
Larva
Olivier Morin
A zigzag pattern created by a young larva, visible at the end of the zigzag
Véronique Martel
Selected references
Blank, S.M.; Hara, H.; Mikulás, J.; Csóka, G.; Ciornei, C.; Constantineanu, R.; Constantineanu, I.; Roller, L.; Altenhofer, E.; Huflejt, T.; Vétek, G. 2010. Aproceros leucopoda (Hymenoptera: Argidae): an east Asian pest of elms (Ulmus spp.) invading Europe. European Journal of Entomology 107: 357–367. https://doi.org/10.14411/EJE.2010.045
Blank, S.M.; Köhler, T.; Pfannenstill, T.; Neuenfeldt, N.; Zimmer, B.; Jansen, E.; Taeger, A.S.; Liston, A.D. 2014. Zig-zagging across central Europe: recent range extension, dispersal speed and larval hosts of Aproceros leucopoda (Hymenoptera, Argidae) in Germany. Journal of Hymenoptera Research 41: 57–74. https://doi.org/10.3897/JHR.41.8681
Martel, V.; Morin, O.; Monckton, S.K.; Eiseman, C.S.; Béliveau, C.; Cusson, M.; Blank, S.M. 2022. Elm zigzag sawfly, Aproceros leucopoda (Hymenoptera: Argidae), recorded for the first time in North America through community science. The Canadian Entomologist 154(1): e1. https://doi.org/10.4039/tce.2021.44
Oten, K.L.F.; Day, E.; Dellinger, T.; Disque, H.H.; Barringer, L.E.; Cancelliere, J.; Somers, L.; Bertone, M.A. 2023. First records of elm zigzag sawfly (Hymenoptera: Argidae) in the United States. Journal of Integrated Pest Management 14(1): 12. https://doi.org/10.1093/jipm/pmad009
Vétek, G.; Csávás, K.; Fail, J.; Ladányi, M. 2021. Host plant range of Aproceros leucopoda is limited within Ulmaceae. Agricultural and Forest Entomology 24(1): 1–7. https://doi.org/10.1111/afe.12463
Vétek, G.; Fekete, V.; Ladányi, M.; Cargnus, E.; Zandigiacomo, P.; Oláh, R.; Schebeck, M.; Schopf, A. 2020. Cold tolerance strategy and cold hardiness of the invasive zigzag elm sawfly Aproceros leucopoda (Hymenoptera: Argidae). Agricultural and Forest Entomology 22(3): 231–237. https://doi.org/10.1111/afe.12376
Zandigiacomo, P.; Cargnus, E.; Villani, A. 2011. First record of the invasive sawfly Aproceros leucopoda infesting elms in Italy. Bulletin of Insectology 64(1):145–149. https://www.researchgate.net/publication/265467634_First_record_of_the_invasive_sawfly_Aproceros_leucopoda_infesting_elms_in_Italy