Introduction
The elm zigzag sawfly, or EZS, is an invasive pest that defoliates native and non-native elm species. It originates from East Asia but has been a problem on elms in Europe since 2003. Elm zigzag sawfly was detected in North America in 2020 when it was discovered in Québec, before being confirmed in the United States in Virginia in 2021.
Figure 1: The range of elm zigzag sawfly in the U.S.
(Map produced by Kelly Oten, North Carolina State University)
Identification
Elm zigzag sawfly eggs are 0.8-1.0 mm long and are found on the leaf margins. They start out as a blue green color but turn black right before hatching. Larvae are about 1.8 mm long and can grow up to 10-11 mm long. They begin as a grayish white color after hatching but turn green with a black band on their head as they mature. They also develop T-shaped black markings on their second and third pair of thoracic legs. They pupate in cocoons, and these cocoons can be found attached to the undersides of leaves, on branches, or in the leaf litter or soil. Adults are a small (7-8 mm long), wasp-like fly. It is shiny black in color with a white patch on the bottom of the thorax, smoky brown wings, and yellow legs.
Elm zigzag sawfly larvae have a black band on their head and T-shaped marking on their legs.
Photo by Matt Bertone, North Carolina State University
Elm zigzag sawfly adults look like small black flies.
Photo by Matt Bertone, North Carolina State University
Life Cycle
Elm zigzag sawflies do not need males to reproduce, and no male EZS has ever been found. They reproduce through parthenogenesis, meaning that females can reproduce asexually. They can have many generations per year, but the number varies, and not much is known about the EZS lifecycle in the U.S. They can have four to six generations per year in Europe, but in the US, two to five generations have been observed.
Each EZS female lives between 1-6 days and can lay up to 60 eggs. Larvae emerge within eight days and begin feeding on elm leaves, creating the characteristic zigzag pattern. Larvae go through six instars in a matter of 15-18 days before pupating and creating a cocoon. Two different types of EZS cocoons are possible. Cocoons created during summer months are loosely woven and net-like, commonly attached to leaves. The cocoons created for overwintering are denser and a solid-walled cocoon often found in the leaf litter or soil. After cocoon creation, summer adults emerge within 10 days. Those that create an overwintering cocoon remain in the cocoon until they emerge the following spring.
Elm zigzag sawfly summer cocoons are loosely constructed.
Photo by Kelly Oten, North Carolina State University
Cocoons can be attached to the underside of leaves (here with a quarter for size reference).
Photo by Eric Day, Virginia Tech
Hosts
Larvae of EZS feed entirely on elm leaves (Ulmus spp.). In the U.S., EZS has been observed feeding on American elm (U. americana), winged elm (U. alata), slippery elm (U. rubra), Siberian elm (U. pumila), Chinese elm (U. parvifolia), English elm (U. procera), and the U. × ‘Cathedral’ (Japanese × Siberian elm hybrid). Anecdotally, EZS preferentially feeds on non-native elm species more readily than our native elms. See HGIC 1011, Elm, for more information about the elms of South Carolina.
Signs of Damage
EZS are defoliators, so their damage targets the tree’s leaves. The most obvious sign of EZS is the characteristic zigzag pattern created on elm leaves by the feeding of young larvae. Feeding by older EZS larvae is broader and typically removes the zigzag pattern produced by younger larvae. Large infestations can defoliate entire trees.
Figure 4: Signs of elm zigzag sawfly damage include a zigzag pattern on leaves (A), broader feeding damage characteristic of older larvae (B), and host tree defoliation (C)
Kelly Oten, North Carolina State University
Economic and Ecological Importance
Because EZS is a defoliator, infestations can affect the health of individual elm trees. Defoliation events can weaken trees and predispose them to other pests and diseases. However, long-term health impacts from repeated EZS defoliation are unknown. In Europe, where EZS is an invasive species, it has been observed outcompeting native butterfly and moth species that feed on elm, leading to a decline in their populations. There is potential that EZS could similarly affect North American species.
Management
Currently, little is known about management for EZS. Pesticide application has had some success in European infestations but only for individual trees or small stands. Ongoing research suggests systemic insecticides can reduce EZS populations, but with EZS being relatively new to North America, we still have a lot to learn about the management of this species.
Preventing the Spread
Monitoring elm trees plays a key role in preventing the spread of EZS. If an insect is suspected to be EZS, capture the insect or take a picture and report it to your local Clemson Extension agent or to Clemson DPI.
Resources
- https://content.ces.ncsu.edu/elm-zigzag-sawfly
- https://forestry.ces.ncsu.edu/2022/09/elm-zigzag-sawfly-found-in-nc-for-the-first-time/
- https://www.invasivespeciescentre.ca/invasive-species/meet-the-species/invasive-insects/elm-zigzag-sawfly/
- https://dof.virginia.gov/wp-content/uploads/FHR-2021-12_pub.pdf
Originally published 04/23
