Introduction
The elm zigzag sawfly, or EZS, (Aproceros leucopoda) is an invasive pest that defoliates native and nonnative elm (Ulmus) species. Originally from East Asia, this pest has affected elms in Europe since 2003. Elm zigzag sawfly spread to North America in 2020 when it was discovered in Quebec, Ontario, Canada, before further spreading to the US with the first observation in Virginia in 2021. This pest was also found in New York, Virginia, Maryland, and, most recently, North Carolina in 2022.
Identification
Elm zigzag sawfly eggs are 0.8-1.0 mm long and are found on the leaf margins. They start as a blue-green color but turn black right before hatching. The larvae are 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 as they mature with a black band on their head. They also develop T-shaped black markings on their second and third pair of legs. They make cocoons to pupate, which can be found attached to the undersides of leaves, branches, or the ground. Adults are a small (7-8 mm long), stingless wasp-like fly. It is shiny black 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 reproduce through parthenogenesis, meaning females can reproduce asexually. In fact, no male EZS has ever been found. Elm zigzag sawflies can have several generations per year, but this number varies, and little is known about the EZS lifecycle in the US. For example, they can have four to six generations per year in Europe, but two generations per year were observed in Virginia in 2021.
Each EZS female lives between 1-6 days and can lay up to 60 eggs. Eggs hatch within eight days, and larvae begin feeding on elm leaves, creating a characteristic zigzag pattern. Larvae go through six instars (life stages) in about 15-18 days before pupating and creating one of two different types of cocoons: those created during summer months are loosely woven and net-like, commonly attached to leaves, while those created for overwintering are denser with solid walls, and often found in the leaf litter or soil. For the non-overwintering generations, adults emerge within ten days after cocoon creation.
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
Elm zigzag sawfly larvae feed on elm leaves (Ulmus spp.). American elm (U. americana) and winged elm (U. alata) have been reported to be affected in the North Carolina infestation. Still, all elms seem to be susceptible hosts, with Siberian elm (U. pumila), Chinese elm (U. parvifolia), Ulmus ‘Cathedral’ Japanese x Siberian hybrid, and English elm (U. procera) all being reported as hosts in the Virginia EZS infestation. For more information about the elms of South Carolina, see HGIC 1011, Elm.
Signs of Damage
Elm zigzag sawflies are defoliators, meaning they feed on tree 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 typically removes the zigzag pattern produced by the younger larvae and consumes every part of the leaf except the large veins. Large infestations can defoliate entire trees.
Signs of elm zigzag sawfly damage include a zigzag pattern on leaves (A), broader feeding damage characteristic of older larvae (B), and whole tree defoliation (C) (Photos by Kelly Oten, North Carolina State University)
Economic and Ecological Importance
Elm zigzag sawfly infestations can affect the health of individual elm trees. Defoliation events can weaken trees and predispose them to other pests and diseases. If defoliation occurs several years in a row, it can contribute to tree death. In Europe, EZS 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 moth and butterfly species.
Management
Currently, little is known about management for EZS. Insecticides have had some success in European infestations but only for individual trees or small stands. With EZS being relatively new to North America, we still have a lot to learn about managing this species. Insecticides labeled for use on other defoliators are likely to work for EZS management, but none are currently labeled for use on EZS in the US. Small infestations can be picked off by hand if they are reachable or possibly removed with a strong stream of water from a hose.
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. For more information about the invasive species of South Carolina, see Clemson Regulatory Services list of invasive species.
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
