Elms Under Attack: How to Spot and Manage the Invasive Zigzag Sawfly

Introduction

The invasive elm zigzag sawfly (EZS, Aproceros leucopoda; Figure 1), also known as East Asian sawfly, elm sawfly, and zigzag sawfly, is a small wasp native to Japan, Russia, China, and the Korean peninsula. In 2003, EZS was accidentally introduced to Europe. Since then, it has spread rapidly to many countries in Europe.

In the United States, EZS has been reported in nine states, where it continues to pose a threat to elm trees. EZS larvae exclusively feed on the foliage of elm trees (Ulmus spp.), creating distinctive zigzag feeding patterns on the leaves (Figure 2). Although EZS is considered a minor pest in Asia, it has caused severe defoliation on elm trees in Europe and the United States.

Distribution

The EZS, native to East Asia, was first detected in North America in Québec, Canada, in the summer of 2020 when a Canadian citizen uploaded the feeding damage to the iNaturalist website. In the United States, it was first found on Siberian elm (U. pumila) trees in Frederick County near Winchester, Virginia, in May 2021. It has since been detected in many states, including Pennsylvania, North Carolina, Maryland, and New York in 2022, and Massachusetts, Ohio, and Vermont in 2023. By 2024, it also was found in Minnesota. It is anticipated that EZS will continue to spread across the elm tree-growing regions of the United States.

Several elm species occur in Georgia, especially in the northern parts of the state. The major species of elm found in urban areas and upland forests of Georgia are American elm (U. americana), winged elm (U. alata), Chinese elm (U. parvifolia), and slippery elm (U. rubra). Chinese elm or lacebark elm (U. parvifolia) is widely planted in urban areas, especially parking lots and residential and public landscapes. Native elms, such as American elm and winged elm, are found in the foothills and mountainous regions of North Georgia, where air temperatures are slightly cooler. These regions are particularly at risk from elm zigzag sawfly infestation.

Description

Adult EZS is 7–8 mm long, with a shiny black body, pale legs, and smoky-brown wings (Figure 1). Eggs, which are laid into the serrated edges of elm leaves, are light green and less than 1 mm across.

Young larvae are green and are easily identifiable by the zigzag pattern they create after feeding. Older larvae have a lateral stripe on each side of their head capsules that crosses their eyes (Figure 3, red arrow). The second and third pairs of true legs of older larvae have brown, sometimes incomplete, T-shaped markings (Figure 3, blue arrow).

Older stage larvae are 10–11 mm long. When ready to pupate, they make cocoons.

Life Cycle

Adult female elm zigzag sawflies reproduce by laying unfertilized eggs. All the unfertilized eggs develop into females. Males are not reported for this species. This phenomenon, called parthenogenesis, is a natural form of asexual reproduction where an embryo develops from an unfertilized egg.

Females begin to lay eggs as soon as they emerge from the pupal stage. They lay up to 49 eggs singly along the serrated margins of elm leaves. Eggs hatch within 4–8 days. Larvae feed for 10–18 days and progress through six larval development stages (instars) before pupating.

EZS larvae pupate in net-like or solid-walled cocoons. During the summer, cocoons are loosely woven and net-like, and typically attached to leaves (Figure 4), whereas overwintering cocoons are denser, with solid walls, and are often found in leaf litter or soil.

The summer adults emerge within 10 days of cocoon formation, whereas overwintering individuals remain in their cocoons until the following spring and then emerge. The number of generations per year varies by region, with four to six generations reported in Europe. In Virginia, two generations were observed in 2021 and one in 2022. This suggests that the number of generations is not completely determined in the United States.

Damage and Symptoms

EZS could threaten elm trees in both urban landscapes and forested regions. The EZS larvae are voracious defoliators, causing serious damage to the elm trees they infest. Initially, the larvae create a distinctive zigzag pattern (Figure 2) when feeding on leaves. However, as feeding progresses, the zigzag pattern disappears when they start eating the entire leaf, leaving only the midrib behind (Figure 5). Severe infestations can result in complete defoliation of trees. Over time, heavily infested trees may experience branch dieback and stunted growth in subsequent seasons. An infestation of EZS rarely kills a tree.

Host Plants

Elm zigzag sawfly infestations can occur on native and commercially grown elms (Table 1).

Table 1. Host Plants of Elm Zigzag Sawfly.

Common Name	Scientific Name
American elm	U. americana
Winged elm	U. alata
Chestnut-leafed elm	U. castaneifolia
Japanese elm	U. davidiana
Mountain elm	U. glabra
Hairy-fruited Glaucescent elm	U. glaucescens
Dutch elm	Ulmus × hollandica
Manchurian elm	U. laciniata
Russian white elm	U. laevis
Hebei elm	U. lamellose
European field elm	U. minor
Chinese elm	U. parvifolia
English elm	U. procera
Dwarf elm/ Siberian elm	U. pumila
Slippery elm	U. rubra
Hybrid elm	hybrid Ulmus × ‘Cathedral’

Monitoring

Both yellow and fluorescent yellow sticky “cloak” traps have proven effective in capturing large numbers of adult elm zigzag sawflies. The adult EZS can spread up to 28–56 miles every year. The EZS can tolerate cold temperatures, with a supercooling point ranging from 10.4 to -11.2 °F in Europe, allowing it to survive harsh winters.

Solitary elms in urban landscapes, newly planted trees, and those near roads or railways are particularly vulnerable to infestation. These characteristics make the EZS an excellent target for detection through community science initiatives.

Management

Currently, there is limited research available on the effective management of the elm zigzag sawfly. In Europe, the application of deltamethrin and teflubenzuron successfully reduced first-generation larvae. Similarly, field trials with the entomopathogenic fungus Beauveria bassiana reduced larval populations. Bioassays using foliar-applied dinotefuran also killed high numbers of larvae.

Management techniques against other sawfly species may also be effective on EZS. For example, applying insecticidal soaps or horticultural oils when larvae are small can help suppress their infestation. Physical methods, such as hand-picking larvae or using a high-pressure water spray to dislodge larvae from trees, are simple yet effective tactics for mild infestations.

Report a Sighting

We welcome reports of suspected sightings of elm zigzag sawfly to help us monitor its distribution and impacts. Please report it to the Georgia Department of Agriculture (https://agr.georgia.gov/form/contact-us?src=pest-control) or your county UGA Extension agent. You also may report sightings in Georgia using UGA’s EDDmapS pest reporting tool (https://www.eddmaps.org/). Please include clear photos when submitting a report online.

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