Elm Zigzag Sawfly

Elm zigzag sawfly have four physical forms through their lifecycle: egg, larva, pupa and adult fly.

• Egg – approximately 0.8 – 1.0 mm long by 0.4 – 0.5 mm wide; blue green in colour until right before hatching they turn black. The eggs are found attached to the leaf margin at the tip of theleaf tooth.
• Larva – initially grayish white and approximately 1.8 mm long by 0.3 mm wide; mature larvae are green with black spots near breathing openings and one triangular black spot on their upper back near the 3^rd body segment; they grow up to 10 – 11 mm long by 1.4 – 1.5 mm wide.

 

Larval Aproceros leucopoda feeding on an elm leaf. Photo by Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

• Pupa – are encapsulated in cocoons that are attached to undersides of leaves, branches, or shoots. In summer, the cocoon is loosely woven and light; overwintering cocoons are dense and found in the duff (decomposing organic material layer) on the ground. During the winter months, the dormant pupae often go unnoticed, buried just beneath the duff and leaf litter.

Net-like summer cocoon of an Aproceros leucopoda eonymph. Photo by Danail Doychev, University of Forestry Bulgaria.

• Adult – shiny black wasp-like fly without wasp waist, approximately 6 – 7 mm long; upper lip is dark brown with a dark yellowish-brown thorax, there is a white patch on the bottom of the thorax, yellow legs ending in white tarsi, with smoky brown wings.

Elm zigzag sawfly reproduces parthenogenetically – meaning that the female reproduces asexually – producing up to four generations per year in its home range but has been known to produce six generations in Europe (Zandigiacomo et al. 2011, Mol and Vonk 2015, Papp 2018). Adult females live for 1 – 6 days and can lay eggs as soon as they emerge from their cocoon. Females lay 7 – 49 eggs along the serrated margin of the elm leaf. After 4 – 8 days, larva emerge and begin to feed on the leaf leaving behind the signature ‘zigzag’ pattern. Once the larva develops through the six larval instars over 15 – 18 days, the life stage in which they consume the entire leaf, it will enter the pre-pupal stage. Once the cocoon has been spun and attached to a structure, pupation will occur over 2 – 3 days and adults emerge 4 – 7 days later.

Elm zigzag sawfly feed and develop on elm trees across a variety of natural and urbanized habitats in its invasive range. It has been noted that populations may decline in abundance with increasing altitude (Zubrik et al. 2017). It occupies temperate deciduous forest zones in the mid-latitudes, ranging from 34° S to 59° N. In winter months, elm zigzag sawfly can withstand temperatures as low as – 30° C, increasing the threat to Canadian forests as the pest is well adapted to overwinter. (CABI Datasheet).

The signature ‘zigzag’ pattern is key to identifying this species in the early stages of the larval form. Since six generations per year are possible, larvae can be found during the months of May through October. Elm zigzag sawfly can go unnoticed for a long period of time after introduction for several reasons; early infestations are often difficult to detect as defoliation is less noticeable in low densities. If found in the later larval stages, elm zigzag sawfly can be mistaken as their signature ‘zigzag’ pattern on elm leaves may have been obscured by further feeding (Vetek et al. 2017). Distinguishing between leaf-miner species and larval elm zigzag sawfly can be challenging as they can produce similar feeding channel patterns. Refer to the photos below for differences between leaf-miner species and elm zigzag sawfly. Their loose, summer lattice-like cocoons are also key indicators as they are distinctive and can be found on the underside of eaten elm leaves. Severe defoliation and branch dieback are also symptoms of significant populations of elm zigzag sawfly.

Leaf damage caused by a larval EZS, this is the only species that makes this distinct pattern on elm leaves. Photo by Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org.

Look-a-like leaf damage caused by an Aspen leaf-miner (Phyllocnistis populiella), feeding occurs within the leaf and not all the way through like that of EZS. Photo by William M. Ciesla, Forest Health Management International, Bugwood.org.

Elm zigzag sawfly is native to Asia and has been present in Europe since 2003, where it has caused minor ecological and economic impacts. It was first detected in North America in July 2020 in Quebec.

Ecological Impacts
Elm zigzag sawfly is a proven highly competitive elm specialist which could impact native elm browsers. In the United Kingdom, these impacts have been observed on native moth and butterfly species including Cosmia diffinis and Satyrium walbum (CABI 2019). North America is home to its own native elm feeding insects which could be outcompeted by elm zigzag sawfly. Significant branch die-back caused by the pest can contribute to weakened overall tree health, and potentially tree mortality. This has potential to further exacerbate the decline of elm trees in conjunction with other pressures including Dutch elm disease (Ophiostoma ulmi).

In Europe where the species has seen wide-spread infestations, there is concern over competition between elm zigzag sawfly and native elm foliage-feeding species. Elm trees previously impacted by Dutch elm disease are now seeing defoliation rates of 74-90%, and complete defoliation in some areas (Forest Research, Government of the United Kingdom).

Economic & Social Impacts

Elm is a commonly planted ornamental tree, especially in urban settings due to their rapid growth and minimal pruning requirements. Severe defoliation occurrences are rare but can contribute to a decline in aesthetic value in certain settings which could cause a minor decline in land value.

In its native range, elm zigzag sawfly is considered a mild pest, natural predators and parasitoids are known but are relatively unsuccessful at containing the populations.

Pesticide application has been effective in controlling larva in some parts of Europe, but re-infestation is likely and year-over-year management is required. Therefore, current pesticide applications are targeted at individual trees or small stands, (Forest Research, Government of the United Kingdom).

Researchers at the Canadian Forest Service are currently working to determine the origin of the detection in Quebec. While the Canadian Food Inspection Agency in collaboration with its partners are trying to determine the extent of the infestation and the appropriate regulatory response. Canadians play an important role in early detection and reducing the spread of elm zigzag sawfly. Monitoring your elm trees for the characteristic ‘zigzag’ larval feeding pattern and reporting any findings, as well as limiting the movement of firewood are all important steps to help prevent further infestations.

Community Action

With only one confirmed detection of this species in North America, EZS is in the early species arrival phase. The prevention of this species expanding across the continent starts with early detection and monitoring. Raising awareness about this species presence is the first step in protecting Canada’s elm trees.

If you think you see the elm zigzag sawfly, there are several ways to report

• Report suspected sightings to the Invading Species Awareness Hotline at 1-800-563-7711.
• Send a photo of your sightings to the Canadian Food Inspection Agency, including your location of sightings to Surveillance@inspection.gc.ca.

Leaf damage caused by a larval EZS, this is the only species that makes this distinct pattern on elm leaves. Photo by Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org.