Elm zigzag sawfly
The elm zigzag sawfly (Aproceros leucopoda), named for the distinctive feeding pattern of its newly emerged larvae, was recently found in Canada. It was discovered in Sainte-Martine Quebec in July 2020 by a nature photographer, who published a photo of an elm leaf with a distinctive feeding pattern on iNaturalist. From there, it was quickly identified, and the CFIA confirmed its presence in Quebec. Since 2020, the sawfly has been found in numerous locations extending from Quebec City to just east of Toronto.
Females start laying eggs immediately after emerging and lay up to 50 eggs singly into the leaf margin. Larvae hatch 4-8 days later, their early feeding creating the distinctive zigzag feeding pattern (see photo in sidebar). As larvae mature, they feed more heavily, consuming the entire leaf except for the mid-rib. Larvae develop over 15-18 days, then pupate on the underside of the leaf, twig or shoot. They emerge as adults 4-7 days later.
The elm zigzag sawfly reproduces parthenogenically (i.e., without mating) and, to date, only females have been found. Because they do not need to mate to reproduce and because their short lifecycle allows for multiple generations per year, populations can build up rapidly.
Trees at risk
As its name suggests, elm zigzag sawfly feeds exclusively on elms, and it will attack any age of elm. It is found in temperate deciduous forests as well as urban forests.
When environmental conditions are favourable and the sawfly population is high, they can cause severe defoliation which may cause crown die-back. With repeated feeding over time, tree vigour may be reduced, and it becomes more susceptible to secondary pests.
The sawfly is native to eastern Asia. It is introduced in Europe, where it has been spreading since at least 2003. It is present in Quebec and Ontario and several northeastern US states.
The most distinctive evidence of the sawfly is zigzag feeding channels located between the veins of the leaf, running from the leaf edge inward toward the midrib. On close examination, larvae may be found feeding in the channel. As larvae mature, their feeding damage is more extensive, and they can obliterate the zigzag pattern made by the younger larvae. Crown flagging and die back follow severe defoliation.
Eggs are tiny and difficult to detect. New larvae are grayish-white and about 1.8mm long. Older larvae are green with T-shaped black spots on their front legs and a stripe on each side of their head that crosses their eyes (sidebar). The mature larvae are up to 11mm long. Summer pupae are loosely constructed, and the insect is visible within; they are located the bottom of leaves or twigs. Overwintering pupae have a denser covering and they are located in the leaf litter or soil. Adults look like a typical sawfly (sidebar) and are up to 7mm long. The thorax is yellow to brown with a white patch on the bottom, and the legs are yellow with white tarsi. Adults can be seen from mid-April to mid-September and larvae from late May to mid-October.
What you can do
If you think you have found elm zigzag sawfly or its damage, report your finding on iNaturalist.