Elm zigzag sawfly
The confirmed distribution in 2020 in North America was restricted to the South of the province of Quebec, in an area of roughly 350 km2 including the City of Montreal, although it might be more widely distributed. Its native range is in East Asia, but it was discovered in Eastern Europe in 2003, and is now distributed in most European countries.
Damage, symptoms and biology
The elm zigzag sawfly can have more than one generation per year. In Europe, four to six generations have been reported. In the spring, first-generation females lay 7-49 eggs on the edges of elm leaves. Larvae hatch after 4-8 days and start feeding, creating the zigzag pattern on leaves. Older larvae can consume the entire leaf. Larvae go through six instars in 15-18 days, after which, they spin their light-coloured cocoons on the lower surface of leaves to pupate. Adult females emerge 2-3 days later and lay eggs of the next generation. No males have ever been observed and the species reproduces parthenogenetically (i.e., asexually with all eggs being unfertilized and developing into females). Larvae of the last generation of the season fall to the ground and spin a double-lined overwintering cocoon, generally in the leaf litter or the upper soil. Overwintering pupae have a supercooling point between -12°C and -24°C, suggesting that they could survive more southerly Canadian winters.
The defoliation caused by young larvae is easy to recognize: zigzag patterns that start at the leaf margin and continue between two veins inwards towards the midrib. As the larvae get larger, they eat almost the entire leaf, except for the veins, making it more difficult to recognize the telltale zigzag signs later in the season or during severe infestations.
Larvae of the elm zigzag sawfly can entirely defoliate elm trees in Europe, and although this can affect the growth and aesthetics of the host tree, no mortality was reported. Heavily defoliated trees sometimes produce a second batch of leaves late in the season. Stress from defoliation, however, might increase host susceptibility to diseases such as the Dutch elm disease. The extent and severity of damage by this insect in Canada is still unknown.
Cocoons are dimorphic with unique summer cocoons and winter cocoons. The former one is located on the underside of elm leaves and has a thin-walled lattice-like structure, with the pupa visible inside. The latter is found on the ground, and has a solid-walled structure protecting the pupa during winter.
There is currently no management strategy deployed in Canada. In Europe, although insecticides can control larvae, they are typically not deployed because of the ability of the insect to quickly reinvade treated areas. Potential natural enemies of the sawfly in Canada are unknown.
Canadian Forest Service Publications
Diet and feeding behaviour
- Phyllophagous : Feeds on the leaves of plants.
Information on host(s)
In Europe, the insect attacks almost all elm species, both native and exotic, including elm cultivars resistant to Dutch elm disease. During the first year of its discovery in Canada (2020), it was found mainly on American elm (Ulmus americana), but all elms species planted in Canada should be considered as potential hosts.