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Elm zig-zag sawfly (Aproceros leucopoda)

Elm zig-zag sawfly (scientific name Aproceros leucopada) is an insect pest of elm trees (Ulmus species). Its larvae feed on elm leaves, and can defoliate whole trees, making the trees vulnerable to other threats, and depriving other invertebrate species of food. Evidence of its presence was first found in the United Kingdom in 2017, and the first specimens were found in 2018.

Distribution

Elm zig-zag sawfly has been reported from South-East England and the East Midlands, and it could spread further.

Identification

The adult is a small, black, wasp-like sawfly with white legs, and is difficult for non-experts to identify.

The larvae are tiny green caterpillars with a characteristic habit of holding on to the inside edge of the leaf tissues as they feed. Older caterpillars have a stripe on each side of their heads.

The easiest way to identify the pest is to look for the characteristic zig-zag or meandering-river pattern of the leaf damage which the young larvae make as they feed in spring and summer. These patterns are made between the leaf veins. The identification can then be confirmed if one or more larvae can be found in association with the distinctive damage.

Identifying the pest by the damage it causes becomes more difficult as the larvae get older. This is because their feeding damage becomes less distinctive, and destroys the zig-zag patterns made by young larvae, so that it looks similar to the damage made by other leaf-chewing species. Older larvae can devour whole leaves.

Report a finding

If you think you have found elm zig-zag sawfly, or evidence of its presence on elm trees, please report it to the Forestry Commission with the Tree Alert on-line pest and disease reporting tool. You will need to upload one or more photographs.

Spread

The pest could spread short distances at a time through the adults’ flight, and by the wind.

Longer-distance spread of the adult sawflies in or on vehicles is possible. There is evidence of spread along major arterial roads in continental Europe, with populations reported in elm trees at some motorway service stations. The discovery of outbreaks in Europe which are long distances from the nearest known populations also suggests this 'jump spread' by ‘hitch-hiking’ adult sawflies.

Long-distance spread could also occur in the transportation and importation of elm plants for planting. In this case, the pest is likely to be moved in the form of pupae in tiny cocoons a few millimetres long in the leaf litter and soil in the plant pots.

The risk

The larvae (caterpillars) of the elm zig-zag sawfly feed on elm leaves, and can affect all the main elm species grown in Britain.

High populations can strip elm trees bare of leaves. This can threaten the viability of native insect species which rely on elm leaves, such as the white-letter hairstreak butterfly (Satyrium w-album) and the white-spotted pinion moth (Cosmia diffinis). The white-letter hairstreak, for example, depends entirely on elm leaves for its food supply.

The species reproduces by parthenogenesis, that is, males have never been found, and the females produce eggs without first needing to mate with a male. Increasing the risk is that more than one generation can be produced in a year.

Repeated defoliations of trees can weaken them and make them more vulnerable to attack by other pests and diseases, and to environmental stresses such as drought.

Control and management

The most effective measures are:

  • limiting movements of elm plants to the minimum number and distance necessary;
  • inspecting elm plants, and any soil and leaf litter with them, on receiving them, before moving them on, and again before planting them;
  • cleaning and disinfecting equipment, machinery and vehicles used in tree and forestry operations before moving to new sites;
  • destroying elm material arising from tree surgery or felling - on site if possible, or if it is not possible, covering it securely before moving it to a place of destruction or burial; and
  • buying British-grown elm plants rather than risk being party to a further introduction by buying imported plants.

Chemical control by pesticide is likely to have unacceptable impacts on other invertebrate species which are harmless or beneficial. It would also be ineffective in the medium to long term because treated trees will almost certainly be re-infested from nearby populations, making repeat applications necessary. Use of chemical control is therefore likely to be limited to the protection of individual elm trees, or groups of elm trees, of aesthetic or cultural importance.

The prospects

This pest could spread within the UK, and it is thought that it could survive in all parts of the country. Government response at the borders includes comprehensive international surveillance, stringent biosecurity and robust contingency planning to contain threats when outbreaks do occur.

How much damage elm zig-zag sawfly will do in the long term to the UK’s elm trees and their ecological and biodiversity functions is currently unknown.

Further research is needed before any potential for control by natural means, such as predators, can be known.

Background

Elm zig-zag sawfly is a native of Japan and parts of China which has spread to Europe in recent decades, most likely with imported plants for planting. It was first confirmed in continental Europe in 2003, in Poland and Hungary, and has since been found in several other European countries, including Belgium and the Netherlands. It has not yet been confirmed in our nearest neighbour, France.

Further information

More-detailed information about elm zig-zag sawfly, including its biology, lifecycle, and European distribution, can be found at the following resources.

Related pages

 

Last updated: 6th September 2018