The Elm Zigzag Sawfly, Aproceros leucopoda, has now been confirmed in Britain following a rapid expansion across Europe from eastern Asia. This page briefly describes the biology of the species, the risk it poses to elms in Britain, and its identification. To report a sighting of this species, please use Tree Alert. All information current as of June 2018.
First recorded in Europe in 2003, the elm zigzag sawfly has spread rapidly throughout Europe, eventually being identified in Britain in 2017. The species specialises on elms (Ulmus spp.) and appears to feed on all three elms commonly found in Britain: U. procera (English elm), U. glabra (wych elm) and U. minor. Eggs are laid by the adults into the serrations at the edge of elm leaves and the larvae hatch within 4-8 days. The larvae proceed to feed on the leaf tissues between the main leaf veins and whilst small, they produce a characteristic ‘zigzag’ pattern of feeding damage. Later, the feeding damage becomes more extensive and some of the smaller veins are eaten. Competition with other larvae and the wide variation in the size of elm leaves means that although feeding traces formed by young larvae are readily identifiable, those formed by larger larvae are less straightforward to diagnose.
A young elm zigzag sawfly larva beginning to produce the characteristic feeding pattern
The larvae pass through 4-7 instars (growth stages) in about 15-18 days and then build either a loose silk cocoon on the underside of a leaf or, later in the year, a stronger solid-walled cocoon that will protect the sawfly over the winter. During the summer, the adult sawflies emerge from the cocoon within 4-7 days. At higher temperatures, e.g. 24oC, the life cycle is completed in 23-24 days, but at lower temperatures the life cycle takes longer. For example, at 11oC complete development requires 85 days. Multiple generations are produced during the summer, although we do not know yet how many generations the sawfly may be able to complete per year in the British climate. No males have been recorded for this species and therefore the sawfly appears to reproduce exclusively by parthenogenesis.
Multiple feeding traces
Zigzag sawfly in Britain
Elm zigzag sawfly was first identified near Dorking in Surrey in 2017. In June this year, 2018, further reports have come in from across a wide area of south east England and the east Midlands. The full extent of the sawfly’s distribution is not yet known, but it is expected that it will continue to spread. How the sawfly made it to Britain is also unknown. Natural spread across the channel seems unlikely for such a small insect, but is possible, and human-assisted movements, e.g. in or on vehicles, may also have played a part.
The current known distribution of elm zigzag sawfly in Britain based on reports submitted to Forest Research (as of 25.07.2018)
The risk posed by elm zigzag sawfly
Elm populations have not recovered in Britain following the introduction of Dutch elm disease, but those that remain in hedgerows and field margins still support a large diversity of insects, most notably the rare white-letter hairstreak butterfly (Satyrium w-album) and white-spotted pinion moth (Cosmia diffinis); elm thus remains an ecologically important tree. Elm zigzag sawfly has the potential to become a major competitor of other foliage-feeding species on elm, but trying to predict the amount of damage it will do in Britain is difficult. In some areas of Europe, the sawfly has been reported as causing severe defoliation (74-98%) or even complete defoliation, but in other countries (such as Bulgaria) defoliation rates appear to be much lower (1-2%). There are no records of trees being killed by the sawfly, although severe defoliation may lead to some dieback of shoots and branches. Britain has a cooler climate compared with the parts of continental Europe where significant defoliation has been recorded, and it may be that the sawfly’s life cycle will be slower here and populations less damaging. The impact of predators and parasitoids is also largely unknown. There are very few observations of predation on the sawfly’s larvae or pupae, and the parasitoid fauna associated with the sawfly is poorly understood.
Elm zigzag sawfly has not yet caused high levels of defoliation in Britain, but as the species population builds up this situation may change. Pesticide treatments have proved an effective means of controlling the larvae in other countries, but the pesticides available for use in the UK are not specific to the sawfly larvae and will kill other insect species. Therefore, pesticide applications are likely to have significant effects on other invertebrates inhabiting the elm trees. The few elms in Britain which are preserved as ornamentals, and do not support rare, native invertebrates, may require the sawflies to be treated with a pesticide to preserve their appearance, but this would not be a long-term solution to the problem.
Feeding traces of mature larvae
The zigzag feeding pattern produced by the young larvae is characteristic and simple to diagnose, especially if the larva is present. The feeding damage caused by older larvae can be more difficult to identify, as the older larvae feed in a more general manner and the zigzag pattern is less obvious. The adult sawfly probably emerges in Britain in April after the winter, and the first damage may be noticeable soon after this time.
Larvae – young larvae are a near-uniform green and are easy to find if present within a zigzag track due to their habit of holding on to the inside edge of the leaf tissues. Older larvae often destroy the original zigzag and they feed more extensively, and damage at this stage can look similar to that produced by other leaf-chewing insects. The older elm zigzag sawfly larvae have a lateral stripe on either side of their head capsule which crosses their eyes, and their second and third pairs of true legs have a brown, sometimes incomplete, ‘T’-shaped mark.
Mature zigzag sawfly larva
Feeding damage – Starting from the outside of the leaf, the young larva begins to feed toward the central leaf vein, winding its way sinuously between the leaf veins. As the larva grows the width of the feeding trace increases. Eventually the larva stops, and moves back toward the outer edge of the leaf to the next leaf vein intersection where it begins to feed in a more ‘normal’ leaf-chewing fashion without the original zigzag. When fully grown, the larva leaves the feeding trace and wanders a short distance to find a secure place to pupate.
Feeding trace from a young larva
Feeding trace from a more mature larva
Multiple feeding traces produced by larvae of different ages
Pupae – after spinning a lattice-like silken cocoon, the larva moults into a pupa before undergoing ecdysis and becoming an adult. Though an isolated pupa would be difficult, if not near-impossible, to identify as an elm zigzag sawfly, a pupa within a lattice-like cocoon on the underside of an elm leaf is almost certainly of this species.
Pre-pupa in the cocoon
Adults – are difficult to identify conclusively without taxonomic expertise and a microscope. Generally, their appearance is that of a small, black sawfly with white legs. Upon finding a suspected adult, check nearby for elm trees, and look for the feeding traces of larvae, or pupal cocoons.
Adult elm zigzag sawfly ovipositing
Feeding damage to elms by other species
Elms harbour a large diversity of herbivorous invertebrates, many of which may produce feeding damage similar to that produced by elm zigzag sawfly, particularly the more general damage produced by the mature larvae. Leaf miners, which feed between the leaf surfaces (‘internal chewers’), produce a darkened feeding area, but don’t eat through the leaf itself. The various leaf feeders (‘external chewers’) which utilise elm often remove small chunks from leaves, but they don’t produce the characteristic zigzag of the sawfly larvae. It is unlikely that there will only be one elm zigzag sawfly feeding trace on a tree, so if in doubt take the time to look for more feeding traces to see if any better fit the expected pattern.
Feeding damage from other herbivorous, leaf-chewing insects
Damage to leaves not caused by insects
The feeding track formed by a leaf miner, possibly Stigmella lemniscella
Alexander, Butler & Green (2006) The value of different tree and shrub species to wildlife. British Wildlife 18 18-28
Blank et al. (2010) Aproceros leucopoda (Hymenoptera: Argidae): An East Asian pest of elms (Ulmus spp.) invading Europe. European Journal of Entomology 107 357-367
Doychev (2015) First record of the invasive Elm sawfly Aproceros leucopoda Takeuchi (Hymenoptera: Argidae) in Bulgaria. Silva Balcanica 16 108-112
Papp (2018) The lifestyle of the invasive zigzag elm sawfly (Aproceros leucopoda Takeuchi, 1939). Doctoral Dissertation, Szent István University
Tuffen (2016) Rapid Pest Risk Analysis (PRA) for: Aproceros leucopoda. DEFRA