The zigzag elm sawfly (Aproceros leucopoda), originally recorded in Japan and also found in China, was observed in Europe for the first time in 2003, when it was identified on various species of elm trees (Ulmus spp.) in Poland and Hungary. Since then, other infestations of this type have been found across Europe. The larvae may cause the complete defoliation of elms, regardless of their age or location. It can be assumed that this invasive species of insect will continue spreading Europe, thereby exerting additional pressure on elm trees, large numbers of which have already been killed by an aggressive fungal disease called 'Dutch elm disease'.
The adult females are 6-10 mm in length, have three-part antennae, are a brownish to black colour and have brownish translucent wings and light-coloured legs. There is no record of males of this species.
The females deposit their eggs, which are light green and less than 1 mm long, individually at the leaf margins.
Larvae are light green, with a dark stripe running down the side of their head and a T-shaped spot on their second and third pairs of legs (see red arrows in fig. 3). Within two to three weeks, they go through six larval stages (4-10 mm). In the final stage, they pupate in a loosely spun, reticulate summer cocoon of 8 mm in length attached to the underside of the leaf. The only cocoon that is more compact and has thicker walls is the one for hibernating pupae. This is found on leaves that have fallen on the ground or in plant litter.
The species appears to only reproduce asexually. To date, no males have been found. The females deposit up to 50 eggs individually at leaf margins. Larvae hatch from these just under a week later.
An initial sign of their presence is the typical zigzag frass of the young larvae (giving them their name) which emerges between the lateral veins of the leaves, starting at the margins and moving towards the midrib. Subsequently, more mature larvae eat away the entire leaf except the midrib, thereby eliminating traces of the zigzag feeding characterising the early larval stages. By continuously feeding on the crown of a tree, larvae can cause high crown transparency, extending to complete defoliation in the worst case. In all, six larval stages are completed in the course of two to three weeks. The larvae are pupated in a loosely spun, reticularcocoon on the underside of the leaf. The only type of cocoon that is more tightly spun is the one for hibernation. Such cocoons spend the winter attached to the leaves on the ground or in plant litter.
The whole cycle from oviposition to hatching of the females of the next generation only takes one month. It has been shown both out in the open and in laboratory experiments that there may be up to four generations of zigzag elm sawflies a year. Imagos hatch continuously from mid-May to mid-September.
All types of elm – even cultivated forms – can be attacked. In Europe, it appears that zigzag elm sawflies are most likely to attack wych and small-leaved elms but less likely to hit European white elms. Even those elms that were cultivated for their resistance to Dutch elm disease are being attacked by these insects.
Zigzag elm sawflies are originally from East Asia and were probably transported to Europe from there with imported plants. However, they are likely to have been propagated not only through infected young trees but also infected parts of plants such as branches, leaves or soil.
In Europe, there is no single identifiable infestation centre or particular direction of propagation. Instead, this species has emerged over a relatively short period in specific locations in areas that are considerable distances from each other. The natural propagation of this species increases between May and September as a result of these insects' outstanding ability to fly. These animals are probably also passively spread by traffic, as zigzag elm sawflies can frequently be observed by the roadside. However, this may also be due to the fact that in the past it was common to plant lines of elm shrubs along roads. Furthermore, the insects are thought to be passively carried by the wind.
The larvae of this invasive species feed on elm leaves, causing high crown transparency, ranging from 75% to 100% even by the time summer arrives. Newly formed replacement leaves are in turn also attacked, in general killing off the affected twigs and branches. If this goes on for a number of years, it reduces tree vitality, thereby making the tree more vulnerable to secondary pests. However, there have not yet been any recorded cases of whole trees dying because of this pest. Damage caused by the zigzag elm sawfly has been observed in both forests and residential areas (gardens and roads where elms are planted for ornamental purposes or in avenues of trees).
Countermeasures are only possible to some extent. In gardens, parks or public green spaces in the autumn, the fallen leaves along with the attached winter cocoon may be raked together and destroyed in the autumn.
The species' use of asexual reproduction means a quick succession of generations (up to four a year). Using pesticides is not recommended due to the lack of success experienced with these in trials in other countries. Furthermore, this invasive species' outstanding ability to fly certainly contributes to its propagation. Its natural propagation velocity has been demonstrated to range from 50 to 90 km per year. In addition, humans transport these insects long distances.
One successful strategy could be to use specific parasitoids with a narrow spectrum of hosts. However, research into this possibility has not yet reached a point where this technique can be applied.
Groups of mixed elms are much less vulnerable to pests than larger planting areas, and mass propagation will occur much less rapidly there. As a result of Dutch elm disease, care should be taken to only plant elms in groups of mixed tree species. This also helps to combat the propagation of zigzag elm sawflies. For both harmful organisms, when planting it would also be advisable to avoid creating linear connections with forest edges or avenues of trees for example, which would be likely to further propagation.
There is hardly any likelihood of getting zigzag elm sawflies mixed up with other species for anybody finding the typical zigzag frass and cocoons on elm leaves. However, undistinctive holes can also be created by various other insects, such as winter moths.
This invasive insect species was found in Europe for the first time in 2003, with the first records coming from Hungary and Poland. Since then, according to the EPPO Alert List, the sawfly has been spotted in 18 European countries. After initially settling in countries in eastern Europe (Hungary, Romania, Poland), they were recorded in Austria and Italy for the first time in 2009, followed by Germany in 2011 and then other countries across Europe.
Timeline of infestations in Europe from the European and Mediterranean Plant Organisation (EPPO) and individual data from the initial reports of the relevant countries:
- Hungary: (North) Bàcs-Kiskun, Békés, Budapest, Csongràd, Heves and Nògràd counties
- Poland: (South) Sandomierz and (Central East) Warsaw "powiats"
- Romania: (East) and (in 2006) Banat (West)
- Moldova: (West)
- Ukraine: (East) Luhansk and (in 2009) Kharkiv 'oblasts'
- Slovakia: (East) and (in 2009) (West)
- Austria: (Northeast) Vienna and Lower Austria
- Italy: (Northeast) Piedmont and the Friuli-Venezia Giulia region and (in July 2013) Grigno and Ospedaletto in the Autonomous Province of Trento (also known as "Trentino"), along the River Brenta (Veneto region) with 70-80% crown transparency
- Germany: (South and Northeast) From summer until November, the species was reported in small-leaved elms (Ulmus minor) in a forest in Bavaria and in Saxony, and in 2013 in two locations in Brandenburg. Here, trees along an avenue were attacked by the sawflies.
- Croatia: Larvae; larval frass; cocoons; and adult insects
- Slovenia: (West) In late September, this invasive species was found in Rožna Dolina near Nova Gorica. Then in early October, typical larval frass was also reported in Volčji Potok Arboretum and Ljubljana Botanic Garden (Central). Mainly small-leaved elms (Ulmus minor) were hit, and a number of wych elms (Ulmus glabra) were also affected.
- Netherlands: Females and empty cocoons were found, and in September 2015 females were discovered again.
- Belgium: (North) In July/August, adult insects were found in small-leaved elms (Ulmus minor) in Brussels but no larvae or cocoons, and the sawflies were also reported in Central Belgium from June to September 2014.
- Czech Republic: (North) In August, 13 trees – small-leaved elms (Ulmus minor) and wych elms (U. glabra) – in the Hradec Králové region (near Hradec Králové) were reported as hosting adult insects, larvae, cocoons and traces of frass. In September, this pest was also recorded in other regions.
- Bulgaria: (West) In June, females, cocoons, larvae and typical traces of larval frass were found in small-leaved elms (Ulmus minor) at five locations in the Western Balkans and Sofia; however, this was all low-level (only 1-2% of the leaf mass affected).
- Switzerland: (North) In August, three wych elms along a stream by a road were found to have been attacked by sawflies (larval frass, empty cocoons, adult insects). In some places there were clear signs of complete defoliation.
- Estonia (North-East): Damaged mountain elm trees were found in the Ida-Viru region. Cocoons and larvae were found on them.