Mark of Zorro: new pest coming soon to woodland near you?

The distinctive “signature” of the zigzag elm sawfly (Aproceros leucopoda) might conjure images of the 1940’s movie Mark of Zorro. But, it heralds the arrival of a new pest in the British countryside. Recently discovered examples of leaf damage indicate the insect has arrived in the UK and could threaten elm-dependent insects around the country.

Originally recorded in Japan, the aptly named zigzag elm sawfly only feeds on elm leaves and has been progressing steadily through Europe. Now scientists at the Royal Botanic Garden Edinburgh (RBGE), one of a UK-wide network of environment-related organisations who work in partnerships to tackle issues of plant health, has confirmed the tell-tale zigzag feeding trail left behind by sawfly larvae on leaves collected in Surrey during autumn 2017.

RBGE mycologist Dr Katherine Hayden explained: “This was one of those chance discoveries that highlight the important collaboration between members of the public and centres of expertise like botanic gardens. Plant samples arrived here to be identified as part of local plant recording activity carried out by experienced amateurs in Surrey. Examination by our elm specialist revealed the curious zigzag feeding damage as the first evidence of the pest in Britain.”

The sample was brought to the attention of RBGE Science Communicator Dr Max Coleman who explained infestation had spread rapidly through Europe. “The zigzag elm sawfly can quickly build up pest populations as females are able to reproduce asexually,” he added. “People will be familiar with how fast greenfly can spread. The sawfly has evolved the same strategy: exploit the speed advantage of asexual reproduction and avoid the risk of not finding a mate.”

As part of ongoing research and conservation efforts into this and other pests, scientists are encouraging the public to help monitor the spread of zigzag elm sawfly. Dr Coleman added: “With spring upon us and the leaf damage pattern so easy to spot, members of the public can provide useful information by sending in their own sightings of the unmistakable zigzag pattern. New records are particularly important as we cannot confirm the presence of this insect until a specimen is collected. Adult insects, larvae or even the pupal cases, where larvae transform themselves into adults, would provide the definitive evidence.”

While zigzag elm sawfly rarely kills trees, large populations can completely defoliate elms. This can be disastrous for elm leaf feeding insects such as the rare white-letter hairstreak butterfly (Satyrium w-album), which suffered in the 1970s through the loss of trees to Dutch elm disease. The caterpillar of this species can only feed on elm leaves and populations are sometimes restricted to single trees.

“The worst scenario would be an outbreak of elm sawfly that could defoliate a tree supporting a colony of the white-letter hairstreak. With help from the public we can monitor the spread and impact of this new tree pest” said Dr Coleman.

Anyone wishing to report sightings of zigzag elm sawfly should use TreeAlert, the online reporting tool developed by the Forestry Commission to track tree health problems available at: www.forestry.gov.uk/treealert

The Royal Botanic Garden Edinburgh is a charity (registration number SC007983)