Elm leaf pest identified in UK for first time

The zigzag elm sawfly (Aproceros leucopoda) has been identified in the UK from the distinctive signature of its feeding, and the hunt is now on for actual flies or larvae to confirm its arrival.

Image: Ewald Altenhofer (CC BY-SA 3.0 DE)
Image: Ewald Altenhofer (CC BY-SA 3.0 DE)

Originally found in Japan, the sawfly has been progressing steadily through Europe since first being identified in Poland and Hungary in 2003.

Now scientists at the Royal Botanic Garden Edinburgh (RBGE), one of a UK-wide network of plant health organisations, has confirmed the tell-tale zigzag feeding trail left by their larvae on leaves collected in Surrey last autumn.

RBGE mycologist Dr Katherine Hayden explained: "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."

She added: "This was one of those chance discoveries that highlight the important collaboration between members of the public and centres of expertise like botanic gardens."

Explaining the threat it poses, RBGE science communicator Dr Max Coleman said: "The zigzag elm sawfly can quickly build up pest populations as females are able to reproduce asexually. The sawfly has evolved the same strategy as greenfly: exploit the speed advantage of asexual reproduction."

Scientists are encouraging the public to help monitor the fly's spread.

Coleman added: "With 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 the sawflies rarely kill trees, large populations can completely defoliate them, which can affect other elm leaf-feeding insects such as the rare white-letter hairstreak butterfly (Satyrium w-album), whose caterpillars can only feed on elm leaves.

The zigzag elm sawfly has been added to the Observatree citizens science project's list of priority pests.

To report possible sightings, please use the Forestry Commission's TreeAlert online reporting tool.

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