Invading ‘mark of Zorro’ bugs threaten elm trees

ELM trees are under threat from a pest that leaves a “mark of Zorro” trail of destruction on its leaves, scientists fear.

The zigzag elm sawfly, originally from Japan, feeds only on elm leaves and has been progressing steadily through Europe.

Now scientists at the Royal Botanic Garden Edinburgh (RBGE) have confirmed the telltale zigzag feeding trails of sawfly larvae were found on leaves collected in Surrey last autumn.

They fear the insect could pose a fresh threat to the UK’S elms, which have already been devastated by Dutch elm disease, and the wildlife that relies on the trees.

Elm leaf-feeding insects such as the rare white-letter hairstreak butterfly suffered in the Seventies due to the loss of trees to Dutch elm disease and could be hit by the invader’s arrival.

While the zigzag elm sawfly rarely kills trees, large populations could completely defoliate elms, which could be disastrous for the hairstreak, whose populations were sometimes restricted to single trees, the experts said.

The first evidence of the pest in Britain was a chance discovery. Dr Katherine Hayden, an RBGE mycologist, said: “Plant samples arrived here … as part of plant recording activity carried out by experienced amateurs in Surrey. Examination revealed the curious zigzag feeding damage as the first evidence of the pest in Britain.”

Scientists have asked people to report sightings of the unmistakable zigzag pattern, to track the sawfly’s spread and help scientists secure definitive evidence of its presence in the form of adult insects, larvae or pupae.

Dr Nick Atkinson, a Woodland Trust senior adviser, said: “This could be another blow to the elm population, following the widespread devastation caused by Dutch elm disease.”