UK's elm trees under threat from pest that leaves a 'Mark of Zorro' trail of destruction on leaves

The UK’s elm trees are under threat from a pest that leaves a “Mark Of Zorro” trail of destruction on its leaves, scientists have discovered.

The zigzag elm sawfly, originally found in 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 trail left behind by the sawfly larvae has been found on leaves collected in Surrey during autumn 2017.

The discovery has raised concerns it could pose another blow to the UK's elms, which have already been devastated by Dutch elm disease, and threaten wildlife that rely on the trees.

Elm leaf-feeding insects such as the rare white-letter hairstreak butterfly, which suffered in the 1970s due to the loss of trees to Dutch elm disease, could be hit by the new pest's arrival.

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

The first evidence of the pest in Britain was a chance discovery, according to scientists at RBGE.

RBGE mycologist Dr Katherine Hayden said: "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."

Scientists are now asking 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.

It has also prompted renewed calls for action to protect the UK's woodlands and wildlife.

Dr Nick Atkinson, senior conservation advisor for the Woodland Trust, said: "This could be another blow to the elm population, following the widespread devastation caused by Dutch elm disease, which began in the 1970s and continues even today."