Elm trees already devastated by Dutch elm disease are being ravaged across south and east England by a new alien pest that leaves the 'Mark of Zorro' on their leaves.

Maps seen by the Telegraph show the Elm Zigzag Sawfly, originally from Japan and named after the Z-shaped holes it eats into leaves, has colonised a 7,000 square mile area of England.

The bugs can consume almost the entire foliage of an elm, not only threatening the future of rare and endangered indigenous insects like butterflies that live on elms but also weakening the trees’ ability to resist the twin threats of Dutch elm disease and hotter British summers.

It has now been sighted across an area covering a seventh of England from Hythe near Folkestone in the south to Ipswich and King’s Lynn in the east and Oxford and Bedford in the heart of England as well as throughout London, according to maps by Forest Research, a government agency.

Matt Elliot, the Woodland Trust’s tree health expert, said there was nothing that could stop it as pesticides would kill other indigenous insects that live on elms including one of the UK’s most endangered butterflies, the white-letter hairstreak.