CONKER trees are under threat because of an alien invader that is winning an insect war in England.
In the past ten years, the horse chestnut leaf-mining moth has spread from London into almost the whole of England and Wales, research has shown.
The moth, which arrived from the continent in 2002, burrows through the leaves of conker trees, causing them to turn brown, shrivel and fall early.
Experts had hoped the tiny parasitic wasps that lay their eggs in the moth caterpillars would act as natural pest controllers.
But the latest research published in the online journal Public Library of Science ONE shows the wasps are being outnumbered, and there are too few of them to keep the moths at bay.
The evidence comes from records of leaf damage collected by thousands of citizen scientists in 2010.
In a follow-up study, volunteers including hundreds of school children followed instructions to rear moths in sealed infested leaves.
Counting how many moths emerged made it possible to assess the impact of the parasitic wasps.
Lead scientist Dr Michael Pocock, from the Centre of Ecology & Hydrology in Wallingford, Oxfordshire, said: "This is the sort of science that anyone can do.
"By taking part the public are doing real science - and the publication of this scientific paper is a demonstration of how seriously citizen science is now taken by the community of professional scientists.
"It seems almost like magic for children and other people to put a damaged leaf in a plastic bag, wait two weeks and then see insects - the adult moths or their pest controllers - emerge, but making these discoveries was a valuable contribution to understanding why some animals become so invasive."
Co-author Dr Darren Evans, from the University of Hull, said: "This work could have been done by paying research assistants to travel the country and collect records, but by inviting thousands of people to get involved we, together, were able to pull this off much more cost-effectively."
A total of 8,000 people took part in the Conker Tree Science project between 2010 and 2011.
The horse chestnut leaf-miner, Cameraria ohridella, was first observed in Macedonia, northern Greece, in the late 1970s and described as a new species in 1986.
Since the 1980s it has spread throughout the whole of Europe, from southern Sweden to Spain and Turkey.
The moth first appeared in the UK in the London Borough of Wimbledon in July 2002.
Moth larvae were found on horse chestnut trees along the southern edge of Wimbledon Common and in nearby streets and gardens.