Scientists are unlikely to be able to find an effective predator for horse chestnut leaf miner, which could lead to the end of the tree as a commercial proposition, according to Dr Michael Pocock, who is leading the pioneering citizen science project Conker Tree Science.
Sales of Aesculus hippocastanum have fallen by up to 98 per cent over the past five years because of the miner, Cameraria ohridella, which disfigures leaves, and bleeding canker, which kills trees.
Pocock said: "In 150 years' time, horse chestnuts on village greens may be a thing of the past. For the time being they will be present on village greens and in parks but will look dead before the end of the summer."
Conker Tree Science (www.ourweboflife.org.uk) is a project that allows anyone to research the insect.
Pocock said the project was a powerful way of engaging the public in biodiversity, but added: "My feeling is there isn't any chance of finding an effective parasitical wasp, mainly because the wasps attacking in this country are generalists, so they feed on a range of leaf-mining insects.
"We were expecting there could be some adaptation, so some strains might be adapted to the seasonality of the leaf miner, but that only has a slight effect."
He added that there was no evidence of behavioural adaptation from continental Europe, where the miner is well established.
But Pocock added: "The current batch of trees, from what is known of horse chestnut leaf miner, would survive happily for a good long time.
"The only issue is whether canker is proved to be associated with the leaf miner. I suspect councils are not planting, so maybe this is the beginning of the end, but it will be a long, slow decline."
He said the project had received mixed responses as to whether the trees were worth saving. "It depends on how important it is to have trees looking pristine. Some people say they hate seeing them looking dead and others say the tree is not native, so why should we value trees more than the moths?"
Meanwhile, Treeworks Environmental Practice is holding a seminar to address disease threats to mature trees at Royal Botanic Gardens, Kew on 16 September.
Organiser Neville Fay said: "We're in a new phase of pressure our trees are under. The disease symptoms we see are symptoms of this pressure."
Oak, plane and horse chestnut were threatened by migratory and exotic pests "introduced through globalisation and exacerbated by climate". Fay added that the introduction of pests and diseases through mass nursery imports needed clear thinking about biosecurity.
"We need to work in a cross-disciplinary way to protect our really important trees because that's our natural heritage," he added. "We need a more integrated view of the problem and to be open to understand different models of how trees function within landscapes."
Barcham Trees managing director Mike Glover said horse chestnut tree sales had declined from 6,000 a year in 2004 to a few hundred this year.
Industry View - Steve McCurdy, owner, Majestic Trees
"Our sales of Aesculus hippocastanum are tiny compared to what they used to be. Five years ago we sold 50 or 100 a year and now it is two or three a year. I haven't bought one for two or three years. We still sell them with a warning label. We spray every year for leaf miner but until they find a predator it will be no good because you can't spray 70-foot horse chestnuts around the whole country. If they find something to treat bleeding canker there is a chance, then they need to find a natural predator, too."