In lowland England the "vast majority of places have leaf miner and it is here to stay", said Dr Michael Pocock, organiser of the Conker Tree Science project and an ecologist at the UK's Centre for Ecology & Hydrology. "The project has been running for a few years and is coming to an end in terms of tracking the main thrust of the spreading of horse chestnut leaf miner." He added: "In the long term, I don't think it's looking good for horse chestnuts."
While horse chestnut leaf miner causes premature leaf loss, it is horse chestnut bleeding canker that is likely to cause the demise of the tree in England, he added. "Bleeding canker is a much more serious problem because it can cause tree death. Once it enters the tree, it has basically got it for life and can hasten death.
"A particular problem is it can cause limbs to die and drop, and that causes issues with health and safety and trees need much more action with management and felling. Because the canker causes trees to be felled and no one wants to replant horse chestnuts because they look so scrappy because of leaf miner, these two working together mean not a great prognosis for our wonderful big horse chestnut trees. Over the next two or three decades it could be the case that horse chestnuts are scarcer in parks and gardens."
Dr Glynn Percival, manager of the Bartlett Tree Research Laboratory at the University of Reading, said: "I don't think the prognosis is good at all, unless we find something to control leaf miner. In trees that have leaf miner we do get an increase in the severity of bleeding canker because they have so little energy to defend themselves.
"Our research shows trees without leaf miner produce conkers twice the weight of those with - 8g against 4g, which is a 50 per cent drop. We planted them out and the 4g germination rate is lower and the vigour is lower. There's a definite knock-on effect, our data shows. The affected trees' conkers are smaller and they have less vigour when germinating because of leaf miner.
"Horse chestnuts have got maybe another five years unless we get the issues under control. The trees are living off their own natural resources. They're brown and crispy when everything else is green. No energy is being produced." He said his research shows that honey fungus is also attacking stressed trees, killing them off quicker.
Percival and Berkshire arboricultural consultant Simon Holmes have assessed the efficacy of three "systemic inducing agents" - Messenger (active substance harpin protein), Phoenix (potassium phosphite) and Rigel (salicylic acid derivative) - and a conventional insecticide, deltamethrin, against the pest, with all providing some control.
But tree consultant Jeremy Barrell said: "I regularly see trees getting over these issues and managing to live with them, so I think it may be a little premature to write the species off after so little time to adjust. But, more strategically, we must be thinking beyond the conventional planting palette because climate change and modern transport of pests means that resilience must be the mantra, not sticking to a narrow view that what we had is what we should be planting. The tree planting environment is changing beyond recognition and we have to adapt to keep up with it. That means many new species, as well as the more traditional, and being open to new ideas."Forestry Commission scientists are still carrying out their long-term study on the impacts of horse chestnut leaf miner and bleeding canker disease. Some background to this is set out on the Forest Research website at http://www.forestry.gov.uk/fr/BEEH-9R2N94 .
This is on-going work and isn’t due to report in any detail until end 2017/2018. The first 10 years of the study were written up in Straw, N.A. & Williams, D.T. (2013) ‘Impact of the leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae) and bleeding canker disease on horse-chestnut: direct effects and interaction’. Agricultural and Forest Entomology Vol 15, 321–333. This is available at http://onlinelibrary.wiley.com/doi/10.1111/afe.12020/full .