The horse chestnut, Aesculus hippocastanum, and its tiny leaf mining moth, Cameraria ohridella, have been in the news recently, in stories expressing fear for the survival of the tree due to damage by the larvae of this moth. The appearance of this tiny moth in Britain is not so recent as might be imagined, it having been identified on a tree in Wimbledon as early as 2002 - Its caterpillar is a leaf miner which eats the leaf while concealing itself between the surfaces;  the leaves develop prominent brown blotches and may be shed earlier than usual. So what's the problem: don't all trees and most wildflowers have moths and flies, often many different species of insect, nibbling away at their leaves, and have been living with the inconvenience for tens, or hundreds of thousands, of years.
The problem stems from the fact that this is a new moth to Britain and doesn't have the usual complement of predators to keep it under control. Blue tits, coal tits and great tits, those renowned caterpillar consumers, especially when nesting, can be relied upon to take a small percentage, but the big caterpillar killers, the parasitic wasps and flies, which lay their eggs inside the body of the moth larvae, and on hatch ing consume it from within, are absent.   One possible reason for this arises from the fact that the horse chestnut and its moth originated in the Balkans and when imported to Britain and other parts of Europe as an ornamental tree, the moth with its specific parasitic wasp was initially left behind.  Now the moth has found its way here, but not the parasitic wasp, so nature is out of balance.
So how do we deal with the problem? To seek out the parasitic wasp or fly in the Balkans and introduce it here might be one approach. Alternatively, there are a lot of parasitic wasps and flies that are less specific and it might be possible to point these in the direction of horse chestnut trees.
It is important to realise that the moths are not currently killing the trees, just making the leaves look unhealthy so there is no immediate threat to their survival. Over the years though it is possible that the trees if faced with other stresses like other diseases and drought might gradually decline in vigour, but this does give us time to seek a solution, whether it involves the use of systemic insecticides, parasitoids, more nesting boxes for blue tits, or a combination. One suggestion is to clear up the dead leaves in which the pupae overwinter and compost them inside plastic bags, so that when the adults emerge the following year they remain trapped and unable to  infect the tree. Though this will not get rid of the moths completely it might reduce the extent of the damage.
One immediate danger is that visible damage to the leaves might be used as an excuse for felling trees which happen to be in an inconvenient position to the landowner. There is absolutely no justification for this. The moths does not carry fungal or viral disease, so  saving the horse chestnut it not such a daunting prospect as with some other trees. It may be that as natural predators find that a rich harvest of caterpillars awaits on the horse chestnut, they will come to the rescue and problem will be resolved.
For more information, go to the the link below.
https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/horse-chestnut-leaf-miner-cameraria-ohridella/