As well as being threatened by a disease called ‘bleeding canker’ caused by a bacterial infection by pseudomonas syringae pathovar aesculi – previously only found in the Himalayas – our British Horse Chestnuts (Aesculus hippocastanum) is now suffering damage caused by the larvae of a moth – the appropriately named

Horse Chestnut leaf miner (Cameraria ohridella).
The larvae of the leaf miner burrow within the leaves of horse chestnut, and the damage caused by large numbers of larvae can be striking. Up to 700 leaf miners have been recorded on a single leaf under favourable conditions. Severely damaged leaves shrivel and turn brown by late summer and fall early, well before normal leaf fall in the autumn.

There are 19 species of the Chestnut Aesculus and almost half of them are favoured by the leaf miner. At present the outbreaks of severe damage are limited to the South East corner of the UK.

Despite appearances, Horse Chestnuts infested with the leaf miner are not permanently damaged. There is no decline in overall tree health, dieback, or tree death and trees have been seen to survive repeated infestations and re-flush normally in the following year. It appears, at least in the South East UK climate, that most of the damage caused by the moth occurs too late in the growing season to greatly affect tree performance. Consequently, there is no reason to fell and remove trees just because they are attacked in this way.

Interestingly, during the autumn of 2006 in the UK, there were a number of reports of Horse Chestnut trees, which had been defoliated in this way re-flushing and flowering again late in the season. The same phenomenon has been observed in other European cities, e.g. in Berlin.

Where the moth is established, the safest and most practical means of control is to remove fallen leaves during the autumn and winter. The Cameraria ohridella over-winters as a pupa in the fallen leaves, and commercial composting of leaves or burning them destroys the pupae and reduces the moth population in the following spring.

Composting is less effective when leaves are collected into smaller heaps, as in gardens, because temperatures in small heaps are too low to kill the pupae. However, in these situations, covering the leaves with a 10cm layer of soil or 15-30cm layer of other plant material, and leaving the heaps undisturbed until the end of May, will prevent adult emergence in the spring and will help to reduce damage.

Horse chestnut trees are native to south Eastern Europe and were introduced into Britain in the 16th century.