Horse Chestnut Leaf Miner

Many of you will have noticed that our horse chestnut trees are looking decidedly sick these days. By late summer the leaves are covered in whitish then brown blotches. These are the leaf mines of the caterpillars of an alien moth species – the horse chestnut leaf miner (Cameraria ohridella). The following information is from Forest Research and Butterfly Conservation.

First noted in Macedonia in the late 1970s, this species has spread rapidly over Europe. It was first recorded in this country at Wimbledon in 2002, since when this moth has spread extremely rapidly and has been found widely over the southern half of England, west to Somerset and south Wales and north to Derbyshire, although areas of high infestation are largely restricted to London, the home counties, parts of East Anglia and parts of the Midlands.

Despite the premature browning appearance of horse chestnut trees infested by the caterpillar mines, there is currently no evidence that infestation leads to a decline in tree health. Trees have survived repeated infestations and produce leaves in the following year, although there may be reduced growth vigour in younger trees.

Sick looking trees can be caused by other problems. Damage by the fungus Guignardia aesculi can give rise to a superficially similar blotch to the mine of the moth but these are not translucent when held up to the light. Significant dieback of branches in the canopy, or death of horse-chestnut trees, is caused by other factors, e.g. bleeding canker of horse chestnut, or a combination of factors. Trees showing these symptoms need to be checked by a qualified arboriculturist so that appropriate management can be undertaken.

Where the moth is established, the safest and most practical means of control is to remove fallen leaves during the autumn and winter. C. ohridella over-winters as a pupa in the fallen leaves, and commercial composting of leaves or burning them (if local regulations allow) 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.

More information can be found at Forest Research and Butterfly Conservation, and the UK Moths website.