Horse chestnut leaf miner (Cameraria ohridella) is an exotic insect pest which lives in horse chestnut trees. It was first reported in the UK in 2002, in the London Borough of Wimbledon, and has since spread north, south and west to most of England and parts of Wales, and there has been one confirmed sighting in Scotland.
Its larvae (caterpillars) mine within the leaves, and at high population densities they can destroy most of the leaf tissues. Although it can cause severe damage to horse chestnut leaves on an annual basis, and discolouration and defoliation before normal autumn leaf-fall, on its own the pest does not significantly impair trees' health, and they will usually flush normally the following spring.
However, it is possible that differences in climate, or interactions with other pests and diseases, might lead to greater impact in the UK. Consequently, the effects of the moth and its interaction with other pests and diseases, especially bleeding canker of horse chestnut, is being studied through the long-term monitoring of more than 300 chestnut trees at several sites in southern England. These trees are assessed twice each year for infestation, disease crown condition, growth and signs of dieback.
Horse chestnut leaf miner was first observed in Macedonia, in northern Greece, in 1985, and was described as a new species in 1986. In 1989, it appeared unexpectedly in Austria, and since then it has spread throughout central and eastern Europe. It was first found in Great Britain in 2002 in the London Borough of Wimbledon.
Dispersal of the moth from infested areas occurs on a broad front through adult flight, assisted by the wind, and through the passive transport of adult moths or infested leaves in or on vehicles. Transportation by vehicles appears to be responsible for the sudden appearance of the moth in towns and cities a long way from known areas of infestation.
The map shows areas where confirmed sightings of horse chestnut leaf miner had been reported up to 2014.
Damage can be reduced by removing fallen leaves during the autumn and winter and either composting them thoroughly, to destroy the over-wintering pupae, or if the leaves are collected into smaller heaps, by covering them with a layer of soil or other plant material to prevent adult emergence the following spring.
There is no requirement to report sightings of this pest to us, but you can help monitor its spread and contribute to other research into horse chestnut leaf miner - see www.ourweboflife.org.uk .