Horse Chestnut Leaf Minor
Cameraria ohridella was first found established in the UK in the London Borough of Wimbledon in July 2002. Leaf mines with larvae were present in high densities on horse-chestnut trees (Aesculus hippocastanum) along the southern edge of Wimbledon Common, and in nearby streets and gardens, which suggested that the first moths had arrived in either 2000 or 2001. From this initial area of infestation, the moth has spread rapidly, and it is now present across most of south-central England, East Anglia and the Midlands. The rate of spread in the UK (40-60 km/year) is similar to that seen on the continent.
Despite the poor appearance of horse-chestnut trees infested with C. ohridella, there is no evidence that damage by the moth leads to a decline in tree health, the development of dieback, or tree death. Trees survive repeated infestations and re-flush normally in the following year.
It appears 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 by C. ohridella.
Managing Horse chestnut leaf miner (Cameraria ohridella)
Damage by C. ohridella is primarily an aesthetic problem, and there is no evidence that infestation, on its own, causes dieback or a decline in tree health, or tree death. Consequently, there is no reason to fell and remove trees just because they are attacked by C. ohridella. Even severely infested trees will re-flush as normal in the following spring.
However, damage to trees in parks, gardens and in other urban situations can be reduced by removing fallen leaves during the autumn and winter, and this can help ensure that trees retain their vitality.
In the long-term, it is hoped that biological control will lead to a permanent reduction in the pest population.
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. 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.