Horse chestnut leaf-mining moth is a relatively new pest in Britain, but it has spread rapidly across the country since it was first identified by the RHS entomologists in Wimbledon in 2002. It had probably been present in that area for at least a year before it was discovered. The effect on the appearance of horse chestnut trees in the second half of the summer by the leaf-mining moth is profound.
What is horse chestnut leaf miner?
Horse chestnut leaf miner is a small moth with caterpillars that feed inside the leaves, causing brown or white blotch mines to develop between the leaf veins.
- Horse chestnuts produce normal foliage and flowers in the spring but the first signs of leaf mining appear during June
- Elongate blotches, at first white but later turning brown, develop on the foliage from mid-June onwards
- Caterpillars, or circular pupal cocoons, can be seen within the mined areas if the leaf is held up to the light
- By August, most of the leaf area may be occupied by leaf mines, giving the impression that the tree is dying, although it will survive
- Heavily infested trees will drop their leaves early, however research has shown that this has no effect on the growth rate of trees
- Collecting and burning fallen leaves in autumn will reduce the overwintering pupae
- Alternatively, the leaves can be composted in sealed bags that are kept closed until the following July, by which time the adult moths will have emerged and died. This can delay the build up of damage during summer but is only worthwhile for isolated trees where most of the fallen leaves can be gathered up
- Some chestnuts, such as Aesculus indica, A. × neglecta and A. chinensis, are not attacked or suffer only slight damage
- Aesculus indica is the closest in size and appearance if a replacement tree is required for A. hippocastanum
- A pheromone trap that attracts male moths is available from Oecos. In some circumstances this may reduce the mating success of the moth and therefore the level of infestation
- Spraying is not feasible on large trees
- New to Britain in 2002, this pest is now widespread in much of England and is spreading into Wales
- The tiny brown and silver adult moths lay eggs on the foliage
- After hatching, the caterpillars enter the leaves and eat the internal tissues
- There are usually three generations during summer, and, by August, the foliage may be extensively damaged, leading to early leaf fall
- This pest overwinters as pupae in the leaf mines
Monitoring the long term impact of horse chestnut leaf mining moth on the health of our trees is key to understanding this pest. Through citizen science, gardeners can help with this work. In collaboration with the RHS, TreeWatch is undertaking a survey to monitor horse chestnut (Aesculus) trees for the presence of leaf miner damage and we are inviting members of the public to help by sending their reports. Visit the TreeWatch survey.