Horse chestnut leaf-mining moth

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.

Symptoms

• 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.

Control

Non-chemical control

• 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. x 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.

Chemical control

• Spraying is not feasible on large trees.

Biology

• 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.

Survey

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.