Horse Chestnut trees and their insect guests

This is a leaf from a horse chestnut (conker) tree. Although native to Southern Europe the species is common in the UK, especially in parks and hedgerows, where they provide children with an easy source of conkers each autumn.

The patches on the leaf are cavities caused by the leaf mining moth Cameraria ohridella. Moth larvae live in the cavities between the outer layers of the leaf, where they feed on sap in the leaf veins. By late summer the majority of horse chestnut leaves will be brown, making it look like autumn has arrived early, at least for horse chestnuts. If you come across a horse chestnut tree in your local park or hedgerow, it will probably been infested with leaf miner, and the patches will be visible from some distance away! The adult moth, which is free-flying, is relatively a non-discript stripy brown colour, much less noticable than the leaf mines.

The moth started spreading from its original home in Macedonia in 1989, and is now widespread throughout Europe. It reached the UK in 2002.  Now it is rare to find a horse chestnut in the UK whose leaves are not curling and brown from leaf miner damage by the end of the summer. The health of infested trees is not affected (although they do tend to produce smaller conkers), which is good news - other invasive tree infestations, such as the fungus that causes Dutch Elm disease, have changed the landscape of the UK,  and because horse chestnuts are big trees that often found in prominent locations, the loss of this species would change the landscape substantially.

Lots of other plants are affected by leaf miners, which are easy to spot by looking for swirling blistered/discoloured patches on leaves.