What is the Horse-chestnut Leaf Miner?
If you’ve noticed gingery brown, dry patches on the leaves of horse-chestnut trees these are the ‘blotch mines’ caused by an infestation of the horse-chestnut leaf mining moth, Cameraria ohridella. This pest has only been recorded in the UK since 2002 but has certainly made its presence felt. Earlier in the year the adult female moth will have laid her eggs into areas of the foliage between the leaf veins. Each egg hatches out into a tiny caterpillar which feeds and develops within the leaf, causing the mine to develop and that area of leaf to turn first a white-green colour and then gingery brown. Each one does a lot of eating and if you hold the newly infested leaf up to a bright source of light you can see the blob-shaped critter inside! Later on they produce a silken cocoon and pupate, still within the leaf. If a tree is really heavily attacked you’ll notice the leaves falling early and eventually the tree may see a slight loss in vigour but in most cases it worries you and I more than it does the tree!
How to combat the Horse-chestnut Leaf Miner
Right now, as more and more trees are losing their foliage, it is well worth raking up and burning the horse-chestnut leaves, particularly those which are showing signs of the characteristic leaf miner symptoms. By doing this you’ll dramatically reduce the number of overwintering cameraria ohridella pupae, and so reduce the potential damage next year. If you cannot have a bonfire where you live, cram the leaves into tough plastic bin bags and keep them tightly sealed until July next year – this way the pupae will hatch but the adult moths will not have been able to escape and do damage!