Horse Chestnut Leaf Miner

Our conker trees are being devoured from the inside, their leaves are being mined, but they don’t mind.

This year’s hot dry summer has caused some of our trees to turn brown as if there is an early autumn, but the conker trees have another reason for the colour change. The leaves are being attacked from the inside by the caterpillars of the Horse Chestnut Leaf Miner moth.

This tiny moth, it is only 5mm long and has a wingspan of 1cm, is a natural pest of Horse Chestnuts in their original home in the Balkans. The moth lays eggs on the leaf and, when it hatches, the caterpillar tunnels into the leaf and spends four weeks mining the green tissue between the veins. This causes the unsightly brown patches. When fully grown, the caterpillar spins a cocoon and pupates inside the leaf. It then emerges as an adult moth bursting through the thin epidermis of the leaf. There may be three generations during a summer, but the last set of pupae fall with the leaves in autumn and over winter in the leaf litter.

Surprisingly, the tree can cope with this. The main damage tends to be done in late summer when the green leaves have already made sufficient food to be stored ready for the following spring. A heavy infestation may reduce the number and size of conkers produced, but there will still be more than enough to grow new generations of tree.

Horse Chestnuts are attractive trees highly valued for their aesthetic appeal. If you want to reduce the impact of the moth, the best thing to do is to clear away the leaf litter under the tree and so remove the over wintering pupae. You can see the difference if you look at the Horse Chestnuts in The Knowle where the leaves are not cleared, and the trees are now totally brown or even leafless. In comparison, the leaves under the trees in Church Street are cleared with the grass mowing and the churchyard trees have far fewer mining scars on the leaves.

Mined Horse Chestnut leaves with the Leaf Miner moth inset. Picture credit “Ian Kimber/UKMoths”