Horse-chestnut leaf miner

Identification

A small moth of only a few millimetres' wingspan with rusty-brown, silvery-striped wings; the damage is caused by its larvae, which mine the leaves of the horse chestnut. Small brown mines form in the leaf blade, later expanding and lying lengthwise between the veins. Heavily infested leaves turn brown, dry out and fall as early as July and August, so that the trees look autumnal in the middle of summer. Repeated feeding over the season (several generations) weakens the trees and worsens their condition from year to year. An invasive species, described only in 1986 by Lake Ohrid in Macedonia (hence the epithet ohridella), it spread across the whole of Europe within about a dozen years. The main host is the horse chestnut (Aesculus hippocastanum); the red horse chestnut (Aesculus × carnea) is distinctly less susceptible. The mines lie on the upper side of the leaf blade and are best seen against the light — inside one can then make out the larva and black granules of frass, which distinguishes the damage from fungal spots. Under Polish conditions the insect usually produces 3–4 generations per season (even more in warm years), and successive generations overlap, so the damage builds up from June right through to autumn. It overwinters as a pupa in the fallen leaves, and the spring flight of the moths coincides with the leafing and flowering of the horse chestnuts. Feeding by the leaf miner alone rarely leads to the death of the tree, but the chronic premature defoliation repeated year after year weakens it, reduces its growth increments and worsens its vigour and ornamental value, increasing its susceptibility to other stress factors.

Treatment

The most effective amateur method is thorough raking up and destruction of the fallen leaves (burning or deep composting), in which the pupae overwinter — this reduces the size of the first generation in spring. Pheromone traps serve to monitor the moths' flight and assess the level of infestation. Supporting the tree's vigour (watering during drought, avoiding salt stress) helps it withstand the annual leaf damage. Collected leaves are best deeply composted or disposed of with biowaste — burning green waste is banned in some areas and may be done only where local by-laws allow it. The effectiveness of raking increases when it is thorough and carried out across the whole area — a single property achieves little if the moths fly in from unraked trees nearby, so the best results come from coordinated action (e.g. at the level of a street or housing estate). Pheromone traps remain above all a monitoring tool; mass trapping of males is sometimes tested, but on its own it does not protect the tree and does not replace leaf sanitation. Natural enemies — native parasitic wasps that attack the larvae in the mines — occur, but the level of parasitism in Europe is usually low and does not provide control on its own. For individual, valuable trees professional endotherapy is used, that is trunk injections of systemic products (systemic insecticides or insect growth regulators) — a treatment carried out with registered products by qualified operators, because spraying tall crowns is in practice impossible. In new plantings it is worth choosing less susceptible species from the outset (e.g. the red horse chestnut). Since the damage is mainly aesthetic and weakening rather than lethal, in a garden setting consistent leaf sanitation combined with care for the tree's vigour is usually sufficient.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.