The horse-chestnut leaf miner, Cameraria ohridella, probably originates from natural stands of the European horse-chestnut in Greece, Albania and Macedonia. It was first observed attacking ornamental horse-chestnut trees in Macedonia in the 1970s, from where it spread to most of Europe. Since then, in all invaded regions, outbreaks have continued unabated, causing aesthetic damage to horse-chestnut, one of the favourite ornamental trees in European cities.
The adult moth is 4-5 mm long and a rich brown colour with bright white chevrons edged with black. The larvae mine in horse chestnut leaves go to four (occasionally five instars) and their size varies form 0.5 mm in the firsts instar up to 3.5 mm in the fourth instar. The pupa is brown and is 2.9-4.5 mm long.
The horse-chestnut leaf miner overwinters in the pupal stage in a cocoon in dead leaves. The emergence of adults in spring occurs in April and May, depending on climatic conditions. After mating, each female may lay up to 180 eggs singly on the upper part of the leaflets. Females of the second and third generations lay eggs on all leaflet surfaces. Larval development lasts 25-35 days. First-instar larvae make only a small gallery. The second- and third-instar larvae develop a round mine of 4-7 mm diameter. The larva then goes through two spinning instars, which may or may not build a true cocoon in the mine. The percentage of larvae which spin true cocoons increases in each of the subsequent generations. Adults of the first generation emerge from mid-June to late-July. Cameraria ohridella has two to four generations a year, depending on the temperature conditions.
Larvae of the horse-chestnut leaf miner damage leaves by feeding between the upper and lower parenchyma. The mine starts to turn yellow and later brown. By this time, the damage is very visible.
Eventually the mines may cover the entire surface of the leaflets, especially from July on, when the second and third generations develop.
Trees with a low number of attacked leaves are not greatly affected, but when the entire surface of leaves is covered with mines, the leaves start to dry out and fall off. However, total defoliation does not seem to affect the growth of mature trees. At sites where dead leaves containing overwintering pupae are not removed in the autumn, trees are usually totally defoliated, year after year.
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