Horse chestnut leaf miner: what the agencies record

This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. The horse chestnut leaf miner (Cameraria ohridella) is a moth whose caterpillars feed inside leaves. Forest Research's page says there is no requirement to report sightings to the plant health authorities, and the other two pages read name no regulation. Each section names its source, all read on October 8, 2026. The parent page, insect pests of trees, compares it with the other pests on the list.

Names

Forest Research, the research agency of the Forestry Commission, calls the pest horse chestnut leaf miner and abbreviates it HCLM. The Royal Horticultural Society, called the RHS, calls it the horse chestnut leaf-mining moth and uses leaf-miner for the same insect. The Woodland Trust uses horse chestnut leaf miner. All three give the scientific name Cameraria ohridella. The RHS notes that the caterpillars are the stage that feeds, and the Woodland Trust points out that the name leaf miner refers to the larvae, while the adult is a tiny moth.

Hosts

Forest Research says the caterpillars are an invasive pest of horse chestnut, meaning trees of the Aesculus genus, and of some maple and sycamore. Its page names the common horse chestnut, Aesculus hippocastanum, as the most widely grown horse chestnut in the United Kingdom. It says the moth can feed on Japanese horse chestnut, red-flowering horse chestnut, Indian horse chestnut and red buckeye, though never as heavily as on the common one, and that Norway maple and sycamore are occasionally attacked, especially when planted close to a horse chestnut. It adds that sweet chestnut is not affected. The RHS says Aesculus indica, A. x neglecta and A. chinensis are not affected or suffer only slight damage. The Woodland Trust gives horse chestnut and its hybrids.

What the sources say to look for

Forest Research describes elongate patches on the leaves that start white and turn brown, and says holding a leaf up to the light in summer can show the tiny caterpillars or their round pupal cocoons inside the mined area. It says the patches appear from as early as June, start in the lower canopy and spread upward, and that by late summer so much foliage can be brown that the tree looks as if it is dying, though it advises owners not to draw that conclusion without checking other factors. The RHS gives the same sequence, with first signs in June, mines occupying most of the leaf area by August and early leaf drop on heavily affected trees. The Woodland Trust adds tracks inside the leaves, dry crisp leaves and a crown that may be bare by autumn.

Forest Research warns that the brown patches can be confused with horse chestnut leaf blotch, a fungus disease, which leaves a distinctive yellow border and a more reddish-brown colour, and with leaf scorch, which browns whole leaves with no mines. A tree can have the leaf miner and the leaf blotch together. For another horse chestnut problem see the bleeding canker of horse chestnut page.

The adult moth is described as about 0.5 centimetres long and brown and silver by Forest Research, and as 4 to 5 millimetres long, rich brown with white chevrons edged with black, by the Woodland Trust. The RHS gives a wingspan of 10 millimetres.

Life cycle as the sources state it

Forest Research says adults appear from April after overwintering as pupae in leaf litter, and that between May and August females lay 20 to 40 eggs near the side veins of the leaves. Eggs take two to three weeks to hatch, the larvae mine between the two outer layers of the leaf for up to four weeks, and adults emerge about two weeks after pupation. It gives two or three generations a year in the United Kingdom, seven to 10 weeks per generation, and five a year in warmer, drier climates. It says pupae survive winter temperatures as low as minus 23 degrees centigrade. The RHS and the Woodland Trust also give an overwintering pupa, with the RHS stating three generations in summer. The Woodland Trust says a female lays up to 180 eggs, a different figure from the Forest Research range, and the pages do not explain the difference.

Where it is reported

Forest Research says the moth is found across most of England and Wales and across central and eastern Europe, with four reported sightings in Scotland, three of them in Edinburgh. The distribution map on that page shows 2014, and the page says the moth will be present in other areas beyond those mapped. The RHS says the moth was first identified in Britain at Wimbledon in 2002, has become widespread in England, is spreading in Wales and has a few sightings in Scotland. The Woodland Trust puts the first United Kingdom record at 2002 in England and says it has spread into Scotland as far as the central belt. It gives the origin as south-eastern Europe, and says some of the larger jumps in range have been put down to imports of infested saplings. Forest Research says movement of adult moths or infested leaves on vehicles is the likeliest explanation for sudden appearances far from known areas, and that further spread in the United Kingdom is likely to be extremely slow.

Damage as the sources describe it

Forest Research says the leaf miner does not significantly impair a tree's overall health and the effect is mostly aesthetic. It reports that up to 75 percent of leaf area can be attacked, while loss of photosynthesis is estimated at 30 to 40 percent over the season at most, because most of the damage comes late. It also says infestation lowers seed weight, germination and seedling vigour, so conkers are affected, and that the moth might add to decline set off by other threats such as bleeding canker. The RHS says heavily affected trees have almost no change in growth rate or health, though conkers may be slightly smaller. The Woodland Trust says it does not kill trees but that years of infestation can leave horse chestnuts weakened and more open to diseases such as bleeding canker. The pages differ in how far they link the moth to long-term weakening.

Regulated status and reporting

Forest Research's page marks the pest as present in the United Kingdom and, in its header, "Reportable in Scotland". The text under its heading on reporting a sighting says there is no requirement to report sightings to the plant health authorities, and that records from Scotland can be sent through TreeAlert for research and monitoring. How reports reach each country's tree health body is on the page for reporting a pest in the United Kingdom. No regulated-pest page is held on this site for this moth.

Management as the sources list it

Forest Research lists raking up fallen leaves in autumn and winter where practical, with burning where permitted or sealed composting, to break the cycle, and says clearing leaves is a sufficient control in many cases. It says birds such as tits and several parasitoids feed on the moth but none is thought likely to change populations much, that there is not a strong reason for pesticides because the tree's overall health is not significantly harmed, and that spraying is unlikely to be completely effective and would harm bees and other pollinators. The RHS says horse chestnuts appear to tolerate the moth, so control is not necessary or practical, and lists clearing fallen leaves and pheromone traps as measures that can delay the build-up on isolated trees. The Woodland Trust says there is no getting rid of the pest, with populations that may rise and fall. This page gives no product names, rates or timings. For who may apply pesticides, see pesticide licences.

Contains public sector information published by Forest Research and licensed under the Open Government Licence v3.0.