The horse chestnut leaf miner moth (Cameraria ohridella) is present throughout the UK following rapid expansion subsequent to its discovery in 2002. This page briefly describes the biology of the species, the risk it poses to horse chestnut trees in Britain, and its identification. There is no requirement to report sightings of horse chestnut leaf miners to the plant health authorities, but records from Scotland can be submitted through Tree Alert. Control of the moth is best done by composting or otherwise removing the fallen leaves in Autumn.
All information current as of April 2020.
Picture: Gyorgy Csoka, Hungary FRI, Bugwood.org
The horse chestnut leaf miner (Cameraria ohridella, henceforth HCLM) is an invasive moth in the UK, first recorded here in 2002. The caterpillars, or larvae, of HCLM are pests of horse chestnut trees (Aesculus), including the common or European horse chestnut, commonly known as a ‘conker’ tree (Aesculus hippocastanum), which is the most populous species of horse chestnut tree grown in the UK.
Adults appear from April onwards, emerging after overwintering as pupae in leaf litter, and will subsequently mate during the early mornings. Between May and August, females will lay between 20-40 eggs near or along the lateral veins of horse chestnut leaves. From here, these eggs will take 2-3 weeks to hatch. The larvae tunnel or ‘mine’ into the leaves, living between the two epidermis layers (outermost layers of the leaf), eating the contents between, producing the symptomatic brown patches on the host leaves. Larval development can take up to 4 weeks to complete before pupal development occurs, where a silken cocoon is formed inside the mine, and adults will emerge around 2 weeks later to repeat the cycle. HCLM pupae can survive temperatures over winter as low as -23oC. In the UK’s climate of mild-wet summers, 2-3 generations per year is normal, with each generational period taking between 7-10 weeks to complete (from egg to adult) during the summer. However, in warmer/dryer climates outside of the UK, 5 generations per year can be achieved. The moth’s high fecundity and multiple generations a year can lead to high population densities; and in mid-late summer coalescence of the brown patches caused by larval feeding occurs until nearly the entire tree has brown leaves, resulting in an ‘early autumn’ (pictured below). The leaves will eventually drop, and the replacement leaves can then be attacked by the following generation of HCLM.
Picture: Fabio Stergulc, Università di Udine, Bugwood.org
Within the UK, it is normally the pupae of the final generation that will enter diapause to overwinter before remerging the following year. However, if the population density is very high, overcrowding can cause individuals of the 1st & 2nd generations to enter an early diapause and delay emergence until the next spring (in severe infestations, over 100 larvae can be found within each leaflet of a horse chestnut leaf). This means there are usually always at least some individuals to survive to the following year and re-establish the population.
The risk posed
Horse chestnut trees (Aesculus), including the common/European horse chestnut, are deciduous trees originally native to the Balkans. They have been planted mostly as an ornamental tree across much of Europe, and were first introduced to the UK in the 1600’s. As the wood is soft and not often used for commercial forestry, the species is usually uncommon in forests; however, European horse chestnut can be frequently found along riverbanks and parks, where they are valued highly for their aesthetic appeal.
At high population densities, HCLM has the capacity to destroy most of the leaf tissue on an individual tree (see Biology), causing decolouration and premature defoliation before the natural autumn leaf fall. These symptoms start in the lower canopy, before spreading upwards to cover the entirety of the tree. They can cause severe damage to horse chestnut leaves on an annual basis. Once established, HCLM will very quickly become ubiquitous across European horse chestnuts in a location.
Picture: Gyorgy Csoka, Hungary FRI, Bugwood.org
However, the effect on horse chestnut trees is mostly aesthetic, as HCLM does not significantly impair the trees overall health. Research has shown HCLM can attack up to 75% of the total leaf area on the horse chestnuts, but that the loss of subsequent photosynthetic leaf tissue only reduces the total carbon assimilation by at most an estimated 30-40% over the growing season. The reduction is much less than the total leaf area affected because, the majority of damage caused by HCLM occurs too late in the season, after the tree has completed most of its photosynthesis for the year. As such the general tree condition and stem radial growth are not affected by HCLM, even over repeated annual attacks. However, infestation decreases the reproductive output of the trees, through a reduction in seed weight, seedling germination and vigour.
Following an infestation, the trees will usually flush normally the following spring, where they can be reattacked by the pest which have overwintered nearby.
Picture: Nigel Straw.
Discoloration of the tree canopy, note the discoloration has started at the bottom and is spreading upwards.
HCLM may exacerbate any decline and eventual death triggered by one or more other threats. These can include bleeding canker of horse chestnut, horse chestnut leaf blotch, leaf scorch, and damage to leaves caused by droughts and floods. Horse chestnut trees with all three of leaf miner, leaf blotch and bleeding canker have been observed. See ‘Our research’ below for details about how we and others are looking into these interactions.
Management and control
The pupae of HCLM overwinter in the fallen leaves of horse chestnut trees. Therefore, damage can be minimised by raking up fallen leaves during the autumn and winter, where this is practicable. They should then be burned (where this is permitted), or thoroughly composted, in sealed bags if possible, until the following July to destroy the pupae. These actions will break the species’ life-cycle and reduce the population of the next year’s generation. Clearing leaves from around the tree is a sufficient control in lot of cases. Horse chestnuts in urban areas are often less affected because the leaves are more likely to be cleared away, and those in exposed park land typically have the leaves blown away or mown over.
Some bird species, such as tits, prey on horse chestnut leaf miner, and a number of parasitoids attack the caterpillars. However, none is thought likely to make a significant difference to populations of the pest.
As HCLM do not significantly harm the trees overall health and can be easily dealt with using the method above, there is not a strong reason to employ pesticide use as a control method. Spraying and other insecticide application methods are unlikely to be completely effective and will have damaging non-target impacts on bees and other pollinators which visit horse chestnut flowers. Removing and destroying the leaves is the cheapest and most environmentally friendly method currently available for dealing with HCLM.
There are 13 species of horse chestnut trees and shrubs (Aesculus), all native to the northern hemisphere. Susceptibility to HCLM, is related to taxonomic and evolutionary relationships, rather than a specific geographic origin.
In addition to European horse chestnut, HCLM is also able to feed on other Aesculus species, though the feeding damage is never as significant as that on the European species. This includes the Japanese horse chestnut (A. turbinata), red flowering horse chestnut (A. x carnea), Indian horse chestnut (A. indica) and red buckeye (A. pavia). It can occasionally also attack Acer species such as Norway maple (Acer platanoides) and sycamore (A. pseudoplatanus), particularly if planted nearby a horse chestnut It is not thought to pose a significant threat to any of these species.
HCLM does not affect sweet chestnut trees (Castanea).
Origins and entry into Britain
The HCLM’s native range is thought to be the Balkan region of South-eastern Europe. Mitochondrial & microsatellite DNA analysis has confirmed that all invasive European populations of HCLM originate from this region, which is also the native range of the European horse chestnut tree.
HCLM was first observed in Macedonia, in Northern Greece, in 1984, and was described as a new species in 1986 (Deschka & Dimić, 1986). However, earlier examples of leaves with leaf miner damage, in one casing dating back to the 19th century (1879), have been reported in herbaria (collections of preserved plant specimens).
It was reported in Austria in 1989, and since then it has spread throughout Central and Western Europe.
In 2002, it was discovered in the London borough of Wimbledon, and has since spread throughout England and Wales. There is no requirement to report sightings from either country.
There have been four reported sightings in Scotland, three of which were in Edinburgh, though a subsequent visit to the tree outside Edinburgh in 2019 failed to relocate any mines on the tree. Any additional reports from Scotland would be welcomed.
Dispersal of the moth from infested areas occurs on a broad front through adult flight, assisted by the wind, and through the passive transport of adult moths or infested leaves, in or on vehicles. Transportation by vehicles is the most likely explanation for the sudden appearance of the moth in places a long way from known areas of infestation. Wherever it has established, as seen in mainland Europe, it has built up high population densities, and spread at a rate of 40-70km per year. Any spread in the UK is now likely to be extremely slow and mediated by climate.
HCLM moths – the adult stage of the species’ life cycle – are brown and silver micro-lepidoptera and are, about 0.5cm (0.2 inches) long, with a wingspan of about 1cm (0.4 inches).
The symptomatic patches on the host’s leaves appear in the summer, sometimes as early as June, and can give the trees the appearance of under-going an early autumn, as in the picture below. They are caused by the larvae (caterpillars) ‘mining’ through the leaves as they feed. Holding affected leaves up to the sunlight in summer might reveal the tiny caterpillars, or their circular pupal cocoons, within the mined areas.
Elongate patches on the leaves, starting white and turning brown (below), are a sign that HCLM might be present in horse chestnut trees.
So much of the foliage can turn brown and/or drop their leaves early by late summer that the tree appears to be dying, but owners should not draw this conclusion without first investigating whether other factors are involved.
Damage to horse chestnuts by other species that might be confused for HCLM:
Horse chestnut leaf blotch is caused by the fungus Phyllosticta paviae (syn. Guignardia aesculin). Phyllosticta blotches have a distinctive yellow border around areas of dead tissue, and are otherwise a more reddish-brown colour. Trees can be affected simultaneously by both HCLM and leaf blotch. The difference between patches caused by horse chestnut leaf blotch and HCLM are pictured below.
Trees in urban areas can be badly affected by leaf scorch, particularly where there is heavy traffic flow, root compaction, spring droughts, or unseasonably high temperatures. This occurs where water loss through the leaves is too high for the roots to compensate, and they die from dehydration. Scorched leaves will be almost completely brown without any mines or halos.
Picture: Left - Milan Zubrik, FRI, Slovakia, Bugwood.org, Right - RTECtreecare.com
Left – Mines caused by horse chestnut leaf miner (C. ohridella).
Right – Blotches caused by horse chestnut leaf blotch (Phyllosticta paviae) with its distinctive yellow borders.
It is possible that differences in climate, or interactions with other pests and diseases, might lead to horse chestnut leaf miner’s having greater impact in the UK. Forest Research scientists are therefore conducting a long-term monitoring study of more than 300 horse chestnut trees at several sites in southern England.
The study aims to determine whether there are any interactions between HCLM and the bacterium Pseudomonas syringae pv. aesculi, which causes most cases of bleeding canker of horse chestnut. The trees are assessed twice each year for infestation, crown condition, growth and signs of dieback.
The study’s objectives include assessing whether one influences the extent of the other, and how they affect the health of affected trees. A paper reporting the results of the first 10 years of the study was published in the journal Agricultural & Forest Entomology.
Currently the research indicates that although there might be a reduction in conker size, long-term defoliation by HCLM does not have an impact on tree growth or health, and nor does it increase susceptibility to bleeding canker.
Data collection has continued, and an update will be published when the next 10 years data have been fully collected and analysed.
Straw, N. A., & Williams, D. T. (2013). Impact of the leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae) and bleeding canker disease on horse-chestnut: direct effects and interaction. Agricultural and Forest Entomology, 15(3), 321-333.
Pocock, M., Evans, D., Straw, N., & Polaszek, A. (2011). The horse-chestnut leaf-miner and its parasitoids. British Wildlife, 22(5), 305.
Koskella, B., Meaden, S., Crowther, W. J., Leimu, R., & Metcalf, C. J. E. (2017). A signature of tree health? Shifts in the microbiome and the ecological drivers of horse chestnut bleeding canker disease. New Phytologist, 215(2), 737-746.
Valade, R., Kenis, M., Hernandez‐Lopez, A., Augustin, S., Mari Mena, N., Magnoux, E., ... & Lopez‐Vaamonde, C. (2009). Mitochondrial and microsatellite DNA markers reveal a Balkan origin for the highly invasive horse‐chestnut leaf miner Cameraria ohridella (Lepidoptera, Gracillariidae). Molecular Ecology, 18(16), 3458-3470.
Straw, N. A., & Tilbury, C. (2006). Host plants of the horse-chestnut leaf-miner (Cameraria ohridella), and the rapid spread of the moth in the UK 2002–2005. Arboricultural Journal, 29(2), 83-99.
Thalmann, C., Freise, J., Heitland, W., & Bacher, S. (2003). Effects of defoliation by horse chestnut leafminer (Cameraria ohridella) on reproduction in Aesculus hippocastanum. Trees, 17(5), 383-388.