Present in UK
Notifiable – No
Scientific name - Cameraria ohridella
Picture: Gyorgy Csoka, Hungary FRI, Bugwood.org
The caterpillars, or larvae, of the horse chestnut leaf miner moth are a pest of horse chestnut (trees in the Aesculus genus). This includes the common or European horse chestnut, or ‘conker’ tree (Aesculus hippocastanum), the most populous species of horse chestnut tree grown in the United Kingdom.
Horse chestnut leaf miner is present throughout central and eastern Europe, and in much of England and Wales. There have been four reported sightings in Scotland, three of which were in Edinburgh. Our map shows where it was confirmed to be present in 2014, although it will be present in other areas by now.
Horse chestnut leaf miner caterpillars ‘mine’ within the leaves to feed, and at high population densities they can destroy most of the leaf tissues by the end of summer. They can cause severe damage to horse chestnut leaves on an annual basis, and discolouration and defoliation before normal autumn leaf-fall (below).
Picture: Gyorgy Csoka, Hungary FRI, Bugwood.org
However, on its own the pest does not significantly impair trees' health, and they will usually flush normally the following spring. It is possible that the seeds, or conkers, might be smaller than they would otherwise be.
The effect is therefore mostly aesthetic. However, horse chestnut leaf miner might 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, drought and flood. (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.
In addition to European horse chestnut, horse chestnut leaf miner is also able to feed on other Aesculus species, including Japanese horse chestnut (Aesculus turbinata), red flowering horse chestnut (A. x carnea) and red buckeye (A. pavia). It can occasionally also attack Acer species such as Norway maple (Acer platanoides) and sycamore (A. pseudoplatanus). It is not thought to pose a significant threat to any of these species.
Horse chestnut leaf miner does not affect sweet chestnut trees (trees in the Castanea genus).
Identification and symptoms
Elongate patches on the leaves, starting white and turning brown (below), are a sign that horse chestnut leaf miner might be present in horse chestnut trees.
Picture: Milan Zubrik, FRI, Slovakia, Bugwood.org
These patches 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.
Picture: Fabio Stergulc, Università di Udine, Bugwood.org
So much of the foliage can turn brown 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.
(At first glance the brown patches can be confused with those made by horse chestnut leaf blotch, which is caused by a fungus called Guignardia aesculus. However, Guignarda blotches have a distinctive yellow border around parts of them, and are otherwise a more reddish-brown colour. Trees can be affected simultaneously by both horse chestnut leaf miner and leaf blotch.)
Holding affected leaves up to the sunlight in summer might reveal the tiny caterpillars, or their circular pupal cocoons, within the mined areas.
Horse chestnut leaf miner moths – the adult stage of the species’ life cycle - are brown and silver, about 0.5cm (0.2 inches) long, with a wingspan of about 1cm (0.4 inches). See top picture.
Heavily infested trees can drop their leaves early, before autumn.
Report a sighting
There is no requirement to report sightings of horse chestnut leaf miner to the plant health authorities, but records from Scotland can be submitted through Tree Alert.
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.
Management and control
The pupae of horse chestnut leaf miner over-winter 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 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.
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.
Background and origins
Horse chestnut leaf miner’s natural range is thought to be the Balkan region of south-eastern Europe, which is also the native range of European horse chestnut. It was first observed in Macedonia, in northern Greece, in 1984, and was described as a new species in 1986. However, earlier examples of leaves with leaf miner damage, in one casing dating back to the 19th century, 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 eastern Europe.
It was first found in Great Britain in 2002, in the London Borough of Wimbledon, and has since spread to most of England and parts of Wales and Scotland.
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 horse chestnut leaf miner 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 horse chestnut leaf miner does not have an impact on tree growth or health, and nor does it increase susceptibility to bleeding canker.