Q What is horse chestnut leaf miner?

A It is the caterpillar of a small moth that burrows between the upper and lower surfaces of the leaf, creating conspicuous pale blotches.

Caption: Horse chestnut leaf miner probably arrived in Britain in 2000

Q How do I recognise it?

A The adult moth is unremarkable and unlikely to be spotted. The damage is easily recognised as it is the only leaf miner to attack horse chestnuts. The mines are circular at first, then develop into long ovals running between the veins. They start off looking pale green, but later turn brown.

Q What plants does it attack?

A It is most frequently found on horse chestnut but where there are heavy infestations, nearby sycamores and Norway maples can be also attacked.

Q Where does it come from?

A The original home of the horse chestnut leaf miner (Cameraria ohridella) is uncertain, but it was first identified in northern Greece in 1985. Since then it has spread inexorably at about 80 miles a year across eastern then western Europe. It probably arrived in Britain in 2000 carried by the wind or on vehicles. The first recorded outbreak was in 2002 in Wimbledon, London. It has rapidly colonised the south, east and midlands though it is still relatively uncommon in the north of England, west Wales and Scotland.

Q What damage does it do?

A Caterpillars burrow into the leaf, eating the tissues and reducing the tree’s ability to photosynthesise. In time the leaf dies and falls early. Despite this, trees do not seem to suffer long term damage and leaf up normally in spring. This is probably because leaf miner numbers don’t build up until quite late in the season, when effects are less severe.

Q Could I mistake it for any thing else?

A From a distance, especially when they start to turn brown, the mines can look like leaf spots. However, mined areas are translucent, if held up to the light, and you can spot the caterpillar, or a dark circle where it has pupated. Leaf spots are opaque.

Q How serious is it?

A With up to 700 mines in a single leaf the damage can be very disfiguring. The leaves then shrivel, go brown and drop early. This is a serious problem given that horse chestnuts are almost exclusively planted for their ornamental value. Healthy, vigorous trees seem to be otherwise unaffected, but those already under stress eg from drought may suffer long-term damage.

Q Why has it become such a problem?

A Horse chestnut leaf miner can build to very high levels of infestation. It has natural predators – parasitic wasps, – but they tend to be present only at low levels.

Q What is its life cycle?

A The first eggs are laid in midsummer. The caterpillars live their whole lives within the leaves and pupate there for between two weeks and seven months. New adults emerge in late summer, and lay more eggs. The resultant pupae remain in the fallen leaves over winter, tolerating temperatures as low as -23C, to emerge in spring. In warm regions the moth may have five generations in a season, though there seem to be only two or three in Britain.

Q How do I treat affected trees?

A It isn’t practical to treat insects in the trees, and spraying could well do more harm by killing off natural predators.

Q How can I prevent it spreading/recurring?

A The insect overwinters as a pupa in fallen leaves. If leaves from isolated trees, or from all the trees in an area, are collected up, this can help reduce the infestation levels in subsequent years. The leaves must be burned, consigned to a commercial composting system, or buried under 10cm of soil until May to prevent the adults emerging. So, for example, if you had a single tree in your garden and no others nearby it would be worth collecting up all the leaves and either sending them to your council composting scheme, or burying them at the bottom of a runner bean trench, filled with other vegetation and soil, before May. The long-term hope is that an effective natural predator can be found.

Q Are there any resistant varieties?

A The common horse chestnut Aesculus hippocastanum is very susceptible, though some individual trees show resistance.

Red buckeye (A pavia) is equally susceptible. However, the red-flowering hybrid (A. x carnea) is very resistant and the less widely planted species A. indica, A. glabra and A. parviflora are also resistant.