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Horse chestnut leaf miners

Photo of a horse chestnut leaf minerThe leaf miner, Cameraria ohridella, is a tiny brown moth only 6-8mm in wingspan. It lays eggs on the leaves of horse chestnuts and the caterpillars make mines in the leaf tissue about 1-2mm long. This sounds a very weak threat to the towering and majestic trees that are not starting to develop the familiar conkers, but Cameraria soon develops huge populations in the early summer. The feeding of the caterpillars turns the leaves brown in mid-season and they drop. The stress on the trees may kill them but at best the trees become extremely unsightly.

The origin of Cameraria is obscure. It was discovered and described as a new species in Macedonia in 1986, where a mass outbreak was found around Lake Ohrid. Then, in 1989, the first leaf mines were found in Linz, Austria. The following years saw a rapid invasion into surrounding countries. The pest now covers Germany, Poland, the Czech Republic, Slovakia, Hungary and has also been found in Eastern France, Italy, Switzerland, the Netherlands and Belgium. From the original site in Macedonia it has also spread to neighbouring Albania, Bulgaria and Romania.

How can such a spread occur?

Horse chestnuts are indigenous to the Balkan Peninsula, although they have been spread to many parts of the Western world by the Romans and others as ornamental and shade trees. We can only guess why cameraria suddenly became a pest: is this a mutant species? Was it introduced from a more distant location in Asia or the USA, where it perhaps feeds on a relative of the horse chestnut?

Certainly the scarcity of parasites in its new environments and its ability to reproduce more or less continuously from April to November mean that populations can increase a thousand-fold in one year. The main suspicion is that the insect is carried around in the summer by cars, lorries and trains. If you park your car with the windows open near infested horse chestnuts, moths can find their way in. They are such poor fliers that they are unable to escape from a vehicle as larger insects can. Hence, they can be transported long distances and a car may then later be parked in the shade – underneath yet more horse chestnuts.

The threat to Britain and the south-east in particular has been increasing yearly. Insects were detected just outside Paris in 2000, and the intense traffic flow across the English Channel is likely to support a UK invasion. The first confirmation of the leaf miners’ presence in the UK was made in July 2002 in Wimbledon, south-west London. It has now spread north of the Humber and west of the Severn.

For the latest on the problem, visit the Forestry Commission’s website which also has information on the creatures and how to spot them.