The leaf-mining moth
The horse-chestnut leaf-mining moth is one of the threats to our horse-chestnut trees. It is a tiny moth about the size of a grain of rice. It's scientific name is Cameraria ohridella (it is called 'ohridella' after Lake Ohrid, in Macedonia where is was first discovered in the late 1970s).
The horse-chestnut leaf-miner first arrived in the UK in London in 2002. Since then it has spread at a rate of about 30km per year. It reached parts of Cornwall and west Wales in 2011 and has continued to spread slowly in those areas. Intruigingly it got to the Newcastle area in 2010 but then in 2014 it was found near Loch Tay in the middle of Scotland. By 2013 it has got across the sea to Ireland, when it was found in Dublin, and was found in several new places on the island of Ireland, including Belfast, in 2014. We invite anyone to submit records of the leaf-miner, especially from north of York-Manchester and from Wales, the west country, Northern Ireland and the Republic of Ireland. Do be careful not to confuse it with the leaf blotch fungus though - discover how to tell them apart.
What is it?
The horse-chestnut leaf-miner (Cameraria ohridella) is a tiny little moth (slightly smaller than a grain of rice). It's larvae live inside the leaves of horse-chestnut trees and 'mine' their way through the green part of the leaf. Each individual leaf mine is about the size of a ballpoint pen lid (1cm x 3cm), although in serious infestations the mines merge together as the larvae search for food.
There are two or three generations of the moth per year. They overwinter in the leaf litter under the tree and emerge in late spring to mate and lay eggs on the horse-chestnut leaves. Each female can lay up to 100 eggs, meaning that populations can increase rapidly through the summer.
The horse-chestnut leaf miner only feeds on horse-chestnut trees, and particularly prefers the white-flowering horse-chestnut.
What harm does it do?
The leaf-mining moth does not kill horse-chestnut trees. There is mixed evidence as to how much it affects the trees.
Working with a secondary school recently, the pupils estimated that there were about 30 000 leaf mines on a single, large horse-chestnut tree in the middle of July. In September, we estimated that the large horse-chesnut tree in front of King's College in the centre of Cambridge had produced at least 150 000 moths during the course of the year. (That is as many people as live in Cambridge!)
Scientific research reveals that the leaf-mining moth reduces the total photosynthetic capacity of a tree by 15-30%. (Total photosynthetic capacity is the tree's ability to make its own food via photosynthesis.)
This has knock-on effects: the mass of conkers is reduced by 50% (which equates to the diameter being reduced by about 10%). There is mixed evidence as to whether it affects the growth rate of the tree.
There are many different species of insect that are leaf-miners, which have caterpillars that live inside the leaves 'mining' their way through the green tissue of the leaf and leaving the upper and lower surfaces intact. Many different insects are leaf-miners, including small moths, flies and beetles. Most leaf-miner species are specific to one or a few types of plant.