One of the most common trees in the UK is the horse chestnut. One of the biggest threats to this beloved tree is the leaf-mining moth. As far as moths go, this one is very small, only about the size of a grain of rice. Part of its Latin name is ohridella, taken from Lake Ohrid in Macedonia where it was first discovered in the 1970s.
The moth was first seen in the UK in 2002 and since then it has travelled around 30km every year to different areas of the country. By 2013, it had reached Ireland and the following year, was found in central Scotland.
The larvae of the leaf mining moth live inside the horse chestnut leaves and damage them by ‘mining’ through the green areas of leaves. A single mine is only small but when an infestation occurs, the mines merge making the damage more obvious. Fungal leaf blotch is another harmful issue that horse chestnuts must deal with and produces similar symptoms to the moth.
The moth will go through 2 or 3 generations in one year. The moths emerge during the spring to mate and lay eggs on the horse chestnut leaves. Populations can increase massively during the summer as each female lays around 100 eggs. These moths will only feed on the leaves of the horse chestnut tree, especially the white-flowering species.
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What damage does it cause?
Thankfully, the leaf mining moths do not kill the horse chestnut trees. What is unclear is the amount of damage that is caused by a leaf mining moth infestation.
During the peak of the summer, it has been estimated that there could be around 30,000 leaf mines on a single horse chestnut tree. One of the biggest horse chestnut trees has been estimated to have produced at least 150,000 moths during a one-year cycle. That is the same number of people who live in Cambridge where the tree is located!
Research has revealed that leaf mining moths reduce the photosynthetic capacity of a horse chestnut tree by up to 30%. This is the ability for the tree to make its own food through photosynthesis. The effects of this reduction include conker production reduces by 50% and the overall diameter of the tree decreases by around 10%.
There are a lot of different insect species that also mine leaves. Caterpillars often live inside leaves and ‘mine’ their way out through the green tissue. Other mining insects include flies, beetles and small moths. Most of these species are specific to a certain type of plant.