Sick Series: Horse chestnut leaf-mining moth

10th July 2019

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Sick Tree Series
As we have seen in previous ‘sick tree’ series blogs, moths really can be a nuisance to our tree population – in the case of the oak processionary moth (OPM), they can even be lethal. While nowhere near as damaging to a tree’s health, the horse chestnut leaf-miner can significantly change the appearance of the horse chestnut trees they feed on, and can cause great concern if you see this happening to the trees in your garden. This article should help you to identify this little blighter so you can relax in the knowledge that while the damage they cause may not look great, the harm they do will be minimal.

First discovered in Macedonia in the late 1970s, the horse chestnut leaf-miner is a relatively new problem to the UK with the first reported incident in England being just 16 years ago in 2002. Preferring the widely planted, white flowering variety of horse chestnut tree, the infestation has spread quite aggressively, at a rate of around 40–60km a year, and is now present throughout East Anglia, the Midlands and across most of south-central England. That’s quite a spread in less than two decades.

Symptoms of horse chestnut leaf-miners
If your horse chestnut tree has shrivelled yellow and brown leaves which it sheds prematurely, it may have a case of horse chestnut leaf-miner. The larvae of the leaf miner feed on the leaves causing brown splodges to appear with a yellow surrounding area, both of which expand to cover the majority of the leaf. Most active between June and September, the infestation causes brown and white blotches on the leaves. The appearance can be quite alarming, so it wouldn’t be surprising if you assumed that your tree was dying, but while the infestation takes its toll on the look of the tree, it doesn’t affect its health. It may be tempting to fell the tree, especially if you suspect a more dangerous cause, but the horse chestnut can survive repeated infestations and produce normal leaves again after shedding. So, the lesson here is, call in an expert to provide you with a more accurate diagnosis of the issues your tree is suffering from and to discuss the best course of action.

Causes
The larvae of the moth Cameraria ohridella, which is yellow and only around 5mm in length, mines through the leaves, munching away so much that the leaves physically close in on themselves, appearing shrivelled, and, having had the life mined out of them, they lose their greenery. The larvae is so tenacious that up to 700 mines have been found on a single leaf.

What are the telltale signs?
In Britain, horse chestnuts produce normal flowers and foliage in the spring. If your tree is infested, the first signs are white elongated blotches on the leaves, which later turn brown, and these will normally appear in and around June. By August the majority of the leaf will be taken over by mines, and heavily infested trees will drop their leaves soon after.

Reduced growth
While infestations do not affect the health of the tree, heavy attacks can reduce its growth, leaving it more vulnerable to attacks from other infestations, diseases and fungi. If you spot any changes in your horse chestnut tree, it’s worth getting them checked out to prevent further problems in the long run.

To book a survey to determine whether your tree has been taken over by moth larvae, call us on 0208 292 8992. We’re always happy to pop round or to help with a little advice.

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