The Tiny Pest Disfiguring our Horse Chestnut Trees

13th July 2012
We are very fortunate, in that where we live there is a very old, large and beautiful Horse Chestnut tree directly outside our house. Watching it come into leaf in the Spring and developing through the Summer, has always been a pleasure. Of course, there are the conkers later in the year, eagerly collected by the local children.

Recently, this statuesque tree has been looking a bit sad. It's seriously affected by the horse-chestnut leaf miner moth, Cameraria ohridella. The moth is small (between 3-4mm) and lays its eggs on the surface of the Horse Chestnut leaves. The larvae enter the leaf and eat away at the inside, producing the characteristic "mines" that turn brown as the season progresses. Fortunately, although the foliage is disfigured and affected trees possibly weakened, the pest does not kill the tree.

This species was discovered near Macedonia in 1985, and has spread rapidly to other countries in Europe, being first discovered in Britain at Wimbledon in 2002. It is now found in many areas of Southern England.

I decided I'd have a closer look and try to photograph the larvae. I picked some affected leaves and the photograph top left, shows a larva inside the leaf structure. There are many in each leaf. These larvae and only 1-2mm in length and it's difficult to see the detailed structure through the external layer of the leaf. I managed (with a considerable amount of difficulty!) to extract one and photograph it on the surface of a leaf (image left). An unremarkable creature really!

Undisturbed, this larva would have completed its development and then pupated. This typically takes place four weeks after hatching. After a further couple of weeks, the adult emerges. The moth can complete several lifecycles in a season, typically three in Western Europe. The final cycle of the season sees the pupae over-wintering. They are very frost-tolerant and can survive very harsh winters.

The images above show the typical appearance of an infected tree (left), a leaf (center), showing "mines" in various stages of development, and then a close-up (right), with individual larvae within the leaf circled. It will be interesting following the tree over the coming years and seeing how it fares. I must also get some photographs of the adult moth.

(Click on any image for a larger version)

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