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Are your horse chestnut trees looking ill?

During the summer, I noticed that all the horse chestnut trees around us were not looking too healthy. Their leaves were turning brown and looked as if they had been attacked by something.

(photo by Grahame Madge)

On doing a bit of research, yesterday, I found out that the culprit is a horse chestnut leaf miner. The damage is caused by a tiny ‘alien’ species of leaf-mining moth, which is invading the UK.  For biologists, an ‘alien’ is a species not naturally found in an area or habitat.  The moth’s caterpillars eat the leaves from the inside.  Infected trees are weakened, and produce smaller conkers. Apparantly, it is not thought that this causes any lasting damage and that they should recover next spring when they come back into leaf.

The actual moth is tiny… with a wingspan of only 8mm – amazing that an insect such as this can cause such devastation to an enormous tree. There must be millions of them on each tree. Here are a few facts I copied from an article I found:

Adults appear from April onwards, initially from pupae that have over-wintered. The adults are up to 5 mm long. The forewings are metallic chestnut brown with silvery white transverse stripes edged in black. The hindwings are dark grey with long fringes. Eggs are laid from May to August along the lateral veins on the upper side of the leaflet; each female lays 20—40 eggs.  Larvae pass through five stages (instars) and complete their development in about 4 weeks. They feed inside the leaf tissue, leaving only the upper and lower epidermises intact.  Pupae develop in a silken cocoon in the mine, and generally complete development in about 2 weeks, but this stage can last for 6-7 months in the overwintering generation. The over-wintering pupae survive among the fallen leaves and are known to be extremely frost tolerant. Throughout the summer it is possible to find all developmental stages present in attacked leaves.

There will certainly be a lack of conkers around this year for the kids. I reckon only about a fifth of the trees near me have any conkers.


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