Horse Chestnut trees, Aesculus hippocastanum , are very large deciduous trees, 36m tall, found in the countryside, parks and gardens.They have bold divided leaves with white spires of flowers in the spring which later, in the autumn, turn into the fruit, horse chestnuts or conkers which have spiny outer casings.For quite some time now we have been reading that there is a problem in the UK with the Horse Chestnut leaf miner moth, Camerarie ohridella. It was first noticed in 1984 in Macedonia, a small country north of Greece, which has spread steadily across Europe , until it reached our shores. In 2002 it was discovered for the first time here in Wimbledon, and has taken another 9 yrs to reach us. The moth causes damage to the leaves but so far there is no evidence that it causes the death of the trees, however the fruits, conkers, are fewer and smaller.
You can imagine my horror one day when I went into our woodland and found that this year our 4 trees have been infected. The worst infestation is on the oldest Chestnut tree that we have at the end of the woodland, the tree must be at least 200 yrs old, going by the girth of the trunk. The moth lays its eggs on the leaf and the caterpillar burrows between the top and bottom layer of the leaf, causing ugly patches to form as it travels around inside the leaf. Apparantly trees survive the loss of these leaves and usually come into leaf normally the following year.
The larva damage the leaves and stunt growth. The leaves are covered in brown patches which spread across the entire tree, making it look as if autumn has come early, the leaves die and drop off. The woodland floor is covered with dead chestnut leaves at the moment, it looks more like November than the beginning of August.
New leaves grow and are again infected, this can repeat itself several times in a season, surely this must weaken the tree. Worryingly, the moth can have 3 generations in one year with the last generation pupating for 6 months over the winter. Our last winter , even though we had -15C for some time wasn’t cold enough to get rid of it, it is capable of surviving temperatures down to -23C.
We do have plenty of natural predators, blue tits, great tits and oak bush crickets eat the pupa in the leaves, but unfortunately this only accounts for a small percentage of the problem. This one is empty, has one of the predators found it or has a moth emerged!
The recommended procedure to eradicate the moth, is to collect and burn all the leaves before March, which is when the new moths will be emerging, this should then kill all the pupa in the leaves, which in turn should mean that there would be far fewer moths the following year. I have just been onto the R.H.S. website and as an alternative to burning, the leaves could be gathered up and put into sealed bags until at least next July. This would mean that the moths would emerge and die in the bags. We will have to have a word with our neighbours, 3 more houses have Chestnuts in their woodland and I can see, when I pass, that their trees are also infected.
We had all better get started collecting leaves, I can see that we are going to be very busy.