There are several horse chestnut trees around St Leonard’s, with a few fine specimens overhanging Matford Lane from the grounds of County Hall. They are trees which make their presence felt at this time of year, when the conkers start falling, and children start collecting them to play with. (Despite rumours, it is not necessary to wear goggles for games of conkers!)
We are also aware of horse chestnut trees in the spring, when their “Roman candle” flowers make a splash of colour.
Something odd has happened to our horse chestnut trees in the last ten years. The leaves are turning brown earlier in the autumn, and the fallen leaves are shrivelled. The trees are being attacked by a moth, the horse chestnut leaf miner (Cameraria ohridella). Its tiny caterpillar (a little smaller than a grain of rice) is responsible for the damage. The first reports of this moth in the UK were on Wimbledon Common in 2002, and after that, the infestation spread very rapidly. It was unknown in Devon until 2005 or 2006, but by last year, it had spread all over the county. Some of the spread has been natural, as the moths multiply and fly, aided by the wind. It is thought that some of the caterpillars hitched rides on dead leaves carried by cars, lorries and trains! Certainly, the early reports of damage included several towns that were well away from other sites with infestation.
It looks as if we will have to live with the moth, its caterpillars and larvae. The trees do not suffer, except for their visual appearance. The species has no known predators or inexpensive way of control, despite intensive study in Britain and across Europe. The moth first came to the notice of biologists in the 1970s and 1980s, and was named as a species in 1986. In the past year, there has been reported success as a side effect of some experiments treating horse-chestnut trees for a bacterial disease. There are some natural predators, but they haven’t been so successful in hitching lifts across Europe. The best advice that can be offered to reduce the effect of the caterpillar and moth without this experimental method is to remove all the dead leaves and burn them, or compost them in a hot compost bed. This reduces the number of moths that emerge in the early spring. However, the gap is quickly filled by moths spreading in from surrounding areas. In September last year, the trees in Matford Lane were badly affected, possibly because there were leaves underneath them all the previous winter. A few trees in Countess Wear, where the dead leaves had been swept away, had not been so badly damaged.
Biologists at the University of Hull are studying the leaf miner and some of those natural predators. If you want to find out more, look for “Conker Tree Science” on the internet.
Meanwhile, this autumn, look at the horse chestnut trees around us, and reflect on the way that a moth and its tiny caterpillar can affect something so large as a tall tree, and spread across our country so rapidly. (September - October 2012)