HAVE you noticed the leaves of our local horse chestnut (conker) trees are turning brown prematurely, as if it is autumn already?

My research has found this is due to the larvae of a particular moth. It is known as the Horse Chestnut Leaf Miner Moth – scientific name Cameraria ohridella. The moth lays its eggs in the leaves. The larvae that hatch from these eggs eat and tunnel their ways through the leaves, feeding on the tissue between the upper and lower surfaces, producing 'mines'.

If a leaf is held up to the light, such tunnels (if fresh) may appear translucent. The loss of the green photosynthetic tissue eventually results in the death of the leaf and this early leaf fall.

The larvae turn into pupae within the leaves and moths emerge in about two weeks. Later pupae will spend the winter in the fallen leaves beneath the tree, with the first new moths emerging in the spring. Cameraria ohridella does particularly well in warm, dry conditions when the tree may already be under stress from drought.

Pupae appear to be extremely frost tolerant, reportedly surviving winter temperatures as low as -23 degrees in Hungary. This can lead to increasing populations from year to year even when winters are severe.

The damage caused by large numbers of larvae can be striking. Up to 700 leaf mines have been recorded on a single leaf under favourable conditions (to the moth). Severely damaged leaves shrivel and turn brown by late summer.

The moth grows up to 5mm long. Their forewings are shiny, bright brown with thin, silvery white stripes. The hind wings are dark grey with long fringes. Each female moth lays between 20 and 40 eggs singly on the upper surface of leaves.

Just about every horse chestnut tree I've seen lately has the tell-tale brown blotches on its leaves, which means they will eventually turn completely brown and fall off early. I say 'just about' because it is apparently the white-flowered horse chestnut trees (Aesculus hippocastanum) that are infested and these are the ones we have most of. I've noticed the few pink flowered horse chestnuts we have round here are not damaged, but unfortunately they don't produce very good conkers. This horse chestnut species is a hybrid and is resistant to the leaf miner larva, causing it to die soon after hatching inside its leaves.

The Horse Chestnut Leaf Miner was first observed in Macedonia in northern Greece in the late 1970s. In 1989, it appeared unexpectedly in Austria and has since spread throughout central and eastern Europe. Its current distribution includes Belgium, The Netherlands, Denmark, southern Sweden, all of Germany, Poland and the central European countries, and it is spreading west through France and south through Italy. Since 2002 it has been reported from Spain, Albania, Turkey, Ukraine, Moldova, Belarus and western Russia.

Cameraria ohridella was first found established in the UK in the London Borough of Wimbledon in July 2002. Leaf mines with larvae were present in high densities on horse-chestnut trees along the southern edge of Wimbledon Common, and in nearby streets and gardens, which suggested the first moths had arrived in either 2000 or 2001. From this initial area of infestation, the moth has spread rapidly and it is now present across most of south- central England, East Anglia and the Midlands. The rate of spread in the UK (40-60km/year) is similar to that seen on the continent.

Dispersal of the moth from infested areas occurs mostly through adult flight, assisted by the wind, and through the passive transport of adult moths or infested leaves in or on cars and other vehicles. Transportation by vehicles appears to be responsible for the sudden appearance of the moth in towns and cities a long way from known areas of infestation.

It takes two to three years after the moth first establishes in a new area for populations to build up to densities high enough to cause severe damage and defoliation. Up to 2006, severe damage was restricted to London and the Home Counties, parts of East Anglia and some locations in the Midlands.

Over the past few years, severe damage has been reported from much further afield, particularly south Wales and many parts of southern England.

There is hope though, because despite the poor appearance of horse-chestnut trees infested with C ohridella, there is no evidence that damage by the moth leads to a decline in tree health, the development of dieback, or tree death.

Trees survive repeated infestations and re-grow leaves normally the following year.

However, continued and extensive attack by the leaf miner is likely to put some trees under stress, which although itself may not be too detrimental, could reduce a trees resistance to more serious diseases, of which there are quite a few.

If leaves are cleared from under trees each autumn, this could help, if only by reducing the numbers of moths.