Anyone walking around the streets of Richmond cannot fail to see the sad state of our horse chestnut trees. Instead of sporting an autumn hue of butter yellow foliage, leaves are brown, crispy and distorted. The culprit is a moth that allegedly hitched a ride on a lorry from Macedonia, around 18 years ago.

The pest, known officially as Cameraria ohridella or chestnut leaf miner moth, started its British onslaught in 2002 and was first spotted in Wimbledon.

Since then, the population explosion has taken millions of them as far west as Devon with a few reported cases in Cornwall, and has romped north almost as far as Scarborough.

As its common name suggests, the larvae of this moth produce mines within the leaves, feeding between the upper and lower surfaces, completely removing any semblance of green tissue. This turns the leaves brown and dry, which eventually curl upwards and fall prematurely. Early leaf-fall can affect 70—100% of the leaves on a single tree. By August, entire trees look totally miserable.

If there was only one generation of the moth per year, the damage might not be quite so catastrophic. But with our hot and dry weather, there can be as many as three generations a year.

The adult moth appears around late April, initially from pupae that have survived the winter among fallen leaves. Its body measures around 5mm in length and its forewings are metallic chestnut-brown with silvery white transverse stripes edged in black. The hind wings are dark grey with long fringes. Quite a striking insect indeed.

It lays eggs from May to August, which hatch in two to three weeks. Larvae pass through five stages (instars) and complete their development in about four weeks. They feed inside the leaf tissue, leaving only the upper and lower leaf surfaces intact.

Pupae then develop in a silken cocoon inside one of the sinuous mines, and this last stage can last up from six to seven months, especially if it is over-wintering.

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The rapid spread of infestation, ironically, has been down to human movement – the pest mainly hitching a ride on vehicles. Experts tell us that although damage can be extremely disfiguring for a horse chestnut tree, it's unlikely to kill it. However, if you consider that a plant uses its leaves to convert light into food, having most of its food factory turned off cannot be good for its long-term health.

The UK doesn't have an effective natural predator for this pest, and where a cold winter will knock out some insects, the overwintering pupae of this one can survive temperatures to around - 20C.

Until the boffins come up with some magic remedy, there can be no doubt that the simple removal and burning of fallen autumn leaves will go some way to minimising its population explosion.

© 2020 Valerie Munro