Matthew Bell: Never mind your woollies – there's a rare species at risk

Our writer flies to the defence of the poor, misunderstood moth

With last week's heatwave it was finally safe to put away the woollies and time to stock up on mothballs. Killing moths has become a priority for me ever since last autumn when I opened the wardrobe to find all my jumpers had been reduced to a concertina of loosely joined rags over the summer. So I'm surprised to find myself now calling for their protection. The moth population is in decline, as I discovered while staying with some ecologically minded friends in the country last weekend.

The first thing to know is that the moths who feast on your cashmere are quite different to the 800 varieties out there in the garden. And the next three months are, apparently, peak moth-trapping season. Lepidopterists around the country are dusting down their traps, reeling out the extension leads, and setting up these weird and expensive bits of kit in the garden. My host had just forked out £300 for a Robinson Mercury trap, which is essentially a very bright light over a shallow bucket half-covered in Perspex, where you place a few up-turned egg cartons. The light stays on all night and by morning, with luck, you have a galaxy of exotic varieties to identify.

To many, the moth is an ugly monster, greyish brown with an off-putting fleshy body. The butterfly is more obviously appealing. But armed with your Field Guide to the Moths of Great Britain and Ireland, the definitive guide, published by British Wildlife Publishing, you soon find yourself marvelling at the extraordinary diversity of nature. Turn over one egg box and there's an elephant hawk moth, achieving the improbable task of looking like both an elephant and a hawk, under another a death head hawk moth, distinguishable by the skull shape on his back.

We were helped by a visit to one of the leading amateur experts in the country, Dr Humphrey Kay, an eminent retired pathologist in the next village. He was immediately able to point out that what seemed like a boring little brown fellow was a spectacle moth, which you can only tell by looking at it head-on, to see the rings round his eyes. Another was a bright line brown eye, not to be confused with the brown line bright eye. If you haven't got Dr Kay on hand, who after years of moth-trapping rattles off their names as he sees them, you'll do a lot of rifling through your field guide.

The pity is that there aren't many people like Dr Humphrey Kay left. He is the County Recorder for Wiltshire, but he is in his eighties, and all his knowledge will go with him. Just as alarming is the state of the moth population itself – according to a recent survey, moth numbers have decreased by a third since 1968, especially in the South, due to agricultural intensification and urbanisation. These are fascinating creatures – they use the sun as a kind of sat-nav, always flying at right-angles to it. They are integral to the fragile and constantly amazing ecosystem, essential to the survival of the birds and bats that feed on them.

The Heritage Lottery Fund is to be applauded for its generous grant to the National Moth Protection Scheme, which is asking amateurs to trap moths and send in their findings. Schools are involved and David Attenborough has lent his support. Moths need our protection. But I'm still off to mothball my wardrobe.