Another Life: Beetle beauty and the beasts

My wellies stand on the slate-flagged floor of the porch, and each morning for days a beetle has been under one of them, black and shiny, quite common-or- garden. What is remarkable is that it runs to hide under the freezer while I’m struggling into the boots but is back under one of them again the next day. I could take bets with myself: left or right? The dark and homely scent of earth must make it hopeful of a journey well begun.

Beetles treat the house as well worth exploring. For several springs a weevil would emerge on my desk, vanishing at times beneath the computer keyboard but never – unlike Don Marquis’s typewriting cockroach, Archy – leaving any poetic messages. And sometimes a dearga daol visits the shower, circling the white pan endlessly until I sluice it back down the plughole.

Since the Eye on Nature panel was launched, in 1988, readers have shown increasing interest in encounters with the teeming beetle world. Colourful shield bugs and ladybirds on summer leaves, longhorns perched on roses, diving beetles gliding through garden ponds: the myriad clan of Coleoptera have brought many queries for a name, first by post, with little drawings on the letters, now by email, with close-up smartphone images.

Among the latest was a big beetle of brilliantly violet iridescence photographed by Donal Rice among heather near the summit of Crohane, in Co Kerry. It reminded me of my own first encounter with the species, on the slopes of the Sheeffrys, here in Mayo. Its colour might suggest Carabus violaceus, the violet ground beetle shown in most books, but it turns out that this is not an Irish native. It is, instead, the delightfully alternative Carabus problematicus, widespread on Ireland's mountain heaths and moraines. The problem, it seems, is telling the difference, but ours, on some acquaintance, has the far brighter colour. My drawing shows C problematicus among wild thyme on the sandy grassland behind the dunes.

The carabids are among the best-known groups of Irish beetles, hunting slugs and other invertebrates in the top layer of plant litter. This makes them a widely used indicator of the general insect diversity of land, and good arable farmers leave “beetle banks” to protect their arable crops.

For multiple usefulness, however, it would be hard to beat the little black scarabs of dung beetles, both for great value as nutrient recyclers and as food for bats and birds. They feed on pellets of dung and lay eggs in cowpats, their larvae breaking them up as they grow and mixing the dung with the surface of the soil. This tunnelling activity opens up the pat to the air and further waves of insect invasion. Earthworms take a final feast from below.

As a 50-strong herd of cows, grazing good grass in May, will drop up to two tonnes of dung daily in about 500 pats, its rapid dispersal and decomposition are clearly necessary as well as valuable.

For more than a decade, scientists at Teagasc and University College Cork have been studying these processes – and, in particular, the impact on dung beetles of the farm "worming" drug ivermectin. This is administered to kill internal parasites in cattle and sheep, but it also emerges unchanged – and potentially lethally – in their dung.

In a recent laboratory experiment on two common Irish species of dung beetle the Teagasc-UCC team found that the chances of their larvae growing into adults were cut from 80 per cent in cattle dung without ivermectin to 15 per cent in dung with a trace of the drug.

Back in 2000 the UCC scientists, Stephen Hutton and Paul Giller, collected nearly 40,000 dung beetles of 24 species from farms in Cork and Tipperary. Unlike drug-using farms, it will not surprise you to learn, organic farms "had significantly greater beetle biomass, diversity and species richness".

Reporting the recent laboratory test to a Teagasc conference, Conserving Farmland Biodiversity, the team said they found continuing uncertainty about the overall impact of ivermectin on wildlife, especially in applying laboratory results to the variables of field conditions. Setting limits to ivermectin use, they suggested, needed further evidence about its harm.

This was despite, as they described, restrictive guidelines issued by Natural England and other bodies, especially for areas of conservation value. Among these are machair grassland, such as that at the foot of the hill, where choughs – the red-billed, aerobatic crow, now scarce in Europe – used to feed on beetles in the cowpats of the commonage. Today it is grazed mostly by sheep, whose marbles of dung are not so beetle-worthy, and the choughs often make do with leatherjacket grubs dug from the machair’s turf. A flock of 27 – a good number – was spotted feeding there the other day.