Private lives of the rock'n'roll love bugs

Another Life: There is a virginal perfection to the darling leaves of May

Another Life: There is a virginal perfection to the darling leaves of May. Fresh out of their wrappers, spread exactly to shape, angled just-so between their neighbours for a fair share of the sun, they offer an infinite pattern-book of lobes and lances, scallops and serrations, without so much as a moth's mark of the wear and tear ahead.

On a still morning, with the big willow shedding a slow, silky blizzard of seed, the acre's billion leaves were dotted with the shadows of sunbathing, meditating flies.

I was looking for ladybirds and not finding any. A reader in Co Wicklow came home from a Sunday forest walk to email me about all the ladybirds she had seen, about five to every small furze bush along the forest path. "I love to see them," she wrote, which reminded me how long it was since I could share her pleasure - a few years, anyway, since I collected a fistful of scarlet seven-spotters and unleashed them on the aphids that were crippling our basil in the greenhouse. They quickly lifted their DeLorean elytra and zoomed off, hating, perhaps, to be shielded from the sky.

Ladybirds are Coccinellidae, a class of beetle (cochineal, the red food dye, sounds as if it ought to be made from dried ladybirds, but it is actually made by grinding up Mexican red scale insects that feed on cactus - something for Trivial Pursuits). Ireland had 26 Coccinellidae species at the last count, many of them absolutely tiny - less than 3 mm and quite dull. Of the bigger, prettier ones we know, the seven-spot and two-spot are the commonest, with black spots on red, but there's an 18-spot with white spots and a 22-spot dazzler with black spots on yellow. The less familiar ones may hunt their aphids on specific kinds of host plant, such as heather, conifers, or even bulrushes.

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Like many bright insect colours, those of ladybirds warn predators - birds in particular - of unpleasant consequences if molested. To handle a ladybird may be to find a small yellow stain left on one's hand by a fluid secreted through pores in the beetle's legs. "It is instructive to taste this so-called reflex blood," Britain's Dr Michael Majerus encourages naturalists, "for it is strongly acrid, a product of a cocktail of chemicals . . ." Dr Majerus, who lectures at Cambridge University, is the ladybird authority in these islands and author of an exceptionally lively book on his subject in the HarperCollins New Naturalist series in 1994. As an evolutionary geneticist, he is fascinated by the beetle's adaptive permutations, which seem almost designed for his craft, but he also writes with relish of the ladybirds' private lives - their extreme cannibalism, for example, and sexual promiscuity and vigour (or "rocking and rolling" as he graphically describes it).

Despite their exceptional sexual stamina, Dr Majerus finds threats to the survival of some of Britain's 42 native ladybirds in the arrival of alien species which can out-compete - or simply eat - them. Some are hitching lifts round the world on imported foodstuffs or timber but others are being brought in for release in greenhouses by organic enthusiasts who don't want to use insecticides against greenfly.

Almost 10 years ago, he was voicing particular worries about the multi-coloured Asian or "harlequin" ladybird, Harmonia axyridis, whose importation into North America had led to a meteoric expansion. The beetle was already freely available in The Netherlands and France and Dr Majerus predicted its imminent arrival in Britain. Last September, the first were seen in south-east England and they have since been found in Devon and Cumbria. Earlier this spring Dr Majerus led the launch of a major survey, appealing to gardeners and wildlife enthusiasts to track the invaders' progress north and west.

In less than 20 years, the harlequin has become the commonest ladybird over large areas of the United States and its swarms have caused panic in city buildings, triggering smoke alarms and even penetrating computers. Its prodigious appetite for aphids reportedly leaves native ladybirds starving, and they themselves are then consumed when the harlequins run out of greenfly.

We must wait to see how vulnerable are the ladybirds of these islands (for Ireland, too, seems likely to bring in the harlequin, if not on an east wind then on potted plants from The Netherlands). Dr Majerus is already planning pheromone traps to catch the beetles when they hibernate together in autumn. Meanwhile, a visit to www.harlequin-survey.org will show how disconcertingly variable is the harlequin's livery. It is rounder and slightly larger than the familiar seven-spot, and can be orange with 15 to 20 spots (see drawing above), black with two orange or red spots or black with four orange or red spots. At least you'll end up knowing more about ladybirds.