A welcome population explosion

IN his column last week, Damien Enright celebrated the butterflies.

Where he lives, cabbage-whites and speckled-woods predominate. In the fields along the coast of north Co Dublin, there are also lots of butterflies but most of them are tortoiseshells. I have never seen so many as we have at present. Our Irish species is the small tortoiseshell, with a wing-span of 4 to 5cm. A bigger cousin, the large tortoiseshell, is found in mainland Europe and occasionally in Britain.

The small tortoiseshell’s colourful markings are immediately recognisable; a glorious design of red, yellow and white patches against a dark background. A row of bright blue dots runs along the insect’s rear fringe from wingtip to wingtip. The undersides of the wings are dark; when they are closed and held vertically above the torso, the insect is well camouflaged. Butterflies, however, can’t afford to hold this posture for long. They must open their wings to receive the sun’s bounty, an activity known as “basking”.

Cold-blooded creatures, such as butterflies, need the heat of the sun. Dark objects take in heat more readily than bright ones, which is why the base colour of the tortoiseshell is dark. White pigments radiate rather than absorb heat, so Damien’s whites are at a disadvantage when it comes to warmth. They do, however, have black circles and dark patches towards the tips of their wings. Their heat gathering technique is rather special; it’s thought that the gleaming white colour deflects sunlight directly towards the flight muscles.

But being bright and colourful has its downside. A glamorous coat attracts predators. In the case of the tortoiseshell, the wings are flicked open repeatedly, the sudden flash of colour helping to scare off enemies. Another colourful butterfly, the peacock, takes flashing even further. The large RAF-style “roundels” on the wings look like the eyes of a bird or prey. The smaller ringlet butterfly has it both ways. Its chocolate-brown colouring provides excellent camouflage while small bright “eyes” on its wings spook potential predators.

Tortoiseshells are on the wing from April to October, sucking nectar from thistles. Tortoiseshells lay their eggs on nettles; the caterpillars which hatch from them feeding gregariously. The caterpillars’ main enemies are birds, frogs and mice. The nettle’s sting may protect the plant but it’s no deterrent to a caterpillar; indeed it may even help the leaf-eaters by discouraging mice.

Tortoiseshells have two broods in a season. The second-generation adults, like those of the peacock, are unusual in that they hibernate. As the days shorten and temperatures fall, butterflies may enter houses searching for a dark corner, a drawer or a hole in furniture, for the winter sleep-over. Misled by the warmth of our homes, they occasionally wake up and fly about indoors in the depths of winter.

Just why this year’s tortoiseshell crop is so exceptional I am at a loss to explain. Perhaps the wet summer was a factor. Did the butterflies which would normally have appeared during the summer months, delay their emergence because conditions were so bad? Another, slightly-insane and off-the-wall, hypothesis occurs to me. Perhaps it has been a particularly good summer for nettles, the caterpillars’ host plant. Nettles grow in abundance near lowbrow licensed premises; discharges of urine by short-taken patrons staggering forth into the night stimulate their growth. Urine contains phosphates which nettles crave. Did the wet summer result in more drinking and nettles this year? Probably not and, in any case, Irish fields receive an excess of phosphates; nettles thrive throughout the country without assistance from the drinking public.

Whatever the causes of the population explosion, the tortoiseshells are most welcome. Butterflies are universally loved. They feature in art and folklore everywhere, even appearing in the wall paintings of ancient Egypt. Some of their associations are sad. Butterflies symbolise the brevity of life and its transience. In some folk traditions, they are the souls of the dead waiting to return to their owners’ bodies on the Day of Judgement. They can be seen as the public relations officers of the insect world and creepy-crawlies need all the PR they can get.

Moths are regarded much less favourably than butterflies, perhaps because most of them fly at night and some invade our wardrobes to dine on clothes. However, biologists don’t recognise a distinction between butterflies and moths. It’s only in our part of the world that the two groups appear to be different. The antennae of Irish butterflies are clubbed whereas those of our moths are not. Elsewhere, this distinction breaks down. Even in Ireland there are exceptions; burnet moths, for example, have clubbed antennae.

There are 750 butterfly species in Europe, a fifth of them threatened with extinction. We have 29 resident species in Ireland and one, the small mountain ringlet, which is extinct. Some species, such as the holly blue, seem to be increasing in numbers while others, such as the marsh fritillary, are declining. The world’s most famous butterfly, the monarch of North America, turns up here most years, blown across the Atlantic on the tails of hurricanes.