Another Life: My favourite colour sings out in the flowers of the chives in the herb garden: fluffy amethyst tufts above a perfect foil of spiky, dark green leaves. A long visit there of a tortoiseshell butterfly, brilliantly orange and yellow, made something worth stooping to admire, writes Michael Viney.
The nectar of the allium family must be especially sweet, or perhaps those dozens of florets packed into every tuft are just too good to pass up: last summer a painted lady butterfly fed greedily at the flower-globes of perennial Welsh onions, and offered a leisurely look at an often fidgety Mediterranean migrant.
That butterflies feed themselves at many kinds of flowers can make the term "food plant" quite baffling, especially when the insects are said to "depend" only on one sort, or a few, closely-related plants. But the food in question is not nectar for the adult butterflies: it's chewable greenstuff for their caterpillars, hatching from eggs the butterflies lay on the leaves. Stinging nettles, for example, are the food plant of several butterflies (and moths), notably the tortoiseshell, peacock and red admiral - but they take no interest in their flowers.
The relationship of butterflies and moths and their food plants is so striking that they are commonly assumed to have evolved together, reaching the peak of their development at about the same time. Some research suggests that the attraction is to particular chemicals in the plant, and caterpillars certainly seem to know what they like.
When the cabbage-white butterfly homes in on the sulphurous whiff of my brussels sprouts, it is following its nose to the mustard-oil glycosides common to all brassicas, wild and cultivated. These give the butterfly a wide choice of food plants. But the distribution of many butterfly species, especially those near the northern edge of their range, is far more constrained by what plants are growing where. One of the reasons the lovely marsh fritillary is our most endangered butterfly, for example, is because its colonies depend so exclusively on devil's bit scabious, growing in habitats vulnerable to drainage and "reclamation". In continental Europe, the same butterfly will also lay its eggs on field scabious, knapweed, plantain and other plants.
Most species that have more than one generation in a season stick to the one kind of food plant. An eccentric exception is a little blue butterfly that seems to have undergone a population explosion in the past few years in the greater Dublin area and other parts of the east coast.
The holly blue, about 3.5cm across, is one of Ireland's three blue butterflies (the others are the common blue, found anywhere that bird's foot trefoil grows, and the truly tiny small blue, found only locally on sand dunes with plenty of kidney vetch). The holly is a pale violet-blue on the upper wings and these have wide black borders in the female, especially noticeable in the summer generation that hatches out next month.
Any blue butterfly seen fluttering around shrubs a few metres above the ground is almost certainly a holly blue. But the butterflies emerging in early spring - the only blue ones then on the wing - lay their eggs singly on the flower buds of holly trees. The later generations of summer and autumn, on the other hand, lay eggs on the flower buds of ivy. While both plants belong to broadleaved woodland, they now also form a substantial share of the green "biomass" of cities and their suburbs - vegetation now burgeoning in rising levels of CO2.
The first big increase in sightings of holly blues in Dublin's parks and gardens, from St Stephen's Green to Dalkey, came in 2001. In 2002, they were reported some 40 square kilometres in and around the city. Last year they appeared exceptionally early - in mid-March - and added new locations (including the grounds of Trinity College).
Bob Aldwell and David Nash, of the Dublin Naturalists' Field Club, who monitor Ireland's butterflies through a national network of voluntary recorders (see www. butterflyireland.com) would welcome records from remaining "blank" areas of the county - Foxrock and Tallaght, Skerries and Chapelizod are just a few. They feel the holly blue "explosion" may be due to both genuinely bigger numbers and greater public interest.
"A few of us have a sneaking feeling," says Aldwell, "that perhaps there were always blue butterflies around, but we paid little heed." This weekend he leads butterfly field trips in Donegal - no holly blues flying, for the moment, but the glorious marsh fritillary and that small, small blue are among probable star attractions. For more information, e-mail: aldwellb@gofree.indigo.ie