Voles away!

Whatever about the well-run compost-heaps of suburban gardens (neatly-layered binfuls of tea-leaves, dead chrysanthemums and fluff from the Hoover), a robust rural compost-heap stacked against the ditch beneath the hawthorn hedge can swiftly become an annexe for wilder life than mere woodlice and beetles.

Under the sheet of marine-plywood that keeps off the rain, the surface of our heap in autumn soon displays a host of tunnel-openings, like those of some super model railway. Down in the fibrous warmth of the interior, certain creatures are burrowing through the cabbage-leaves to chew banana skins and lick at eggshells.

Tunnels are actually good for compost-heaps - they let in the air and help decomposition. Up to a point I ignore them in the chore of taking down the bucket of peelings from the kitchen. A loud clearing of the throat, a bar or two of some favourite aria or a preliminary bang on the lid, has usually averted any actual encounter with the occupants.

The other morning, however, with something else on my mind, I neglected these precautions, with the result that Rattus norvegicus shot out of a tunnel and leaped clear across my arm. I had time to be surprised that a rodent burrowing in kitchen waste, however vegetarian, should emerge looking so immaculately clean, dry and well-groomed: such is the injustice of stereotypes.

But the time had obviously come, etcetera . . . There is, after all, the dreaded Weil's disease, leptospirosis, which many rats carry in their kidneys and excrete in their piddle. This has always been a hazard for farmers and smallholders (though in Britain, by a bizarre twist, canoeists on rivers also seem peculiarly at risk).

Garden compost is supposed to be the epitome of green goodness, the very stuff of life, not of a malevolent, red-eyed 'flu. So, no more rats, for a while.

A few weeks ago I looked at some photographs of a rodent "smaller than a rat but larger and darker than your average mouse" which "sat up like a squirrel" to be photographed by a reader in Co Monaghan. In a fit of aberration, and quite misreading the animal's size, I identified it in Eye on Nature as a bank vole, thus sending a frisson of alarm and astonishment through the relevant experts in the Forestry Service. The bank vole is one of their more troublesome tree-nibblers, and an alien invader which, advancing from the south at an average of 4.5 kilometres a year, shouldn't have reached anywhere near Monaghan.

Ireland doesn't - or didn't - have voles of its own. In the tangled story of the island's acquisition of its mammals after the last Ice Age, the absence of voles has been given great importance in the arguments about land-bridges from Britain - where these bridges were (if at all), at what point in time, how inviting they were to cross, and so on. The other island has four sorts of vole, all regular prey for carnivores, raptors and owls, while our predators have had to do with fieldmice and baby rabbits and one not-very-nice-tasting shrew.

The bank vole, Clethrionomys glareolus, was discovered in Ireland in 1964 by a student trapping rodents in Listowel, Co Kerry. It is dumpier than a field-mouse, with a tail half the length of the body and ears just peeping above a rich, chestnut-coloured fur. Its front teeth are prominent little chisels, upper and lower, and its voice, says James Fairley, is "a rather flat, querulous squeaking".

In 1970, Dr Fairley launched an intensive trapping programme to map the animal's distribution through Kerry and beyond. By "reasoned interpretation of the data", he concluded the vole had arrived somewhere about half-way along the south shore of the Shannon Estuary. Foynes harbour, with its incoming ship-loads of timber and animal feeds, was a logical choice as entry-point, and the rate of spread suggested 1950, or thereabouts, as the year.

By 1982, the voles had colonized the whole of Co Limerick, most of Kerry (but not much of the Dingle Peninsula), large areas of Cork and Tipperary and the south-east of Clare. It has taken since then for them to reach Galway and Kildare, nudged along by the production of four or five litters each summer, each with around four young. This fairly hectic birth-rate just about keeps up with the demand of being one of nature's take-away foods.

Bank voles are out and about in daylight but keep to good cover - brambly ditches and hedgerows conceal them from kestrels and owls. Thus the map of their advances shows an irregular front, avoiding open mountain and bog. But where the vegetation suits them, they can be surprisingly abundant, at densities up to more than 70 per hectare.

The voles share with field mice an appetite for fruit and seeds. But when these get scarce in winter they turn to stripping bark off young trees and feeding on the cambium underneath - the layer that carries nourishment up the stem. The damage is often slight, but when the voles start at the base and girdle the tree for a foot above ground, it can die.

In Coillte's conifer plantations, the overall damage to Sitka spruce is less than one per cent, but exceptionally and locally it can be quite heavy. The voles like plenty of cover, so clearing the forest floor and spraying with herbicide can help prevent an invasion. But the cost is only worth it if the areas of high risk can be predicted, and that is demanding further study of the little rodent's behaviour.

There are more nature-friendly alternatives that Coillte is quite ready to consider (if the cost is right). Experiments with odour repellents have been successful in north-east China and providing alternative food for the voles has had some success in Canada. These may suit the smaller grower, but Coillte is still thinking: perhaps making the trees taste nasty, or finding ways to encourage more long-eared owls.

If we'd had voles to start with, there might be a lot more long-eared owls, so that's the one I favour.