Rains of summer a catastrophe for wasps

On any morning with sun and a bit of a breeze the garden dances with rainbows as a chorus of redundant CDs spin, flash and jiggle in the apple trees. They were hung up to disconcert the blackbirds, but too many slow-ripening Jonathans have already been disembowelled aloft or lie as hollowed windfalls in the grass.

In any ordinary autumn, the wasps would be at work on them all, licking up a last feast of sugar. But there are no wasps to be seen in my garden, and few enough, it seems, across the rest of the island.

Like much of northern Europe, Ireland had a very wet spring. In May, in particular, rainfall was twice the month's average across most of the island.

The west and south-west, in particular, had the wettest May in more than 20 years, and the 200mm at Valentia was the most for the month since records began. The rain-gauge in our orchard collected only 170mm, but, spread across 21 days, it was quite enough to wreck the lives of wasps in the busiest weeks of their season.

Our common social wasps usually build underground, in empty holes (and sometimes, of course, in people's roofs). The queen wasp, not long out of hibernation, does all the early work, suspending a wood-pulp umbrella from the roof of the hole and moulding the tiers of paper cells beneath it, then laying an egg in each. She has to feed all the grubs with chewed-up caterpillars and other insects. It's five weeks or so before the first brood grow into workers and take over the nest-building, foraging and feeding, leaving the queen to go on laying eggs.

Heavy rain suggests flooded holes and drownings, but the impact of so much wetness is probably more subtle. Wasps are slow to fly in rain and so are the bluebottles and other insects they prey on (though caterpillars probably don't mind). The workers are more likely to settle for the "honeydew" excreted by aphids, to fuel their own energy, than to hunt high-protein insects the grubs need for growth. Rain dilutes the honeydew's sugar and makes it run down the stems, so the wasps need to spend more time lapping it up.

Such intimate detail comes from research in New Zealand, where European wasps are under close study as recently imported and difficult pests that compete with native insects and birds and have no predators. From there, too, comes the (to me, reassuring) conclusion that wasp populations can suffer 99.9 per cent mortality among the new season's queens and still recover their populations in the following year. Next May will find me keeping an eye on a cotoneaster tree in the orchard: its tiny, shallow flowers are pollinated almost exclusively by wasps seeking nectar, an alternative to honeydew as an energy source, and the consequence is masses of autumn berries for the birds.

The commonest social wasps in both Ireland and Britain are the German wasp and the common wasp, whose physical differences are slight. Both like nesting in cavities, but the common wasp seems happier in urban situations and the German wasp is missing from our north-western counties. We also have a couple of social wasps that prefer to nest in trees: there's the tree wasp proper and the Norwegian wasp, neither with more than a few hundred workers in their nests.

As climate warms, Ireland seems certain to share in the northward spread of new wasp species from Europe. Wasps don't migrate or travel in swarms, but some species seem to be natural colonisers. Two new social wasps established themselves in the south of England in the 1980s: the large, dark median wasp, which hangs its nest from a branch, mostly in leafy gardens, and the Saxon wasp, which often builds its aerial nest under eaves or in garages. The first reached Scotland some years ago and the second is nearly that far north.

Southern England also has hornets, the largest and loudest European wasp (the queens can be 3.5cm long) and certainly the most feared. They are "amazingly peaceful animals, even shyer than honeybees", one expert German website assures the visitor, offering photographs of smiling researchers with hornets perched on their noses. But hornets do object to trespassers on their nest-space (often in the holes of old woodland oaks, but also old sheds and wooden balconies) and their sting is made extra-painful by a 5 per cent dash of acetylcholine, a neurotransmitter said to have a certain Viagra-style action if one weren't too busy running to notice.

Robert Lloyd Praeger, the great Irish naturalist, confessed to a special liking for wasps, "so active and daring, dainty and cleanly". But resting once on the mossy bank of a Galway river, he found himself sitting on a big wasps' nest. Two swift skips took him head-over-heels into the river and he swam off underwater, drowning five wasps inside his shirt.