The wasps plundering resources for native ecosystems
Wasp species common to Ireland are making much trouble for insects and birds in New Zealand
When I remembered to go looking, the first windfall apples were already on the ground, most of them sharply pitted by blackbirds but with plenty left for my own, more tentative bite. I looked around for wasps, not grudging them their September sugar but wondering if windfalls could conjure a buzz in a singularly waspless year. There was none.
It can’t all have been our western weather, so often shaded by Atlantic cloud while the rest of the island is sunbathing. We did have a rather fine spring – a whole dozen days without rain in May. And that, by most scientific wisdom, should have been significantly good for wasps.
But they do seem to follow population cycles of their own. One 20th-century study at Wisley, a famous garden in England with the national collection of apples, pears and plums and masses of ripening grapes, had a 25-year record of local wasps’ nests to match with seasonal numbers of the insects.
It showed that each season of wasp abundance was followed immediately by one of scarcity, then one of abundance again even with a wet spring. A more recent English study confirmed the two-year cycle and suggested an “overcompensating” mechanism in the wasps, producing pairs of exceptional years of abundance and scarcity.
That chimes with a finding from New Zealand that wasp populations can suffer a near-total wipeout of the new season’s queens and still recover their number in the following year.
New Zealand takes a special interest in the same two social wasps that are most numerous in Ireland: the common wasp (Vespula vulgaris) and the German wasp (Vespula germanica). They both arrived in New Zealand as invaders, probably with machinery, and are among more than 2,000 alien invertebrates now abroad in the islands’ wild habitats.
Like so many introductions, they make trouble for native ecosystems quite beyond their impact on people. They have multiplied hugely in the native beech forests, lapping up sugar-rich honeydew that native sap-sucking scale insects dribble down the trees from their rear ends.
Wasps have the same appetite in Ireland, supplied by aphids feeding in trees such as sycamore and willow. But in New Zealand, it seems, they are plundering a key resource of carbohydrate, important to native insects and birds.
The common and German (really European) social wasps are the most plentiful in Ireland, building nests underground or in roof spaces. We also have a couple of others, the tree, and Norwegian wasps. The latter does choose trees for its small nests, with only a few hundred workers, while the tree wasp, perversely, may also build at ground level or in an empty bird-box.
Reports of supposedly aggressive continental species making in-roads in southern England have raised some alarm here this summer, in which the big wood wasp, or horntail – actually a solitary sawfly – has been feared as a hornet. Its big rear ovipositor is not a sting, but it drills holes in trees for the insect’s eggs.
At the other end of the scale are the egg-drilling activities of solitary wasps that can be nearly too small to see. Readers have sent Eye on Nature photographs of caterpillars split open by masses of grubs or shrouded in fluffy cocoons.
These are the victims of parasitoid wasps that have injected eggs inside the caterpillars, along with a dose of virus-like particles that freeze any further development. The wasp’s young use the caterpillar’s substance as a larder, taking their time in growing and eating their way out, then pupating on the surface.
This reproductive manoeuvre horrified Charles Darwin, who nonetheless accepted it as yet another process evolved by ungodly natural selection. As he wrote to a colleague: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Icheumonidae [the parasitic wasps] with the express intention of their feeding within the living bodies of caterpillars.”
He may have felt happier about a similar strategy that allows the tiny Cynipoidea, the gall wasps, to use the living, often woody, tissue of plants as food and shelter for their young.
The growths, or galls, that swell around their injected eggs vary in character enormously according to the plant, and some of the most striking in Ireland are the “oak apples”, or globular, tough-walled growths induced in oak buds to house the grubs of Cynipid wasps.
Among these, unfortunately, is Andrícus quercuscalis, the one alien species that does real damage to oaks in Ireland. Its galls invade acorns as they grow, distorting them to weird and infertile shapes rather like the bare kernels of walnuts. These are the “knopper galls” whose sudden appearance on pedunculate oaks in the Phoenix Park in the 1990s confirmed the wasp’s presence in Ireland.
Michael Viney’s Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks; firstname.lastname@example.org