Hovering jewels on the wind

FLUTTERING like shreds of some faded oriental carpet, painted lady butterflies have brought a special glamour to the hedgerows' this month, alighting on the richest, creamiest feast of hawthorn blossom in anybody's memory. Red admirals, too, are here in jostling numbers, the cocks competing for the females in spirals of brilliant wing.

Exceptional migrations of moths from continental Europe have been worrying the brassica growers of England and recent southerly winds will have brought fresh waves of white butterflies to the cabbages of both these lands. If the winds persist, what else might turn up?

Two biologists' were in a boat off Courtmacsharry Bay in Co Cork last July when they saw long skeins of insects flying towards the coast. The big, dark ones were red admirals but airborne with them were great swarms of dainty looking flies so many that dead specimens were forming drift lines on the sea.

These were hover flies, generally regarded as the "goodies" of the garden, since their larvae feed on aphids while they themselves drink nectar. They sometimes hover for minutes on end, hanging in the air like bright little Jewels, but also dart about too quickly for the eye to follow. Many of them have bodies striped with yellow bands, like wasps a device meant to fool birds and other enemies, and one that worries people, as well.

We are familiar with the migration of large insects to Ireland from southern Europe and northern Africa red admirals, painted ladies and clouded yellows, and hummingbird and other hawk moths. But the migration of flies in any self directed way is an idea that takes some getting used to.

Dr Martin Speight entomologist with the National Parks and Wildlife Service, discusses last summer's Cork fly past in the current issue of the Irish Naturalists Journal its first record of migrating hover flies. Yet, as he points out, long distance migration by some species of this insect is well recognised and noted in Europe from Scandinavia to the Mediterranean.

The two main species in the Cork swarms were Episyrnhus balteatus and Eureodes corollae (there are 5,000 odd species of hover fly and very few have common names). Both of them are among the most persistently recorded migrants among European species and E. balteatus has even arrived alive on polar ice floes.

The species also stars, as it happens, in one of my favourite accounts of airborne migration.

An English ornithologist, David Lack, once climbed with his wife to a high and extremely narrow pass in the Pyrenees, on the border between France and Spain. They were checking a theory that migrating small birds would find the mountains too high to cross.

The theory was wrong. The Lacks counted hundreds of finches, linnets and other small birds skimming southwards over the pass, often only inches above the ground. With them came butterflies, mostly clouded yellows and red admirals, then dragon flies, even closer to the ground, at a rate ok several thousand an hour.

And finally they realised that outnumbering all the other insects and birds put together, was a little, hoverfly with a black and yellow striped body, later identified as Episyrphus balteatus.

"The hoverflies, like the drags on flies," wrote back "flew steadily up to and over the pass, keeping their heads to the wind but they flew even closer to the ground, and that was why, together with their small size, we at first did not even notice them. Shortly before we left at 3 p.m., the whole surface of the pass, at about ankle height above the rocks, was a shimmer of iridescent light, due to the reflections of the autumn sun on myriad tiny wings."

At that time 1950 a lot less was known about insect migration. The northward flight of red admirals and clouded yellows was put down simply to overpopulation in the south. The idea of a return passage in autumn had not yet registered, still less that it is carried out by the offspring of the butterflies that come north.

That some dragonflies, too, make regular seasonal migrations, "out and back, was another surprise. Today, the migration of dragonflies is still quite mystifying. It is rarely a mass take off but more a continuous movement of individuals in one direction.

The Irish naturalist Robert Lloyd Praeger once found himself a sand spit in the Baltic while "the air was dark for hours, with millions of great dragonflies, drifting, through the pine woods and high overhead, as thick as snowflakes.

Ireland has nothing to match this sort of movement, but one native dragonfly sometimes seen in a migration is Libellula quadrimaculata, the "four spotted chaser" of Connacht's boggy pools. One arresting theory is that this species is "irritated" into flight by an internal, parasitic worm that wants to get the dragonfly and itself eaten by a bird, the parasite's ultimate host.

Some migrations are more passive than others. On any warm day in summer, the air at 1,000 to 5,000 feet contains billions of insects clouds of aphids, flies, mites and spiders (flying on threads of silk), all waiting to see where the wind will take them.

With hotter summers, changing wind patterns, droughts and scorched crops in southern Europe, who knows what insect plagues await us?

IN 1688 and a decade thereafter, mass migrations of cock chafer beetles arrived in Connacht, on warm winds from the continent. Their depredations were recorded by the naturalist Thomas Molyneaux.

Towards evening or sunset, they would all rise, disperse and fly, about, with a strange humming noise, much like the beating of drums at some distance, and in such vast incredible numbers that they darkened the air for the space of two or three miles square.

The grinding of the leaves in the mouths of this vast multitude all together made a sound very, much resembling the sawing of timber."

If we must have hordes, let them be of hoverflies, hungry for aphis and nectar and for all their waspish livery, quite without harm to humans.