Marmalade hoverfly at tomato flowers. Illustration: Michael Viney
My casual mention last week of flying swifts feeding on aphids could have surprised readers who know these tiny insects only as sap-sucking garden pests. But aphids are just one group of insects that populate the sky, sometimes passing overhead in billions per month, at heights of up to a kilometre.
The study of insect life in the sky has grown with the use of radar. In early radar technology, baffling shadows on the screen had yet to gain recognition as flocks of migrating birds. But refinements in vertical radar and analysis have now distinguished the varying flights of insects, making equally purposeful journeys.
In the fast-developing science of radar aeroecology, the mass of invisible insects once regarded as “aerial plankton” has been resolving into identifiable species, many far from passive in taking off and landing and choosing a direction in the wind.
The regular migrations of insects with larger bodies, such as day-flying butterflies, dragonflies and night-flying moths, have been intensely studied, but the flight behaviour of migrant hoverflies, more vital to control of crop pests such as aphids, is now yielding to radar research.
In Britain, it is led by Dr Jason Chapman of Exeter University. This entomologist estimates that, in skies above Britain alone, up to 16 billion small insects, with up to 450 tonnes of biomass, “undergo to-and-fro migrations each year, with impacts on energy flows, pollination, pest control, crop damage and disease spread.”
After a decade of studying the flight of hoverflies using fast-moving airstreams more than 150m above the ground, his latest survey was of their migration south in autumn. They kept to their course, using the sun as a compass, even though they were heading into unfavourable winds.
This has been matched by a ground-level observation by an eminent UK ornithologist, the late David Lack.
Checking on a theory that Europe’s migrating small birds might find some mountains too high to cross, he climbed with his wife to a high and extremely narrow pass in the Pyrenees, on the busy migrant route to Spain.
There they counted hundreds of finches, linnets and other small birds skimming the pass southwards to Spain. With them came butterflies — clouded yellows and red admirals — and dragonflies at the rate of several thousand an hour.
At first they didn’t notice the hoverflies, tiny black-and-yellow ones, pressing on at ankle-height into the wind. What caught their eye at last was “a shimmer of iridescent light, due to the reflection of the autumn sun on myriad tiny wings”.
They were those of Episyrphus balteatus, or the marmalade hoverfly as it is known to many Irish gardeners. It is one of the most important and abundant of hoverflies and a target species in the surveys by Dr Chapman, who filters radar data for this insect’s body shape and size.
E. balteatus kills billions of aphids. This is not in the sky but when the hoverflies have landed, found mates and bred. Then their larvae hunt voraciously for aphids clustered on plant stems.
Food, sex and space are all reasons for aerial migration, but it’s still uncertain which apply to the millions of tiny spiders that also travel the sky. They are borne up on long strands of buoyant gossamer, extruded for the purpose.
In northern Europe it’s mostly the Linyphiidae family, the little “money spiders”, that disperse like this, along with young spiderlings of the Lycosidae, or “wolf” family. They perch on tiptoes on a high point, perhaps on a fence post, to catch the wind. Then they squeeze out silk threads, typically half a dozen, from the spinnerets in their abdomen.
Entomologists have differed on whether they let the wind keep pulling the threads out, or if the spider anchors them first and pays them out in stages. When they are long enough for lift-off, perhaps a metre or more, up the spider goes. It has some control of its flight, by tugging at its threads, which may make it more of a parachutist than a passive balloonist.
Almost entire populations of spiders, young and adult, can be prompted to leave home, given the right conditions and pressed by food shortage or overcrowding. In one remarkable experiment, at a sewage works in Birmingham in 1980, dispersal was closely measured for a week after two filter beds were deliberately dried up. The spiders, bereft of flies, climbed bicycle spokes planted in the gravel and took off in their millions.