Hoverflies: Britain's overlooked migrant heroes of the garden

On a warm autumn afternoon, a small yellow and black insect hovers motionless over the last flowers of the garden, then darts sideways in the blink of an eye. At first glance it looks like a wasp, and many people instinctively flinch when it approaches. Yet this little visitor has no sting and cannot harm you in the slightest. It is a hoverfly, one of the most astonishing and useful insects in Britain, and at this very moment countless millions of its kind are crossing the sky above us.

Not a wasp, not a bee, but a fly

Despite their convincing disguise, hoverflies are neither wasps nor bees, but true flies, close relatives of the common housefly. Their yellow and black stripes are simply a clever costume, borrowed from more dangerous insects to fool hungry predators into leaving them alone. A bird that has once been stung by a real wasp will think twice before snapping at anything that wears the very same warning colours.

Telling a hoverfly apart from the insect it mimics is easier than it seems. Bees and wasps have two pairs of wings, while every fly, including the hoverfly, has only one. Add to this a pair of very large eyes, short antennae and the complete absence of a sting, and the disguise soon begins to unravel. Best of all is their flight, for hoverflies can hang perfectly still in the air, then shoot sideways with breathtaking control.

Britain's billion-strong migration

What few people realise is that many hoverflies are extraordinary long-distance travellers. Using special radar designed to track insects, scientists have discovered that up to four billion hoverflies cross to and from Great Britain every single year, a living river of insects weighing around twenty-eight tonnes. It is one of the greatest and least known animal migrations happening quietly right above our heads.

Each spring, hoverflies such as the marmalade hoverfly fly north from continental Europe into southern Britain, where they settle and breed. When the new generation matures, these young flies head south once more in the autumn, back across the English Channel to the mainland. Remarkably, around a billion more hoverflies leave Britain in autumn than ever arrived in spring, which makes the country a net exporter of these tiny migrants.

Tiny farmers' friends

Far from being a mere curiosity, this great movement of insects brings enormous benefits to the land. Hoverflies are among the most important pollinators after bees, visiting countless flowers as they feed on nectar and pollen. Researchers estimate that the spring arrivals alone carry somewhere between three and eight billion grains of pollen into southern Britain each year, quietly helping to fertilise both wild plants and valuable farm crops.

The real surprise, though, lies in their young. The larvae of many hoverflies are ravenous hunters of aphids, the sap-sucking pests that plague gardens and farms alike. Across Britain, locally born hoverfly larvae are thought to devour something in the order of six trillion aphids every year. For farmers and gardeners, this amounts to a vast and completely free service of natural pest control, delivered without a drop of chemicals.

A life of two halves

The life of a hoverfly is really a tale of two very different creatures. The adult is a graceful flower visitor, sipping nectar and gathering pollen in the sunshine, harmless and gentle to everything around it. The female, however, has a shrewd instinct, laying her eggs right beside colonies of aphids so that her hungry offspring will hatch surrounded by an almost endless supply of food.

Those offspring look nothing like their elegant parents. Many hoverfly larvae are small, legless and slug-like, creeping over leaves and stems as they hunt down aphids one by one. The family is astonishingly varied, with roughly two hundred and eighty different species recorded in Britain alone, some whose larvae even live in stagnant water or in the damp heart of rotting wood.

An unsung pillar under threat

Given everything they do for us, it is troubling that hoverflies, like so many other insects, appear to be in decline. The loss of flower-rich meadows, the heavy use of pesticides and ever more intensive farming all make life harder for them. When these small workers disappear, we lose not only their beauty but also the free pollination and pest control on which our own food supply quietly depends.

The good news is that helping them is simple and within everyone's reach. Growing a wide range of flowers that bloom from early spring right through to late autumn gives hoverflies the fuel they need, especially the migrants passing through. Cutting back on garden chemicals and leaving a few wild, untidy corners provides both food and shelter for these remarkable travelling insects on their long journeys.

Look again at the little hoverer

So the next time a striped little insect hovers over the ivy or the last asters in your garden, resist the urge to swat it away in alarm. Take a closer look instead, and you may well be watching a harmless fly that has crossed the sea, or is just about to. In that small hovering shape lies one of the hidden wonders of the British autumn, unfolding right on your doorstep.

For all their modest size, hoverflies deserve a far greater place in our affections than they usually receive. They pollinate our plants, protect our crops and undertake journeys that would humble far larger animals, all without ever asking for anything in return. To notice them, and to make a little room for them, is to honour one of the quiet and unsung pillars of the natural world.