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The marmalade hoverfly, Episyrphus balteatus, pollinates many kinds of plants.

Konrad Wothe/Minden Pictures

Half a billion hoverflies migrate to the United Kingdom each year. The benefits to farmers are huge

Each year, hundreds of millions of hoverflies cross the English Channel from continental Europe, according to a new radar-based study. Most migratory insects around the world are pests, such as locusts, but luckily for U.K. farmers, the hoverflies are friends.

“The potential benefit is quite large,” says Ben Woodcock, an entomologist with the Centre for Ecology & Hydrology, in Wallingford, U.K., who was not involved in the study. Many hoverfly species pollinate crops, he notes, and their larvae consume aphids, which are pests of wheat and other crops.

Most insect migrations are invisible to the naked eye. But researchers can track and identify them with narrow radar beams. In 2016, a group using the technology and led by ecologist Jason Chapman at the University of Exeter in the United Kingdom reported that trillions of insects migrate in and out of the country each year.

Now, Chapman and several colleagues have analyzed the data further to sift out patterns for hoverflies. Like larger insects, hoverflies—a common group of centimeter-size insects, many with colorful striped abdomens—can determine where they migrate, the team found. Some insects seem to get blown around by whichever wind they are caught in, but hoverflies appear to be strategic; they climb to an altitude where the predominant winds are blowing in a particular direction. Then they use this powerful tailwind to travel hundreds of kilometers per day. In the spring, hoverflies fly north from continental Europe into the southern United Kingdom. They lay their eggs, and when the new generation grows up, the young hoverflies fly south in the fall.

The benefits to farmers are huge. Combined, the populations of the two most common species of hoverfly, Episyrphus balteatus and Eupeodes corollae, transport about the same amount of pollen as do all the honey bees in the United Kingdom, Chapman and colleagues report today in Current Biology.

Perhaps more importantly, the larvae of hoverflies eat about 20% of the aphids in an average wheat field—a total of 6 trillion aphids, the researchers estimate. “The numbers really blew my mind,” Chapman says.

And unlike many kinds of pollinators, the populations of the two migratory hoverfly species seem stable, the team found. “It’s quite nice to have a good-news story,” Chapman says.

Still, he says, farmers should avoid spraying insecticides when they are present. The skies are filled with more than birds and airplanes, he says. “Insect migration is happening on a massive scale and it’s really important. It’s this huge underappreciated phenomenon.”