Half a billion hoverflies emigrate to the UK every year. The benefits for farmers are huge | Science

By Erik StokstadJune 13, 2019, 11:00

According to a new radar-based study, hundreds of millions of hoverflies cross the Channel from mainland Europe. Most migratory insects in the world are pests, such as locusts, but fortunately for UK farmers, hoverflies are friends.

"The potential benefit is quite significant," says Ben Woodcock, an entomologist at the Center for Ecology & Hydrology in Wallingford, UK, who did not participate in the study. Many species of hoverflies pollinate crops, he notes, and their larvae consume aphids, harmful to 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 technology and led by ecologist Jason Chapman of the University of Exeter in the UK reported that billions of insects migrated into the country and to the United States. Outside the country every year.

Chapman and several colleagues analyzed the data in more detail to examine hoverfly trends. Like larger insects, hoverflies – a group of insects the size of a centimeter, many of which have a colorful striped abdomen – can determine where they migrate, the team discovered. Whatever the wind, some insects seem to be carried away by the wind, but hoverflies seem to be strategic; they rise to an altitude where the prevailing winds blow in a given direction. Then they use this powerful downwind to cover hundreds of kilometers a day. In spring, hoverflies fly from northern continental Europe to the southern United Kingdom. They lay their eggs and as the new generation grows, young hoverflies fly south in the fall.

The benefits for farmers are enormous. Together, the populations of the two most common species of hoverflies, Episyrphus balteatus and Eupeodes corollae, carry about the same amount of pollen as all honey bees in the UK, today report Chapman and his colleagues in Current biology.

Perhaps more importantly, hoverfly larvae eat about 20% of the aphids in an average wheat field, for a total of 6,000 billion aphids, say the researchers. "The numbers have really hit the spirit," says Chapman.

And unlike many types of pollinators, the team found that the populations of the two migratory species of hoverflies seem stable. "It's great to have good news," said Chapman.

However, he says, farmers should avoid spraying insecticides when they are present. The sky is filled with more than birds and planes, he says. "Insect migration is happening on a large scale and it's really important. It is this huge underestimated phenomenon. "