Migrating birds are a familiar sight for many people, with flocks of feathered travelers often dotting the sky as they fly north for summer or south for winter.
That same sky, however, is also the scene of another mass migration: the seasonal ebb and flow of flying insects. Aside from a few icons like the monarch butterfly, these tiny migrants are widely ignored by earthbound observers, including some scientists. But as a new study shows, their odysseys are no less impressive than many bird migrations — and no less important, given the ecological services they offer.
Published in the journal Science, the 10-year study focused on high-flying insects over southern England, using a mix of netting and specialized radar techniques to count them. It found that about 3.5 trillion insects migrate over the region every year, representing an incredible 3,200 tons of flying biomass. That's more than seven times the mass of all 30 million songbirds that depart the U.K. for Africa every fall.
And as the study's authors point out, England isn't known for having a particularly insect-friendly climate. Other parts of the world may host even wilder insect migrations, although more research will be needed to know for sure.
"If the densities observed over southern U.K. are extrapolated to the airspace above all continental landmasses," co-author and University of Exeter ecologist Jason Chapman says in a statement about the study, "high-altitude insect migration represents the most important annual animal movement in ecosystems on land, comparable to the most significant oceanic migrations."
That's worth knowing about, since mass insect travel can have major consequences for humans, both good and bad. Some insects kill our crops and trees, but a variety of others protect and pollinate the plants we depend on.
"Many of the insects we studied provide important ecological services which are essential for maintaining healthy ecosystems, such as pollination, predation of crop pests and providing food for insectivorous birds and bats," says co-author Gao Hu, a visiting scholar with Chapman from Nanjing Agricultural University in China.
There's the marmalade hoverfly, as Chapman tells NPR, an easily overlooked speck that migrates between the British Isles and the Mediterranean.
"It's only about a centimeter long, it's orange with black stripes, but it's a hugely abundant migrant, and it actually does some very important jobs," he says. On top of eating aphids that damage economically valuable plants, the marmalade hoverfly serves as an important pollinator of food crops as well as wildflowers.
The marmalade hoverfly, pictured here in Japan, exists across much of Eurasia and North Africa. (Photo: harum.koh/Flickr)
The researchers used radar sites in southern England to record larger insects flying more than 150 meters (492 feet) overhead. They counted smaller insects with netting samples, which they sent into the air via small blimps.
Insect migrations have been measured by radar before, the researchers note, but only for relatively few nocturnal farm pests. Their study reveals a flurry of daytime migrants, generally moving north in spring and south in fall. There were seasonal variations from year to year, but over the decade-long research period, the northward spring movement of larger insects was "almost exactly cancelled out" by the net southward shift each autumn, the study found.
On a wing and a prayer
A cloud of monarch butterflies complete their migration in Mexico's El Rosario Sanctuary. (Photo: Luna sin estrellas/Flickr)
While insect migrations aren't on most people's radar, the monarch butterfly has at least helped popularize the concept — including how complex, and how fragile, such journeys can be. The monarch's annual adventure spans 2,500 miles of North America and four generations of butterflies, with adults passing the baton to caterpillars who instinctively carry on their parents' mission. Adaptations like this take a long time to evolve, possibly as a way to buffer against parasites or other threats, yet the recent decline of migratory monarchs is widely blamed on human activity.
Many beneficial insects are ill-equipped to keep pace with modern habitat loss, pesticide use and human-induced climate change, even those with simpler travel plans than the monarch butterfly. Their ancestors placed evolutionary bets on certain habitats and migration routes, yet those places are now changing faster than some insects can adjust. And that can create openings for other insects to exploit, potentially upending ancient ecosystems in which humans are also heavily invested.
"Animal migration, especially in insects, is a very complex behaviour which takes millions of years to evolve and is very sensitive to climatic condition," says co-author Ka S (Jason) Lim, of the Radar Entomology Unit at Rothamsted Research in England. "Global climatic change could cause decline of many species, but equally other highly adaptable species thrive and become agricultural crop pests."