Birds and human vacationers aren't the only creatures that take to the skies each year to migrate north or south. An analysis of a decade's worth of data from radars specifically designed to track airborne insects has revealed unseen hordes crossing parts of the southern United Kingdom—2 trillion to 5 trillion insects each year, amounting to several thousand tons of biomass, that may travel up to hundreds of kilometers a day.
The numbers, reported in this week's issue of Science, are "stunning," says Silke Bauer, an ecologist at the Swiss Ornithological Institute in Sempach. "Wow," adds Larry Stevens, an evolutionary ecologist at the Museum of Northern Arizona in Flagstaff. "Can you image what these numbers look like in tropical settings, say, over the basins of the Amazon or the Congo?"
Although some insect migrations are well known (think monarchs), the new work takes a systematic approach to flying insects and hints that such mass movements are surprisingly common. These airborne invertebrates, their bodies packed full of nitrogen and phosphorus, could move significant amounts of key nutrients across the globe. "Insects are little creatures, but collectively they can have a big impact; comparable in magnitude to large ocean migrations [of plankton]," says Lael Parrott, an environmental geographer at the University of British Columbia in Kelowna, Canada.
In the 1970s, U.K. entomologists began to use mobile radars to assess movements of locusts and other pests in developing countries. By the late 1990s, they had designed a permanent upward-facing radar system, based at Rothamsted Research in Harpenden, U.K., that automatically logs insects of different sizes. In one early discovery, Jason Chapman, now at the University of Exeter in the United Kingdom, and colleagues found that certain large butterflies and moths that dwell in northern Europe in the summer and in the Mediterranean in the winter take advantage of favorable winds to migrate
Now, Gao Hu, from Nanjing Agricultural University in China, Chapman, and colleagues have surveyed data from 2000 to 2009 collected in Harpenden and two other U.K. radar sites. The radars recorded medium-sized insects (hoverflies, ladybird beetles, and water boatmen) and large ones (hawk moths, painted lady butterflies, and aquatic beetles) flying between 150 meters and 1200 meters high; balloon sampling flights helped provide estimated counts for smaller insects.
Among the medium and large insects, the radar documented 1320 mass migrations in the daytime and 898 at night over the course of the decade. These streams of insects, heading south in the fall and north in the spring, usually coincided with favorable winds, which swept them along at up to 58 kilometers an hour. That insects "have an idea of where they want to go to, when they want to go, and what winds are good [is] surprising for these tiny creatures," Bauer says.
It will take more data, from other sites, to convince some entomologists that many insects migrate seasonally like birds and mammals. A European initiative is tracking birds using weather radars, and its scientists hope to get the funding to monitor insects as well. Such studies could be critical, notes zoologist Eric Warrant of Lund University in Sweden. "If, due to human influence, a large fraction of the [insect] migrant population is wiped out, it might have catastrophic consequences for those particular ecosystems."