It's one of the biggest mass migrations on Earth, and you probably don't even know it was happening.
Each year, from May to September, nearly 3.5 trillion insects traverse the skies above southern England, according to a new study in the journal Science.
Together these winged travelers make up 3,200 tons of biomass. That's the mass equivalent of a herd of 800 fairly large elephants marching past the clouds.
And if you live in sunnier climes, like Southern California, the number and mass of insects flying over your head each year are likely to be even greater.
"I believe the numbers in the southern UK are close to minimum values for the rest of the world," said study author Jason Chapman, an entomologist at the University of Exeter in Cornwall. "Almost anywhere I can think of will likely have much higher values, especially in the hotter parts of the world."
Chapman and his colleagues say that if the densities they observed over southern England are indeed similar or larger over other parts of our planet, this insect migration could represent the most important annual animal movements on land.
Chapman has been studying various aspects of high-altitude windborne insect migration for 15 years, but for most of that time his focus has been on large nocturnal species — specifically, large moths.
In this study, however, he and his coauthors tallied migrating insects of all sizes, both day and night. All the insects in the study were flying at least 500 feet off the ground (a bit higher than a 30-story building).
To collect the data, the team used a special entomological radar that allowed them to determine body mass, flight altitude and a host of other information about individual insects with a mass of 10 milligrams or more (about the size of a housefly and bigger).
The radar cannot accurately measure the number of smaller insects, which are much more numerous than the larger traveling bugs. To solve that problem, the researchers used aerial netting to monitor the tiniest migrators (see the photo above).
The authors found that 99% of the individuals that migrated through the study area were in the smallest group of insects (those smaller than a housefly). But these little guys made up only 80% of the total migrating biomass. The 15 billion medium-sized insects and 1.5 billion large insects were responsible for 12% and 7% of the mass, respectively, they write.
In addition, the team found that 70% of that biomass movement from migration occurred during the daytime and that the migration was generally northward in the spring and southward in the fall.
Although it's neat to think about great swarms of bugs moving from one place to another high over our heads, the authors add that the study has real-world implications as well.
They note that insect bodies are typically composed of 10% nitrogen and 1% phosphorus by dry weight and therefore represent rich sources of nutrients that may be essential for plant productivity.
"The 3,200 tons of biomass moving annually above our study region contains 100,000 kg of N and 10,000 kg of P," they write.
They also found that over the 10 years of the study, the net northward movement of migrating insects in the spring almost exactly canceled out the net southward movement in the fall.
But on an annual basis, the net flux could be up to 200 tons or greater in either direction, they said.
"Such insect movements represent an underappreciated mechanism for redistributing nutrients and energy," they conclude.
Chapman has already started to investigate whether other areas of the planet experience similar or greater insect migrations. So far it seems they do.
"We are just stating similar work in Texas, and we are swamped by migrants there, so numbers and biomass will be much higher in parts of North America," he said.