Counting the number of bugs whizzing high overhead annually may seem all but impossible, but researchers in Britain have completed the most comprehensive tally ever conducted. And the headcount they came up with was almost un-bee-lievable.

A total of 3.5 trillion insects weighing a combined 2,900 tonnes migrated annually over a region in south-central England monitored with specialized radar and a balloon-supported aerial netting system, the scientists said on Thursday.

"High-altitude aerial migration of insects is enormous," said University of Exeter entomologist Jason Chapman, whose research was published in the journal Science. "These aerial flows are an unappreciated aspect of terrestrial ecosystems, equivalent to the oceanic movements of plankton which power the oceanic food chains."

The researchers tracked the migration of insects at altitudes between 150 and 1,200 metres over a 10-year period. They suspect even more migrating bugs could be found elsewhere.

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Researchers tracked the migration of insects at altitudes between 150 and 1,200 metres over a 10-year period. (Carlos Jasso/Reuters)

"The numbers will be considerably higher in most parts of the world, but we lack the data to extrapolate the total numbers yet," Chapman said.

In terms of biomass, the insects greatly exceeded migratory birds in Britain. Their biomass was seven times that of the 30 million songbirds flying from Britain to Africa each autumn.

While the study did not plot the departure and destination points for the migrating bugs, they were thought to be traveling back and forth numerous kilometres, and sometimes over the English Channel and North Sea.

"Some of the butterflies and moths we studied migrate hundreds of kilometres in each generation, and thousands of kilometres over the course of the year, which may include six generations," Chapman said.

'We could not function without them'

Insects play important roles, pollinating plants, facilitating productive soil through decomposing, serving as food sources for birds and bats, spreading disease, and serving both as crop pests and predators of crop pests.

"We could not function without them," Chapman said.

The mass insect migrations generally headed north in spring and south in autumn.

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The recorded insects' biomass was 7 times that of the 30 million songbirds flying from Britain to Africa each autumn. (Carlo Allegri/Reuters)

The most abundant day-flying insects in the study included cereal aphids and the tiny parasitoid wasps that attack them. The most abundant medium-sized day-flying insects included hoverflies and ladybirds, also called ladybugs, and the most common big ones included large butterflies such as the painted lady.

At night, abundant small insects included midges and other flies, while medium-sized ones includes lacewings and large ones included noctuid moths and hawkmoths.