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Last week, Ladybugs briefly resumed the news cycle.
The National Weather Service meteorologists were looking at radar images in California on the night of June 4, when they spotted what looked like a broad band of rain. But there were no clouds.
Meteorologists contacted an amateur weather observer directly during the mysterious disturbance. He was not soaked by the rain. Instead, he saw ladybugs. All over.
The radar apparently collected a cloud of migrating ladybugs spread over 80 miles, with a dense core of 10 miles wide floating between 5,000 and 9,000 feet in the air. As gigantic as the swarms are, meteorologists have lost sight of it. Ladybugs disappeared in the night.
Compared to other animal migrations, insect migrations are a scientific mystery. It's easy to spot a herd of wildebeest crossing the savannah. Insects, even in very large numbers, move from one place to another without notice. One day, you look around and ladybugs are everywhere.
"The migrations themselves are totally invisible," said Jason Chapman, an ecologist at Exeter University in Britain.
Dr. Chapman and colleagues use a radar to highlight insect migrations. Scientists help manage a unique network of small radar stations in southern England, designed to sweep the sky 24 hours a day, spotting insects flying over their heads.
"These radars are fantastic," said Dr. Chapman. "We have a lot of information about each individual flyer that flies over, including a measure of shape and a measure of their size."
Recently, Chapman and his colleagues decided to digitize their images to look for a specific type of insect, known as a hoverfly.
You've probably seen a hover fly, but you may have thought about watching a wasp or a bee. Overflights are harmless, but they have evolved to mimic biting insects as a means of scaring predators.
Fly flies offer two great ecological benefits. As larvae, they defend gardens and farms by eating aphids. In adulthood, they pollinate the flowers when they eat nectar and pollen.
"I do not think you can create a more useful insect," said Dr. Chapman.
Scientists had clues about the migration of some species of hoverflies, but they knew little about the timing or extent of their movements.
To take into account hoverflies migrating into the south of England, Dr. Chapman and his colleagues presented a species, the Marmelade Hoverfly, in the laboratory and bounced radar. They determined the distinctive radar signature of the insect, and then searched for it in scans performed by their instrument network.
The findings, published Thursday in the journal Current Biology, revealed hordes of insects that slipped over their heads. Scientists estimate that nearly 4 billion hoverflies migrate each year in southern England.
But the actual number is probably higher because the radar system can only detect insects flying above 450 feet. (In comparison, there are an estimated 5 billion honey bees in managed hives in Britain.)
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Each flight can travel 50 to 100 km per day and, in a year of migration, hoverflies can cross several generations. This mass migration has a huge influence on the environment. Dr. Chapman and his colleagues estimate that larvae produced by migratory hoverflies each year in southern England consume an average of 6,000 billion aphids, which together weigh about 7,000 tons.
Flies visit billions of flowers every year. And while insects travel on their migration route, they carry pollen over long distances. The researchers estimate that hoverflies import between 3 and 8 billion grains of pollen in southern England in the spring, then take away between 3 and 19 billion grains of pollen when flying south to the island. ;autumn.
Insects are important not only for the prey that they eat and for the pollen that they carry. They are themselves tiny packets of nutrients. Many are eaten by predators; the others fertilize the soil after their death. According to the researchers, they contain about 80 tons of biomass containing 35 million calories.
"I think people will be amazed by the scale of migration and the importance of ecosystem services," said Dr. Chapman.
Other species of insects, such as ladybugs, can also carry out gigantic migrations in other parts of the world. But very few researchers use radars to track them. In Australia, scientists are looking for locusts and Dr. Chapman is currently in China to help researchers set up a new insect tracking network.
Understanding these migrations is even more important, says Dr. Chapman, because many non-migratory insect populations are declining. On the other hand, hoverfly movements have remained stable over the last decade.
They may be less vulnerable because they are not stuck in one place, vulnerable to pollution or climate change.
"These migratory flyovers, in our part of the world at least, are going pretty well," he said. "And they could play an increasingly important role in the future."
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