So many dragonflies are migrating this year that their swarms have been showing up on weather radar.

When the National Weather Service’s Cleveland location tweeted out a video in September showing some huge clouds of purple, teal, and green moving across Ohio, Western Pennsylvania, and Indiana, the logical assumption would be that it was, well, weather. But no: “This is not rain,” the tweet said. That information, combined with reports from people on the ground in those areas, led to what experts are fairly sure is the right answer: massive migratory swarms of dragonflies, heading south for the winter.

Green darner dragonfly on grass
Credit: Evy Lipowski/Getty Images

“Some, but not all, dragonflies migrate, usually between July and October,” says Dr. Sally Entrekin, an entomologist at Virginia Tech who specializes in aquatic insects. (Dragonflies qualify; many species spend much of their lives as nymphs—the larval form, which looks kind of like a wingless dragonfly—underwater before emerging as the adults we see around our gardens.) This year's fall migration is dense and big enough that many people have never seen anything quite like it.

Entrekin says that it’s not confirmed exactly which species of dragonfly is migrating, and that it’s pretty likely there are multiple species in these swarms. Still, she’s comfortable making an educated guess that most of these are common green darners, so named because they look something like darning needles. The green darner is the most commonly found dragonfly in North America, according to the Migratory Dragonfly Partnership, and also one of the largest at about 3 inches long.

These dragonflies actually migrate twice in one year. In springtime, the adults fly northward to breed and lay eggs. When the adults of that generation emerge from the water in late summer and early fall, they head south for the winter to lay eggs for the next generation. Each individual adult lives only about 4-7 weeks, so most of a green darner's life is spent underwater as a nymph.

For Entrekin and other entomologists, the fact that these swarms are now showing up on weather radar maps is a huge boon for studying insects in general. Starting in 2012, radar technology has been able to separate weather details from animals. “This technology has helped meteorologists remove the unwanted signals from flying animals, but has also given ecologists a powerful tool for identifying and tracking animal activity in the airspace,” says Phillip Stepanian, a professor at Notre Dame who studies how weather affects animals. 

Radar and satellite imagery has been used to keep track of birds for years now, but the technology just hasn’t been detailed enough to detect insects until now. Entomologists now hope they can gather valuable data on large swarms of dragonflies and other bugs on the move such as where they are and how big the group is. It’s still not perfect; Stepanian says that it’s not detailed enough yet to distinguish specific species, but scientists are able to fill in the blanks, thanks to reports on the ground. If you want to help out, Entrekin recommends using the iNaturalist app, which allows you to fill in reports of the wild world in your backyard.

As to why the swarm this year is so intense, Entrekin suggests "it was more at one time because of the particular weather patterns.” Migratory insects often start their migrations just before a cold snap, and summer has been particularly long in some parts of the country this year. Perhaps this gave the dragonflies more time to emerge and build up their numbers before they were ready to head south for the winter.

Green darners, like all North American dragonflies, are not dangerous. They could theoretically bite, but unless you’ve caught one and won’t let it go, the odds of that are exceedingly low. They are, as a matter of fact, friends: they’re some of the most effective predators of mosquitoes in the country. The adults, the shiny flying guys we’re used to seeing in the summer, eat adult mosquitoes, and the underwater nymphs feast on mosquito larvae, keeping the total number of mosquitoes down. Entrekin says that dragonflies are sometimes even shipped to places with severe pest problems, sort of like bringing in a cat to take care of a mouse problem.

So don’t be alarmed if you see hoards of dragonflies around. Instead, know that they are helping to knock back mosquito populations, and they are a vital part of the ecosystem as a whole. They’re just all making their way to warmer climates for the winter, and wouldn’t we all like to do that?


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