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Curious City

The Great Chicago Dragonfly Invasion, explained

They came like a plague — thousands upon thousands of them. They rose from the murky waters of Lake Michigan and, when the time was right, they molted in the open Chicago air. Call it the Great Chicago Dragonfly Invasion of 2015.

Questioner Debbie Yoo noticed it happening as she jogged along the city’s lakefront trail. She wasn’t expecting to encounter the massive swarms of large-winged insects along the trail that day. But then again, who was?

“They appeared out of nowhere!” she says. “It was crazy, like one of those fables where dragonflies or frogs drop out of the sky. It was like that.”

She wasn’t scared of them, per se. But she was curious, enough to send along this question:

Why is there such an influx of dragonflies at the lakefront right now?

We put the question to Doug Taron, curator of biology at the Peggy Notebaert Nature Museum. And the first thing he said was … don’t be alarmed. For the most part, those dragonflies are just green darners, the most common of the city’s 50-or-so dragonfly species.

But, Taron says, they do seem to be having a larger-than-usual convention in Chicago. And he was kind enough to give us the lowdown on why that’s happening now.

People who've been enjoying the city’s lakefront aren’t crazy? This is a thing?

That's right, several factors have come together at the same time and I know the results have been quite dramatic. I've been hearing about things all along the lakefront, from the South Side of Chicago all the way up into Wisconsin. And so there are a lot of feuding swarms that are being observed at the moment.

This is definitely one of the larger populations that I have seen in the last 10 or 15 years here.

Why is there a boom now?

It’s in the nature of insects to have their populations fluctuate a lot from year to year. ... One thing that might be contributing this year is that the mosquitoes have been really terrible this year and mosquitoes are one of the main foods of many species of dragonflies. Even the young dragonflies that are still aquatic and living underwater eat mosquito larvae, and there were almost certainly lots and lots of them earlier this year. So, it provided a very good food base for the young dragonflies.

These feeding swarms that everybody has been seeing around the last week are often associated with migration. ... This is a little early for that, but I would anticipate that there would be a large migration this year.

Why is the green darner species living around the city’s lakefront?

Green darners do well in this type of environment because they're not one of the species to get really, really fussy about water quality. ... The young are aquatic and for some species the young need really, really clean pristine water. Green darners can experience and cope with a degree of pollution, so they tend to be a species that has remained more common in the modern environment.

Do these swarms do any good?

The dragonfly are a species that’s easy to love because they do something that we consider helpful and they do consume a lot of mosquitoes. When they migrate they are also a great seafood source for certain migrating birds, especially hawks.

A dragonfly can consume thousands of mosquitoes over the course of its lifetime. ... They’re mosquito vacuums. ... It's easy to love something that helps with mosquito control.

They're not going to strain or bite or anything like that, so it can be kind of alarming to see these very large insect zooming around you, but they're not going to hurt anybody.

Will they stick around?

I have seen large feeding swarms a number of times in the last decade or so. You see them a lot on the lakefront. Again this is because they are associated with migration and the lake comes to concentrate a lot of migrating organisms right along the shore.

Green darners migrate every year further south and no one knows exactly where they're going. But it's a very regular phenomenon.

They're extremely powerful flyers and in fact that's one of the things that's made it difficult to study their migration. Most people are aware of the monarch butterflies being the other insect that's well known to migrate and a lot of that migration has been tracked by putting little tags on their wings and then seeing where they get recovered. It's much harder to catch dragonflies, so it's much harder to apply the tags in the kinds of numbers that you need to use that as a tool to study migration.

We will at some point see them head on out and move south, and we won't see as many. Generally that happens about a month from now. But that's also generally when you start seeing the feeding swarm, so I'm not really sure what's going to happen this year.

Sean Kennedy is a reporter in Chicago. Follow him @stkennedy. Logan Jaffe is Curious City's multimedia producer. 

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