The Washington Post

Watch swarms of Mississippi River mayflies so thick they showed up on radar like a storm

This July 20 radar loop image from the National Weather Service shows mayflies that hatched along the Mississippi River as they morph into a green band over about a four hour period. The bugs become blue dots as they disperse. (NWS)

“Almost every night in the summer, there’s some sense on the radar that there’s something coming off the river,” Dan Baumgardt, science and operations officer for the National Weather Service in La Crosse, Wis., tells the Associated Press. “We don’t know what kind of bug it is … until we have people calling or saying, ‘Oh my gosh, there’s mayflies all in the La Crosse area.’ ”

They were indeed mayflies. They showed up on the weather radar as the equivalent of a light to moderate rain storm, according to the National Weather Service. As the service described it, “the Mississippi River produced a massive radar echo as mayflies emerged from the water and became airborne. The mayflies were detectable on radar around 845 pm and reports in the towns and cities began rolling in of the swarming and piles of mayflies. … The radar detected the flies about 845 pm, emanating from the river (the source) with echo values similar to that of light-moderate rain (35-40 dBZ). ”

“By late evening,” says the weather service, “mayflies were swarming in La Crosse, La Crescent, Stoddard and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season depending on the species, this particular emergence was that of the larger black/brown Bilineata species. The radar loop [above] shows the reflected radar energy (reflectivity) from 8:35 pm to just after midnight. The higher the values (greens to yellows) indicate greater concentrations of flies. Note how the swarm is carried northward over time.”

Here’s the deal on them from the Fish and Wildlife Service:

As nymphs, these aquatic insects proceed through one or two years of larval development as filter feeders, consuming decaying organic matter at the river bottom. In summer, large numbers of nymphs synchronously emerge from the water at dusk and take flight as sub-adults. Within 36 hours of emergence, the sub-adults metamorphose into adults that subsequently swarm in the air to mate before returning to the water surface to lay their eggs and die. Some of these emergence events are so large and widespread that swarms can be detected by Doppler weather radar.

They’re supposed to be the bearer of good tidings ecologically. In some years, they’ve failed to breed because of pollution, so their re-emergence is supposed to be a sign of environmental health.

Unfortunately, they’re attracted to lights, so they often head for lighted highways after they hatch. Millions of them get run over or squashed, leaving behind a green slime that’s so slippery it can cause accidents. Only one accident was reported last week.

According to the Minneapolis Star-Tribune:

Sunday’s accident happened about 10:25 p.m., when a northbound car driven by Theresa L. Hunt, 24, of Ellsworth, Wis., lost control on the slickened roadway as the blizzard-like flurry of insects cut visibility, according to the Pierce County Sheriff’s Office. Hunt’s car hit another vehicle heading south driven by Scott Pauly, 52, of Rochester, and then a second vehicle, a van driven by Tanya Lapointe, 35, of Baldwin, Wis. Hunt was slightly injured, and Cynthia Pauly, 51, a passenger in one of the struck vehicles, was taken to Mayo Hospital in Red Wing for treatment of undetermined injuries.


A second, smaller swarm recorded Thursday night starts as a green band before exploding like fireworks into blue dots. The radar loop below covers the time between about 8:45 pm and 11:10 pm.

This loop covers about a two and a half-hour period. (NWS)


Fred Barbash, the editor of Morning Mix, is a former National Editor and London Bureau Chief for the Washington Post.
Show Comments

Sign up for email updates from the "Confronting the Caliphate" series.

You have signed up for the "Confronting the Caliphate" series.

Thank you for signing up
You'll receive e-mail when new stories are published in this series.
Most Read



Success! Check your inbox for details.

See all newsletters

Close video player
Now Playing

To keep reading, please enter your email address.

You’ll also receive from The Washington Post:
  • A free 6-week digital subscription
  • Our daily newsletter in your inbox

Please enter a valid email address

I have read and agree to the Terms of Service and Privacy Policy.

Please indicate agreement.

Thank you.

Check your inbox. We’ve sent an email explaining how to set up an account and activate your free digital subscription.