They came en mass on Sunday and then, almost as suddenly as they emerged, they were dead, leaving a huge mess behind.

A mayfly hatch in Wisconsin over the weekend was so large that it was picked up by weather radar and even caused a three-car accident, not to mention resulted in stunning photos documenting their sheer quantity.

The mayflies emerged on June 20, blanketing the Upper Mississippi River area in Wisconsin. (Image source: NWS)

The mayflies emerged on July 20, blanketing the upper Mississippi River area in Wisconsin. (Image source: NWS)

While it might look like a lot of insects, entomologists said the quantity isn't unusual. (Image source: NWS)

While it might look like a lot of insects, entomologists said the quantity isn’t unusual. (Image source: NWS)

According to WCCO-TV, the mass hatch blanketed the upper Mississippi River and the surrounding area in La Crosse, Wisconsin, Sunday.

“It’s not surprising. They’re just such large numbers that you can see their bodies on radar,” entomologist Petra Kranzfelder told the news station, noting that the size of the hatch is not unusual either.

Though Kranzfelder said that the number of insects that emerged was relatively moderate as hatches go, the National Weather Service still called it a “massive emergence.”

The mayflies emerge around the same time because they were laid as eggs at the same time as well. The insects mate, lay their own eggs and die within a couple of days.  (Image source: NWS)

The mayflies emerge around the same time because they were laid as eggs at the same time as well. The insects mate, lay their own eggs and die within a couple of days. (Image source: NWS)

“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),” NWS said. “With a general south-to-north wind flow above the surface, the mayflies quickly moved north once in the air. As the flies dispersed moving north-northeast, they also gained altitude with some of the echo being detected as far north as Black River Falls and as high as 2,500 feet above ground.”

This radar image shows the mayflies as they hatched. (Image source: NWS)

This radar image shows the mayflies as they hatched. (Image source: NWS)

A few hours after their emergence, the roads near the hatch were so slimy and visibility so low that it caused a car accident on a bridge linking Red Wing and Hager City, which left one person hospitalized with injuries, the Minneapolis Star Tribune reported.

Mayflies covering a bridge caused poor visibility and slippery conditions, which resulted in a three-car accident.  (Image source: NWS)

Mayflies covering a bridge caused poor visibility and slippery conditions, which resulted in a three-car accident. (Image source: NWS)

While the mayflies, which die a day or two after hatching, might be an annoyance, Kranzfelder said they are an indicator of good water quality and are an important food source for fish and other animals.

“And the other nice thing is they transfer nutrients, like phosphorus and nitrogen, from the river to the land,” Kranzfelder told WCCO.

He also said that since their emergence is so fleeting that people should “take some time to enjoy it.”

Watch WCCO-TV’s report:

Those who were around during the hatch recommend people “keep your mouth shut and kind of cover your nose,” Glen Siewer told WEAU-TV.

(H/T: Channel 3000)

This story has been updated.