Massive mayfly swarm forces bridge shutdown, inspires nightmares (video)

Mayfly swarm
CC BY 2.0 Wikimedia Commons

"Knee-high" piles of the aquatic critters had to be removed with snowplows.

The swarm of mayflies that overtook Iowa’s Savanna-Sabula Bridge was piled so high that two motorcycles took spills while trying to cross. “It’s like ice on this bridge," said reserve officer Stephen Thayer of the Sabula Police Department when he went to investigate the situation. But of course this wasn’t ice during the waning evenings of July, it was just another day for mayflies who often hatch in fascinating (and nightmarish) syncopation.

Also called shadflies, the North American species emerges in massive throngs from the Mississippi River each year; the total number reaching into a stratospheric 18 trillion. That’s more than 3,000 times the number of people on the planet. But their numbers are more boom than bane. With some species living only a few hours, the fleeting flies mate and drop dead, providing nourishment to all kinds of other aquatic life. (In East Africa they even provide protein for people who turn the newly emerged midges into a patty called Kunga cake.)

This year’s hatch required snowplows to clear the mass of corpses. “Biggest thing I noticed was after cars were stopped and sitting for so long while we’re trying to get cars moving again is they were probably piled knee high in front of their headlights from just sitting there,” said Thayer. But it may not have topped the Mississippi mayfly mayhem from last year, where, according to the National Weather Service, the swarm reached heights of 2,500 feet above the ground in the La Crosse, Wisconsin. It could be seen on radar and had the signature of a significant rain storm.

So what does it look like to be caught in a storm of frantically mating and dying mayflies? Thayer’s mother Teena Franzen accompanied him on his investigation, here’s her video. If you suffer from fear of insects, consider yourself warned.

Massive mayfly swarm forces bridge shutdown, inspires nightmares (video)
‘Knee-high’ piles of the aquatic critters had to be removed with snowplows along the Iowa crossing.

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