Dead mayflies leave massive mounds at Lock and Dam in Welch, Minnesota

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Millions of dead mayflies on Lock and Dam 3 in Welch, Minnesota. Photos by Jeff Ferguson / U.S. Army Corps of Engineers.

The crew at the U.S. Army Corps of Engineers Lock and Dam No. 3 on the Mississippi River in Welch, Minnesota had a bit of a bug problem to deal with on Wednesday. It’s mayfly season, and mounds of dead mayflies resembling piles of mulch littered the lock and dam.

Every summer, millions of mayflies lay their eggs on the Mississippi River in Minnesota and Wisconsin, creating a swarm that’s big enough to be seen on weather radar. The swarm only lasts about 24 hours before the insects die.

According to the National Weather Service, a large mayfly emergence was seen on radar July 11. Downtown La Crosse, Wisconsin had to contend with a thick swarm of mayflies and they drifted north, away from the river.

The quick infestations can cause serious problems, especially on highways. In 2012, millions of mayflies left created slick conditions on the Highway 61 bridge near Hastings, causing a head-on crash. In 2014, mayflies were blames for a three-car crash on the Highway 63 bridge between Red Wing, Minnesota and Hager City, Wisconsin.

Mayflies spend about 99 percent of their lives as nymphs. Once emerged, the insects rest on nearby vegetation until becoming a mature adult a few days later. Mating takes place in flight as the female flies through a swarm of males. The female then returns to the water to lay eggs and dies.

Mayflies have an important role in the aquatic food chain. As nymphs they are eaten by fish and birds, and the adults are often taken by birds and bats in flight. Once the swarm dies, there is an all-you-can-eat buffet on the surface of lakes and rivers.