Rochester, MN (KROC AM News)  - The National Weather Service office in La Crosse, Wisconsin has carried out what is becoming an annual ritual - tracking the emergence of tens of thousands of mayflies along and in the Mississippi River.

The Weather Service says the 2015 emergence occurred on Saturday, July 4th.  For the crowd that had gathered for the July 4th fireworks show at Riverside Park in La Crosse, this meant sharing space with the insects. According to the Weather Service, adult mayflies are often attracted by lights and can fly several miles inland, commonly creating a slippery mess as they pile up and die on the pavement below the lights.  Some only live for a few hours.

At times they have been so thick on bridges that they had to be cleared with snowplows.

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