Pierce County Sheriff Everett Mulhausen says the mayfly explosion on county roads and highways next to the Mississippi River is the most he's seen in more than 30 years.
The pesky, short-lived mayflies are causing problems on roadways along the river. Last week a motorcyclist skidded out of control on Wisconsin Highway 63 between Hager City near Red Wing because the bugs were up to four inches deep on the road.
Eventually a snowplow was used to clear away the flies.
But, the bugs may be a good thing. Environmentalists say their numbers are a sign of clean water.