Large swarms of mayflies show up on Wisconsin radar

LA CROSSE, Wis. (WXOW/CNN) – A skin-crawling sight in Wisconsin… swarms of mayflies coming out of the Mississippi river on July 20.

Take a look.

You can see them at this gas station, covering the pumps.

NOAA animated radar image
NOAA animated radar image

There are so many mayflies, the National Weather Service was able pick them up on radar.

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). 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 2500 feet above ground.

You can see the mayflies becoming airborne by the river and then being carried northward by the wind.

They say that’s common, but this year’s swarm is bigger than normal.

The reason mayflies tend to emerge around the same time in late June to early July has to do with water temperature and when the eggs were laid.

Since all the the eggs are laid around the same time, the adults are generally ready to emerge at the same time as well.

According to the USGS the large swarm is good news:

Mayflies are sensitive to gross organic pollution and their presence is good news ecologically because it means that gross organic pollution such as sewage is not present in large amounts. Because they live in the sediment for most of their lives, mayflies have also been extensively studied as potential accumulators and indicators of trace contaminants such as heavy metals and PCBs.

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