Mayflies have emerged from the sediment of the muddy Mississippi River and are swarming upon some areas of the Midwest, appearing on radar just as if they are thunderstorms approaching and ready to pour forth rain. The remains of dead mayflies, crushed by cars and other passing vehicles, are coating some roads. Their slimy remains on roads have made them slick, credited with causing at least one car crash in Trenton Township, Wisconsin this past week.
Upon hatching, mayflies bury themselves into the river sediment. They emerge the following summer from the river that marks the borders of Minnesota and Wisconsin, but after that, their lives are hectic and short. Mayflies spend the last 48 hours of their lives find suitable mates, laying eggs and dying.
The mayflies use up 24 hours of the 48 they have remaining to them shedding their exoskeletons. This is necessary before they take to the air and mate.
When mayflies congregate in huge swarms, they can be a nuisance, but the insects are crucial to the diets of many fish. Also, they will only lay their eggs and hatch from rivers that are relatively free of pollutants and that rich in oxygen. Biologists like Mark Steingraeber, of the U.S. Fish and Wildlife Service, consider them to be like “sentinels,” giving researchers an idea of the water quality of rivers.
As an example, the disappearance of mayflies from a 70-mile stretch to the south of the Twin Cities in Minnesota from the 1920s to 1978 was likely due to water pollution. In 1978, mayflies once again began to appear, coinciding with the time when the Clean Water Act ensured that adequate wastewater treatment was taking place.
On Sunday night, the U.S. National Weather Service radar picked up an enormous swarm of mayflies as they came out of the river and were carries northward by the wind. The radar detected energy that was reflected off of the mayflies. The density of the swarm of bugs could be seen by the intensity of the image on the radar.
When the mayflies emerged on Sunday, the radar showed yellow patches above the river. Then, theses yellow patches became a green-looking band as they were transported to the north by the wind. After that, as the swarm broke up, the groups of mayflies leaving it became blue dots.
The U.S. Weather Service generally will pick up several swarms of the mayflies every year, from the month of June through August. Biologist Mark Steingraeber explained that this summer’s first big swarm of mayflies occurred last Sunday because water temperatures have been cooler than usual this year.
Mayflies are tiny insects, but they are attracted to the lights that illuminate roads and bridges. They can accumulate in piles on these road surfaces that are practically two feet high. When cars run over the insects and crush them, green liquid in the eggs of the females coat the roads and make them slick and potentially dangerous. A slick road surface in Trenton Township, Wisconsin, caused by the mayflies, caused a car to slide and crash into another car and a van. Two people were injured as a result.
Soon after laying eggs, the adult mayflies die. Then, after their offspring hatch and spend another year buried, awaiting the next summer, when it will be their turn to swarm and repeat the cycle. Swarming mayflies often congregate in numbers so vast that they appear on radar like an approaching thunderstorm, but they are harmless other than causing road surfaces to become slick. They are also important in the diets and life cycles of fish and other animals like frogs, as well as being indicators of the health of rivers.
Written By Douglas Cobb
Courtesy WKBT TV: Mayflies hatching in LaCrosse video