Mayfly Mess

Mayfly Mess

It is that time of year in the  midwest when mayflies emerge en masse from the depths of the Mississippi River, traveling in swarms that have been known to cause car wrecks, coat roads and leave a slimy mess in their wake. The swarms are so large that they can be seen on weather radar appearing very much like thunderstorms! Radar picks up on the energy reflected off of flies, with the swarm’s density being indicated by the intensity of the image. On Sunday night, in Wisconsin, the National Weather Service picked up on a massive swarm of mayflies emerging from the river and drifting north. June through August is typically a very active time for swarm emergence, the weather service usually records several large swarm events during this period, however, the atypically cool air and water temperatures have caused Sunday’s event  to be the first of the year large enough to be captured on radar.

In this photo provided by the U.S. Fish and Wildlife Service a female "Hexagenia limbata" mayfly. The National Weather Service captured a massive swarm on radar Sunday, July 20, 2014 as thousands of the flies, commonly found in tributaries of the Upper Mississippi River, came out of the river and drifted north on the wind. (AP Photo/U.S. Fish and Wildlife Service)

In this photo provided by the U.S. Fish and Wildlife Service a female, Hexagenia limbata, mayfly. The National Weather Service captured a massive swarm on radar Sunday, July 20, 2014 as thousands of the flies, commonly found in tributaries of the Upper Mississippi River, came out of the river and drifted north on the wind. (AP Photo/U.S. Fish and Wildlife Service)

Flies hatch, then spend the next year burrowed in the sediment at the river bottom, emerging the following summer to mate, lay eggs and die-in that order, within 48 hours. Mayflies serve as an indicator species; a species whose presence, absence, or relative well-being in a given environment is a sign of the overall health of its ecosystem. In the 1920s mayflies had completely disappeared from approximately a 70 mile radius south of the twin cities, Minneapolis and St. Paul, MN. Mayfly populations were heavily impacted by low dissolved oxygen levels and high pollution levels in the river. They did not reappear in statistically significant numbers until 1978 when actions from the implementation of the Clean Water Act began to take effect.

As with many insects, mayflies are attracted to lights, converging on roads, bridges and other inconvenient surfaces in piles that can reach 2 feet high. When these throngs are inevitably crushed by cars, a dangerous, slick mess is created which is blamed for several car accidents each year. Surviving flies spend the remainder of their 48 hour lifespan shedding their exoskeletons and finding a mate. After which, it is a mad dash for females to find water where their eggs can be laid. Lacking the appropriate waterbody, females have been known to lay eggs in any liquid they come across, even beer. Adults die post egg-laying, their offspring will hatch within hours and begin the cycle anew.