Mayfly, Vol. 4, No. 11

Contrary to popular conception, mayflies are relatively long-lived insects.  While it is true that adult mayflies do not feed and only live a day or two—just long enough to mate and lay eggs, most mayflies have an annual life cycle, which means that most adult mayflies are about one year old when they die.  That’s pretty old for an insect, and a few species take several years to develop from egg to adult.  Mayfly taxonomy and identification is pretty complex.  Worldwide there are thousands of species, representing dozens of families.
 
Immature mayflies, known as naiads, live in the water, where they feed on algae and other organic materials and breath by absorbing oxygen through their skin and abdominal tracheal gills.  When mature, they move to the surface of the water to emerge, and the non-feeding adults spend a day or two mating and laying eggs to repeat the cycle.
 
Like periodic cicadas, adult mayflies tend to emerge en masse, so fish, birds and other predators can’t eat them all before they have a chance to mate and lay eggs.  Numbers can be huge, millions and millions, and heavy emergences of mayflies show up clearly on weather radar—like a light to moderate thunderstorm.  Some summers the number of mayflies around the Ross Barnett Reservoir and other large lakes is so high they cause safety hazards, general consternation and other problems by accumulating on walkways, roads and buildings.  Mayfly emergences happen every year, but populations are higher some years than others.  Physical controls, such as light management, physical exclusion, leaf blowers, vacuums, and brooms and dust pans are usually more effective than insecticides for coping with these types of mass emergences.  Fortunately, such emergences are short-lived, usually lasting only a week or so.
 
Mayflies are well known to trout fishermen, who tie flies that mimic different species and stages of the mayfly life cycle so they can “match the hatch.”  No trout here in Mississippi, but savvy bream fishermen are always alert for mayfly hatches on any of our larger bodies of water.  Mayflies play an important role in lake and stream ecology, and healthy mayfly populations are a sign of a healthy aquatic ecosystem.
 
Here’s a link to a site that shows examples of heavy flights of mayflies being detected on weather radar along the upper Mississippi River (https://www.weather.gov/arx/mayfly_tracking).  Check out the 2014, July 20 emergence.  This site also shows photos of heavy mayfly emergences.