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Mayflies in August

Bill Meredith

"And God said, ‘Let the waters bring forth abundantly the moving creature that hath life…." Genesis 1:20.

"When the wind is in the south,
it blows your bait into a fish’s mouth."
…Izaak Walton, The Compleat Angler, 1653.

(August, 2014) The Editor’s deadline was fast approaching, and I was sitting in front of my computer with a completely blank mind and a growing sense of desperation when a barely audible beep informed ne that an e-mail message had arrived. It was from my daughter, who shares my love of the odd and arcane aspects of life and often sends me articles from the Minnesota newspapers when such things occur. This time it was a report about a swarm of mayflies that emerged from the upper Mississippi River. There were literally billions of them; they started coming out of the water around 8:30 one morning and continued until nearly ten o’clock that evening, and the air was so full of them that they were mistaken for a rainstorm by the radar at a local weather station. Visibility on local highways was cut nearly to zero, and the roads became slippery from the bodies of the insects. At least one collision, involving three vehicles, occurred.

For the past several decades, mayflies were rarely seen by anyone except ecologists, trout fishermen, and readers of Izaak Walton’s 1653 classic, The Compleat Angler, but there was a time when most people were aware of them. Back when nearly half of the population still lived in the country and there was no television or air conditioning, we used to spend evenings playing in the yard while our parents sat on the porch; and one of our favorite things to do was to keep an eye on the window screens. Most of the lights in the house would be off, but there were always a couple of 20-watt lamps left on so we wouldn’t trip over things when we came in, and as the dusk deepened an amazing variety of insects would accumulate around any source of light.

In those days the term, "insect," was rarely used; most folks just called them bugs. Window screens and spider webs would be full of them, and we would catch the most colorful or bizarrely shaped ones in Mason jars. Moths were the favorite prizes; they varied from tiny brown "millers" to giant Lunas and Cecropias, but there were also hordes of beetles, horseflies, craneflies (which everyone thought were giant mosquitoes), crickets, katydids… and, occasionally, mayflies.

I remember the first one I saw; it was over an inch long, and so beautifully fragile that it looked as if it would fall apart. Unlike the other insects, it did not try to get away; it sat on my finger and allowed me to look at it. I was anxious to see it in daylight, and carefully put it in my jar; but it was dead the next morning. My father had grown up on the bank of Prickett’s Creek in the days before it became polluted, and he knew what mayflies were. So my knowledge of insects began early.

In my first college Zoology course, Prof. Davisson began his lecture about mayflies by telling us the word "ephemeral" meant "short-lived." He then explained the irony that while immature mayflies live in streams for one or two years, they are called Ephemeroptera because the adults of most species live less than a day. They develop wings at their last molt, emerge from the water, mate, lay their eggs, and die. I dutifully made drawings of them and memorized their general features, but it wasn’t until I started graduate school that I really got to know them.

At West Virginia University, my major professor was a specialist in the ecological distribution of fish. Although we were in Morgantown, he had somehow become interested in the Roanoke River in Virginia, and every two weeks he took a carload of his students there to collect samples of their populations. He assigned me to study the life cycle and food habits of a minnow called the rosy-faced shiner; it was a small fish, no more than four inches long, and abundant in that area.

In the course of the next two years I examined and dissected some 750 individuals, and determined their age, the numbers of eggs produced, and what they had eaten. It was tedious work, but the result was that most of the food these minnows ate was insects that were caught at the surface of the stream. About 8% of them were adult mayflies. This surprised me, because I knew that the immature mayflies (naiads) were among the most abundant insects in the stream. Where the water flowed rapidly, they lived under rocks; in quiet water, they dug shallow burrows into the sand or mud. I was amazed to learn that there were over 30 species of fish living in that stream, and they were able to reduce the competition among themseves by living in different zones and eating different things.

My minnow was a "top-feeder;" it stayed up near the surface and fed on small insects that fell into the water from surrounding trees. That included adult mayflies, but not the naiads. Naiads were eaten by other species of fish which stayed near the bottom of the stream. So I finished my Thesis and got my Master’s degree. But as I look back on it, I am embarrassed to admit that it was years later before I came to understand that this pattern of dividing the habitat into different zones is the basis of biodiversity, and my work had contributed part of the evidence for why ecosystems are able to support so many different animals and plants.

The recent outbreak of mayflies in Minnesota and Wisconsin provides an interesting case study. The species that live in lakes can reproduce in unbelievable numbers, and in the 19th century when street lights became common in cities like Chicago, the mayflies that emerged each summer would accumulate around lampposts in piles four or five feet high every night.

The old biology books had photos of workers shoveling them into horse-drawn wagons to be carried away; it was noted that if they were not removed every day, the stench of their rotting carcasses became unbearable. But the naiads require clean water, and as lakes and streams became more polluted, their numbers decreased. By the mid-1900s, the great swarms became less common, and disappeared completely in many areas. The general public were not much concerned about losing mayflies, but they did demand clean drinking water, and when Environmentalism became popular in the 1960s and ’70s, both local and federal laws began to restrict the release of pollutants into streams.

Today, our streams are not pristine by any means, but progress has been made; pollution levels have dropped, and populations of "indicator species" like mayflies have rebounded in many areas. In recent years several localities in the Midwest have noticed increasing swarms of mayflies; two years ago they came in such numbers that the highway department had to bring out snowplows to clean the roads.

Ecologists don’t see many victories, so we like to celebrate when the chance arises. So as soon as I can find the time, I will dig out the fly-tying kit a friend left to me several years ago, and spend an afternoon with it. And if my hands are too unsteady for fly-tying, I can still spend an evening or two with old Izaak Walton. It matters less whether I catch any fish with my ersatz mayflies; it’s the thought that counts.

Read other articles by Bill Meredith