Mayflies

Sometimes, the mass emergences of adult mayflies can involve extremely large numbers of animals. During the brief times when mayflies are very abundant, some people may view them as a nuisance, because the insects seem to be flying everywhere and can be bothersome because of this, or because their bodies accumulate abundantly on beaches, streets, window screens, and in other places. Rarely, the accumulated biomass of mayfly bodies can represent a traffic hazard, by making roads rather slippery. However, this is quite a rare event, and in general mayflies should not be thought of as a nuisance. Mayflies are rarely abundant enough to be a bother. Almost always, mayflies are a harmless part of the natural world.

Mayfly nymphs are an important component of many freshwater ecosystems. Grazing by mayflies is important in preventing the build-up of a large biomass of aquatic algae and detritus, and in nutrient cycling. Because mayflies can be quite abundant in many habitats, they are an important food for many species of predators.

Mayfly species are rather particular in their choice of habitat, and in their tolerance of environmental conditions, such as the temperature and chemistry of water. Because of their specific habitat requirements, mayfly species are often studied by aquatic ecologists as indicators of water quality, for example, in studies of pollution. A famous study involving mayflies was conducted in Lake Erie during the 1950s and early 1960s. Lake Erie was badly polluted at that time, especially by organic debris associated with sewage and algal growths, the decomposition of which consumed most of the oxygen in the waters of deeper parts of the lake. The development of anoxic conditions resulted in mass die-offs of nymphs of the mayflies Hexagenia rigida and H. limbata, which were previously extremely abundant. The virtual collapse of the populations of mayflies in Lake Erie was widely reported in the popular press, which interpreted the phenomenon as an indication that the great lake was "dead," and had been rendered as such by pollution caused by humans. Today, the waters of Lake Erie are much cleaner, and its populations of mayflies have recovered somewhat.

Both nymphal and adult mayflies are a very important food for economically important sportfish, such as trout and salmon. Many sport fishers are highly skilled at tying mayfly "flies" as lures for use in fishing. The more realistically the lure portrays the species of mayfly that the trout or salmon are interested in feeding upon in a particular stream at a particular time, the greater is the fisher's success in catching fish.