Mayflies are aquatic insects known for their sudden, synchronized emergence from fresh water sources. These events can involve massive swarms near lakes and rivers. Their unique life cycle dictates that the winged stage is extremely short, existing only for reproduction.
The Timeline of Emergence
The timing of a mayfly swarm is closely dictated by the environmental conditions of their aquatic habitat. The emergence of the nymphs from the water is primarily triggered by reaching a specific water temperature. For many species, this temperature threshold is reached during the late spring and early summer months in temperate zones.
For instance, the emergence of some large species is reliably anticipated once the water temperature hits approximately 20 degrees Celsius (68 degrees Fahrenheit). The “mayfly season” can extend over several months because different species mature at varying rates. Most of the mass emergence events occur around or shortly after sunset, initiating the short-lived adult phase.
The Adult Lifespan and Decline
The rapid decline of a mayfly swarm is a direct consequence of the adult insect’s non-feeding existence. Mayflies are unique among insects in that they undergo a two-stage winged phase: the subimago (or dun) and the imago (or spinner). The subimago emerges first and is the only insect form that molts after developing functional wings, transforming into the final imago stage within a day or two.
The adult mayfly is built only for mating, possessing non-functional mouthparts and a digestive system filled with air. They rely entirely on the energy reserves they built up during their much longer underwater nymph stage, which can last up to two years. The adult lifespan can range from a mere few hours up to two or three days, depending on the species.
Males form dense mating swarms that females fly into for reproduction. Once mating and egg-laying are complete, the insects die almost immediately. This synchronized death causes the rapid end to the swarms, leaving behind piles of bodies near the water source.
Managing the Aftermath
Once the mayfly swarm has died off, the primary concern shifts to managing the resulting accumulation of insect bodies. The dead insects often gather in large piles on surfaces like roads, bridges, and walkways, which can create a slick, hazardous environment. Cleanup typically involves sweeping up the dry carcasses or using water to wash down surfaces, sometimes requiring a power washer to remove the sticky residue.
Preventing the swarms from gathering around homes and businesses involves managing light sources, as the insects are strongly attracted to light. Bright white, incandescent, or mercury vapor lights are particularly appealing to emerging mayflies. Switching outdoor fixtures to yellow “bug lights,” sodium vapor lamps, or warm-spectrum LEDs can significantly reduce attraction. Minimizing the use of outdoor lighting during peak emergence times at dusk and early evening will also help to keep the resulting mess away from structures.