Mayflies, so named because of the month that they emerge en masse, can be nuisance insects. Mayflies live between 36 and 72 hours, and when they die, their bodies break up and are carried on the wind. These bodies cause allergic reactions in people with asthma. The arrival of these swarms is extremely dramatic and can cause quite a sight when viewed in the darkening sky.
Mayflies in their larval stage are important in the aquatic food chain. Fishermen often use them as bait, since they are a favorite of fish. Mayflies have existed, virtually unchanged in their makeup, for over 350 million years.
Mayflies are extremely sensitive to pollution and must lay their eggs in water that is pollution-free and highly oxygenated. Mayflies are often used as indicators in water quality studies. Mayflies lay their eggs either by flying low over the surface of the water and depositing eggs, or entering the water and laying the eggs on an object under the surface. Some mayflies will also retain their eggs and then, as the mayfly juveniles, or nymphs, are born they will drop them in the water. The nymphs will molt through many stages, sometimes up to 60, before they are adults. These molting stages completely transform the mayfly from an aquatic crawling insect to a terrestrial flying insect. When the adults emerge, they swarm in extremely large numbers.
The mayfly is attracted to lights at night. This is when they become a nuisance as they can arrive in huge swarms. Changing your outdoor white light bulbs to yellow light bulbs can help reduce mayfly swarms. Also, reducing the amount of light that shines through the windows and doors will help keep the mayfly numbers down.