Mayfly Invasion: Why They Happen and What to Expect

Mayflies are delicate insects known for a natural event where millions appear at once. This phenomenon, a mayfly invasion or hatch, is a synchronized emergence of adults from the water. The sky can fill with these insects, creating an overwhelming spectacle. This mass appearance is the culmination of a life spent mostly underwater, driven by the need to reproduce.

The Mayfly Life Cycle and Mass Emergence

Mayflies spend most of their lives, from a few months to two years, as aquatic nymphs in freshwater habitats. The presence of mayfly nymphs indicates a clean, well-oxygenated aquatic environment, as their survival depends on these systems. When mature, environmental cues like rising water temperatures trigger their journey to the surface where they transform into a winged form called a subimago. Uniquely among insects, they are the only order to molt again after having functional wings.

The subimago flies to a sheltered spot and molts a final time into the imago, or sexually mature adult. This mass emergence is a survival strategy, as the sheer number of mayflies overwhelms predators like fish and birds, increasing the chances for individuals to mate. The adult lifespan is extremely short because their sole purpose is to reproduce.

Geographic Hotspots and Timing of Invasions

Significant mayfly emergences are concentrated around large, healthy freshwater ecosystems. In North America, the Great Lakes region and the Mississippi River basin are well-known hotspots for massive hatches where billions of insects can emerge, sometimes appearing on weather radar. These events occur where water quality and sediment are suitable for large nymph populations.

The timing of these invasions is seasonal, occurring from late spring through mid-summer in the Northern Hemisphere. The specific timing can vary by a few weeks depending on local weather patterns that influence water temperature, as a warmer spring can lead to an earlier emergence.

Impacts of Mayfly Swarms

The emergence of billions of mayflies has noticeable effects. The insects are attracted to lights, congregating on buildings, bridges, and roads near the water. When they die, their bodies can pile up, creating slippery conditions on roadways that sometimes require snowplows for removal and producing an unpleasant odor as they decay.

Despite the nuisance, these swarms have a positive ecological role. The mass emergence provides a feast for fish, birds, and other animals, transferring a large amount of nutrients from the aquatic to the terrestrial ecosystem. This process enriches the surrounding environment and supports local food webs.

Navigating a Mayfly Event

During a mayfly hatch, it is wise to take some precautions. Visibility can be reduced at the peak of a swarm, and the accumulation of dead insects can make roads slick, so reducing driving speed is recommended.

To minimize mayflies around a home or business, reduce outdoor lighting. Turning off unnecessary lights or using motion-sensor lights can help, as can yellow-tinted “bug lights.”

Cleanup involves sweeping or hosing down surfaces; the phenomenon is temporary and usually lasts only a few weeks.