Each summer, Michigan experiences fish fly season, a natural event that both fascinates and inconveniences residents. These insects signal a healthy aquatic environment. Understanding when and where they emerge helps people anticipate their brief presence.
Understanding Fish Flies
Fish flies are a common name for Hexagenia limbata, also known as the giant or golden mayfly. These insects spend one to two years as aquatic nymphs, burrowed in mud at the bottom of freshwater bodies. During this stage, they feed on organic material and algae, contributing to the aquatic food chain. As adults, they emerge as winged insects with large, transparent wings and bodies up to an inch long, with tails extending up to three inches. Adult fish flies lack mouthparts and live only 24 to 48 hours, reproducing before they die.
The Timing of Their Emergence
Fish fly emergence in Michigan typically begins in late June and continues into July, sometimes extending into August. Water temperature is the primary trigger; mayflies hatch and become adults once the water reaches approximately 68 degrees Fahrenheit (20 degrees Celsius). This temperature-dependent emergence means timing varies slightly yearly based on weather patterns. While most emerge in June and July, some late emergences occur in late summer or early fall. Swarms can be large enough to appear on weather radar.
Where They Are Most Prevalent
Fish flies are most common near large, clean freshwater bodies in Michigan. Their nymphs require muddy or silty bottoms for burrowing, making the Great Lakes, especially Lake Erie and Lake St. Clair, key habitats. They are also found in major inland lakes and slow-moving rivers with muddy substrates. Communities along Lake Erie, Lake St. Clair, the St. Clair River, and the Detroit River often experience significant emergences. The presence of many fish flies indicates good water quality and sufficient oxygen, as these insects are sensitive to pollution.
Dealing with Fish Fly Season
Fish flies can be a temporary nuisance due to their numbers. They are attracted to light sources, congregating around streetlights, windows, and outdoor lights. When they die, their bodies accumulate in piles, creating a fishy odor and making surfaces like sidewalks and roads slippery. Fish flies do not bite or sting and are harmless to humans. To manage their presence, residents can turn off outdoor lights at night and close blinds or curtains. Sweeping or leaf blowing helps remove dead insects from surfaces. While an inconvenience, their emergence indicates healthy aquatic ecosystems.