What do mayflies have to do with water quality?

Mayflies. We heard about them a lot this summer. Images and videos of them under streetlights, blanketing cars, and clouding sidewalks went viral on social media. They often get a bad reputation and are called a nuisance “river bug,” but the presence of mayflies along the Cannon and Straight Rivers actually indicates improved water quality. 

Mayflies in Minnesota - Photo by Brian Hoffman

What are mayflies?

A mayfly is an aquatic insect that spends most of its life as a nymph in freshwater habitats before developing into a winged adult, only to mate, lay eggs, and die within a few days or up to a week. Mayflies play a crucial role in the food web of inland waters. As nymphs, they are preyed upon by other invertebrates, amphibians, reptiles, fish, birds, and mammals. Birds and bats often catch the adults while they are alive. Once the swarming, mating adults die, they create a literal feast on the surface of many lakes and rivers, where they often number in the millions.

How are mayflies connected to water quality?

Mayflies are found in almost all freshwater systems with enough oxygen. Not long ago, many of Minnesota's rivers and streams no longer experienced mayfly hatches. Because they are sensitive, chemical pollution, increased dirt in the water from erosion, and lower oxygen levels make it hard for them to survive.

The leading cause of the decline in mayfly populations from the 1960s to the 1980s was the lack of sewage treatment; in many parts of Minnesota, people were literally flushing their toilets directly into the river. (Gross, we know.) However, the presence of sewage wasn’t the biggest problem; it’s what happened when the bacteria broke down and consumed large amounts of dissolved oxygen. Mayflies and other insects like them couldn’t survive without enough oxygen. 

Thankfully, that specific problem was addressed, and due to modern sewage treatment facilities and regulations on the disposal of toxic chemicals, mayflies have returned to most of our waterways. They still face challenges from stormwater pollution, but with the surges of mayfly hatches, it feels like we’re heading in a good direction. 

What you can do to support aquatic insects

Many parts of Minnesota continue to face water quality challenges, and there are steps you can take to be part of the solution. One of the main threats to aquatic insects is stormwater pollution. Stormwater pollution occurs when excess water from rain or melted snow and ice flows over hard surfaces, such as roads and concrete. It picks up chemicals and trash along the way and directly enters our waterways through storm drain systems. 

What we do on land affects the water. Picking up pet waste, keeping leaves and grass clippings out of the street, using minimal or no lawn fertilizers, and following best practices when spreading manure or applying agricultural chemicals can help keep our waterways in better condition for aquatic life. 

Resources

Fremling, Calvin. “Hexagenia Mayflies: Biological Monitors of Water Quality in the Upper Mississippi River.” Journal of the Minnesota Academy of Science Journal of the Minnesota Academy of Science, vol. 55, no. 1, 1989, pp. 139–143, digitalcommons.morris.umn.edu/cgi/viewcontent.cgi?article=2634&context=jmas. Accessed 29 Aug. 2025. 

“Species Profile - Mayfly.” Minnesota Department of Natural Resources, www.dnr.state.mn.us/minnaqua/speciesprofile/mayfly.html.