Mayflies – the ‘Circle of Life’

Thousands of long-tailed mayfly in sunset light

The emergence of the mayfly is a spectacular but very short-lived sight to see. Each year from May through Augustadult mayflies emerge from lakes and rivers. In some parts of North America, swarms of emerging mayflies are so large (up to 80 billion) that scientists have been able to track their presence in the skies on weather radar systems that are usually used to track rain and snow (National Geographic, 2020) 

While we may not see swarms of 80 billion mayflies here in the Credit River Watershed, mayflies are very much an important presence.

Adult mayfly in Credit River Watershed. 

The transformation into the adult form is fascinatingThe mayfly experiences two separate winged adult forms during their lifecycle.

Credit: Art Station, Jana Růžičková      

1. First, a mating female called an imago, also more commonly known as a spinner, lays her eggs in the water. These eggs then settle to the bottom of the stream and stick to stones and plants 


 2. The mayfly egg then hatches into a nymph at the bottom of stream, river, lake or wetland where it spends most of its life. 


3. The first winged-stage, known as the dull-coloured sub-imago or more commonly known as a dun, lasts from 30 minutes to a few hours. During this stage, the nymphs make their way to the water surface and shed their skin. They rest momentarily to dry off their newly exposed wings. They may also look for shelter among streamside plants or trees. During this adult stage, mayflies play an important role in the ecosystem as they serve as food for a wide variety of animals, including fish and birds.

Adult Mayfly emerging from cast skin. Credit: Frost Fly 

1. If the sub-imago survives, they will enter the second winged-stage, known as the brightly coloured imago. This is when mating and reproduction takes place. Once the females have dispersed their eggs, they will fall to the water surface tired out where they become food for fish. The male will typically go to nearby land where they also return to the food chain.

Through our Integrated Watershed Monitoring Program (IWMP), we study the community of mayfly nymphs and other aquatic invertebrates that live on the bottom of the stream. Aquatic invertebrates serve as bio-indicators. That means their presence, numbers and species diversity can tell us a lot about the health of a waterbody and the surrounding environment. Abundant, healthy and diverse communities of aquatic invertebrates typically indicate healthy water and a thriving local environment.  

Typically, we spend about two months each year collecting samples from over 50 stream sites throughout the Credit River Watershed. 

CVC staff collecting aquatic invertebrates at a West Credit River Tributary.

Much of the fall and winter are spent in the lab putting all the pieces together. We work on correctly identifying the organisms using a microscope.

A view through the microscope at CVC. Left: Caddisfly larva. Right: Mayfly nymph.  

This year, we’ve had to postpone our IWMP field work due to COVID-19 in order to ensure the safety of our staff and the communitySo, while we may not be outside right now our work continues. We have a tremendous amount of data collected from 2019’s field season that we have been working to analyze and summarize so that we can accurately report on conditions of the Credit River Watershed’s ecosystems. We also continue to learn more about aquatic invertebrates and sharpen our identification skills.  

If you are interested in learning how to identify aquatic invertebrates, a great resource we have been using is The Atlas of Common Freshwater Macroinvertebrates of Eastern North America. Here you can learn about the different families of aquatic invertebrates that you may discover across our watershed, including the many different types of mayflies. 

Happy learning! 

By CVC’s Chantalle Jacob-Okorn, Technician, Watershed Monitoring

Feature photo by Michigan Radio, 2019.

Comments (8)

  1. I was wondering how the population of green drake mayflies is doing in both the main Credit and the west Credit above the Forks? I know that there has been a severe reduction in the main Credit, but understood that there was still a viable population in the West Credit between the Forks and Belfountain.

    1. We don’t monitor specifically for the Green Drake Mayfly in our long-term monitoring program. To identify the exact species in our samples takes additional effort and resources. We identify samples to the family level – for example the Green Drake is in a family group called burrowing mayflies. This family-level detail gives us information about the overall condition of aquatic invertebrate communities to help us understand conditions and trends in our stream ecosystems.

      Other agencies from across the province also use this community-level approach to aquatic invertebrate monitoring. It is also quite rare that we come across the Green Drake in our aquatic invertebrate samples. The Forks of the Credit area does, however, support a diverse community of aquatic invertebrates including many sensitive families of mayflies, stoneflies and caddisflies. While there is research happening outside of CVC looking at Green Drake populations in the watershed, we do not detect species-level changes in our long-term monitoring.

  2. I am interested and I know that fishermen use flies that imitate May flies. When I was young in England I would take frog spawn home and watch the evolution of frogs. It was also possible to catch newts and they would produce eggs. My father in law would take frog spawn into ponds where the frogs had been eliminated
    to maintain their existence.

  3. What effect will the Town of Erin’s effluent (7400 cubic metres per day, plus urban run-off from a population growth of 18,900, and industrial lands that are the size of 78 football fields) have on Mayflies? Treated effluent containing chlorides and other toxic substances will be released into the West Credit River at the Wellington-Caledon border. What is the tolerance of Mayfly populations to urban drainage and effluent? Could the CVC please reevaluate there “no-objection”, but still “with concerns”, stance on the Town of Erin’s urban sprawl growth in a pristine Headwaters region?

    1. There are multiple species of mayflies in the Credit River Watershed and each has its own unique tolerance to various pollutants. Many species of mayflies are sensitive to urban pollution such as nutrients, trace metals or high water temperature. For this reason we use mayflies as indicators of general water quality conditions in our monitoring program. When we evaluate the condition of a stream, we look at particular metrics such as the number of sensitive mayfly, stonefly and caddisfly families that are present.

      CVC has participated in the Town of Erin Wastewater Treatment Plant Environmental Assessment process since 2007. CVC will continue to be involved in the next phases of the process and will review plans through our regulatory role. We will provide comments on our areas of interest, including water quality.

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