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Millions of mayflies descended on Gimli Harbour. What's happening here?

Wednesday, July 29th 2020, 10:53 am - Mayflies come in large groups, but they don't last long.

This was the scene in Gimli, Manitoba earlier this month.

In the days surrounding July 20th, millions of mayflies descended on Gimli Harbour. Some of the insects were dead, while others wandered around the ground, creating an impassable swarm that smelled like rotting fish.

Stephen Marshall, a Professor Emeritus at the University of Guelph, says large swarms of mayflies aren't uncommon in this part of Manitoba, due to to the presence of Lake Winnipeg.

Worldwide, there are more than 3,000 species of mayflies. The insects in the images above are called burrowing mayflies, and their presence serves as an indicator of good water quality and a healthy ecosystem.


In the immature stage, mayflies are called nymphs. You won't see them, because they live in shallow bodies of water.

Nymphs make a U-shaped burrow in the bottom of the lake, and wiggle their bodies drawing a current down. They feed on what filters down through the burrow in a process that also helps keep lakes clean.

While mayflies live for anywhere from an hour to a couple of days, dying after they mate, nymphs can live for up to a year.

A large presence at the bottom of a lake indicates a healthy ecosystem.

"They disappeared from Lake Erie in the mid-1950s when we were suffering from peak pollution and it was just wonderful when the lakes cleaned up to the point where the burrowing mayflies came back," Marshall says.


In some water bodies mayflies live in enormous densities, such as Lake Erie and, in this case, Lake Winnipeg.

They emerge once a year and the timing depends on the season and the weather.

"Some researchers at the University of Windsor have determined emergence appears to happen when water temperatures reach about 20 degrees Celsius," Marshall says.

The reason they all appear is a survival tactic.

"This is a strategy called predator saturation," Marshall explains.

"If you emerge all at once, as millions and millions of adults, predators can't put a huge dent in the population. Also when you emerge all at once, it's a great way to meet a mate."


Mayflies are unique because they're the only known insect with two wing phases. When a nymph reaches maturity, it flies away from the water and molts, signifying the beginning of the second wing phase.

Mayfiles don't eat or bite but they can be a nuisance when they move in large swarms, and if too many land on a powerline, they can cause an outage.

Aside from that, mayflies won't bother you, and they aren't around for long.

While they're here, midges act as an important food source for aquatic life, waterfowl, invertebrate, birds, bats, and insects.

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