So many mayflies they show up on weather radars?

Updated: February 5, 2020

Mayfly on a person's hand Phillip Stepanian, University of Notre Dame

Mayfly swarms are often so large that they show up on weather-predicting radar. Researchers using that radar to try to accurately count the number of mayflies in two North American watersheds discovered an alarming decline in their populations, potentially leaving a dangerous void in the food chain.

The innovative study of mayfly populations, a primary source of food for a wide array of insects, birds, and fish, recorded dramatic population declines according to their report in the Proceedings of the National Academy of Sciences in the U.S.

Mayflies spend most of their lives in the water as nymphs, then briefly emerge en masse as adults to breed in astronomical numbers. “Nearly 88 billion mayflies can emerge from the water in a single night,” says Phillip Stepanian, a research assistant professor at the University of Notre Dame, and lead researcher on the project. “This [translates] to 12 trillion calories of food resources for insectivorous animals…so this is an important resource for the local food web.”

Traditionally, accurately tracking such large numbers of creatures has been impossible. But Stepanian, who has a background in meteorology, knew that clouds of mayflies show up on weather tracking radar.

The researchers analyzed radar observations of mayfly “emergence flights” in the Upper Mississippi River and Western Lake Erie Basin to observe changes. In both areas, over the course of study mayfly populations dropped by at least 50 percent, with the Lake Erie population decreasing by 84 percent between 2015 and 2019. “The idea that populations could be experiencing such rapid declines was both surprising and confusing,” say Stepanian.

Why are mayflies important?

Since they spend most of their lives submerged in freshwater, and are particularly sensitive to pollution, mayflies are considered an indicator species of the health of a waterway. While the research didn’t specifically look at what caused the population declines, potential factors include rising levels of neonicotinoid pesticides washing into waterways from farmland, fertilizer-fuelled algae blooms, and climate change slowly rising water temperatures, resulting in less oxygen flow in lakes. In a highlighted “Significance Statement” in the paper, the researchers wrote that “if current population trends continue [it] could cascade to widespread disappearance [of mayflies] from some of North America’s largest waterways.”

It’s a mayday call for both mayflies and the countless species of higher order insects and animals that rely on them for food.

Read more: 3 easy ways to make a wildlife-friendly garden at your cottage

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