Massive Mayfly Swarms Are Getting Smaller—and That’s Bad News for Aquatic Ecosystems

The drop is a sign that the insects’ populations are threatened, which could negatively affect the animals that feed on them

Adult mayflies following an emergence on Lake Erie. Phillip M. Stepanian

A single mayfly is a delicate creature—with its long, slender body, translucent wings, and two or three thread-like tails. But in the summertime, hordes of mayflies emerge from lakes and rivers to form swarms so dense that clouds of the insects have been known to blanket cars and envelop gas stations.

These swarms are as impressive as they are annoying. Because mayflies thrive in unpolluted waters, their appearance en masse is also a good sign that aquatic ecosystems are functioning as they should. It is disconcerting, then, that a new study in Proceedings of the National Academy of Sciences has found that mayfly swarms are not as large as they once were.

In North America, mayflies exist primarily around the Great Lakes and in the Mississippi River Basin. Juvenile critters, known as nymphs, typically live in the water for one year, moving onto land as they reach adulthood. They molt twice before they are able to fly, subsequently taking off into the air to mate. Reproduction is now the mayfly’s single purpose: adults don’t eat—their mouthparts and digestive tracts are not even functional—and they die promptly after mating and laying eggs.

Because swarms of mating mayflies can number in the billions, they are often visible on weather radar. The authors of the new study analyzed radar data from between 2012 and 2019, using the size of the swarms to estimate populations along the the Upper Mississippi River and Western Lake Erie Basin. Their calculations suggest that the number of mayflies in the Mississippi River region has declined by 52 percent since 2012. Around Lake Erie, populations have gone down by 84 percent.

“We were really surprised to see that there was a decline year after year,” Phillip Stepanian, a bio-meteorologist at the University of Notre Dame and lead author of the study, tells Douglas Main of National Geographic. “That was really unexpected.”

The drop in mayfly numbers suggest that water quality in these regions is not optimal. According to the study authors, a number of factors could be contributing to the decline. First, warming water temperatures caused by climate change may be disrupting oxygen circulation and the insects’ life cycle, according to National Geographic. Another possible culprit is fertilizer runoff from farms, which have been triggering algal blooms in Lake Erie. Algal blooms release toxins into the environment, to which mayflies are “highly sensitive,” according to the researchers.

Pesticides are also flowing into the Great Lakes tributaries. One 2018 study, for instance, found that concentrations of the neonicotinoid class of insecticides was up to 40 times higher than acceptable limits set by U.S. Environmental Protection Agency Aquatic Life Benchmark. The mayfly species Hexagenia, on which the new study focused, “are among the most sensitive aquatic insects to a suite of these commonly applied pesticides,” the researchers write.

The reduction in mayfly numbers is also disconcerting because these insects play a crucial role in the food chain. As underwater nymphs, they act as a crucial food source for fish and wading birds; once they emerge onto land, they are eaten by other insects, birds and bats.

Mayflies are hardly alone in their decline. A report published last year found that more than 40 percent of insect species are at threatened by extinction, due to factors like habitat loss, chemical pollution, invasive species and climate change. This alarming trend has been dubbed the “insect apocalypse”—and it is sure to have ripple effects.

“It is likely that other aquatic insect species may be undergoing the same declines for the same reasons,” Francisco Sanchez-Bayo, an ecologist at the University of Sydney in Australia who was not involved in the mayfly study, tells National Geographic. “The inevitable consequence is the decline of populations of insect-eating birds, frogs, bats, and fish in those regions.”

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