Mayfly hatches are signs of healthy waters, good fly fishing

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Callibaetis like this one are typically found hatching in the shallow water and weedbeds along the edges of lakes. Since 1996 Robert K. D. Peterson, associate professor of Entomology at Montana State University, has been photography insects like mayflies. A collection of his work can be found online at

If you’ve ever wondered about the water quality in a stream, river or lake you are fishing, just ask a mayfly.

Mayflies are winged insects that spend the early stages of their life underwater before swimming to the surface, shucking their skin and taking flight to mate. Because of their aquatic youth the insects are sensitive to changes in water temperature and water quality.

“They have gills on the outside of their body,” explained Dave Stagliano, an aquatic biologist at Montana Biological Survey in Helena. “They need clean water to not foul up their gills.”

Insect sampling

Sally Entrekin, an associate professor in the Department of Entomology at Virginia Tech, samples a stream in search of aquatic insects, including mayfly nymphs.

Low oxygen levels, silt, chemicals, heavy metals and other pollutants can kill or reduce the number of mayflies.

“One of the big, heavy metals indicators are Heptagenia,” Stagliano said.

Known as the flatheaded mayfly, Heptagenia hug the river bottom. Because their gills are on the bottom of their abdomen they are easily affected by water that contains heavy metals, like the Clarks Fork River. The Clarks Fork has been contaminated by runoff from mining waste in the Butte and Anaconda areas.

In an assessment conducted last October, Stagliano found that zero to less than 5% of all the species of aquatic insects found in the upper Clark Fork River were Heptagenia.

“Whereas in healthy rivers that could be 20 to 30% of the taxa present,” he said.

The number of Heptagenia doesn’t increase in the Clarks Fork River until after Rock Creek flows in and boosts the river’s flow. That’s about 100 miles downstream.

Dainty flies

Mayflies spend much of their life under water. They swim to the surface, squeeze out of their nymphal shell, and become adults to mate. Females then lay their eggs back in the water and die. 

Nowhere is water quality’s effect on mayfly populations more evident than near Lake Erie, the fourth-largest of our five Great Lakes, covering almost 10,000 square miles. Lake Erie is home to the burrowing mayfly, one of about 2,500 mayflies known worldwide.

The burrowing mayfly is well-known to Lake Erie-area residents because they hatch all at once in huge summer swarms — estimated at 88 billion insects — that can be seen on radar. Between 2015 and 2019, scientists saw an 84% decrease in the number of burrowing mayflies hatching.

This decline has an effect on other species like birds, fish and bats who eat those mayflies — an estimated 3,000 tons of food — as well as to the soil that the dead insects fertilize. Some birds even hatch their eggs to coincide with the mayflies, ensuring they have ample food for their hatchlings.

“Multiple stressors in these waterways attributed to human activity could be a reason for the reduction in mayfly populations,” according to a recently published study written by Virginia Tech researchers. “A warming climate puts more stress on certain aquatic environments, leading to decreased oxygen levels, which can result in fewer mayflies coming out of the water. Runoff from rivers into the warmer surface waters of Lake Erie, for instance, can cause algae blooms, which release toxins that these mayflies are especially susceptible to.”

Pesticides applied to crops, especially those containing chemicals similar to nicotine called neonicotinoids, also find their way into the water and can kill or hamper the development of mayflies.

Anglers should take note of the decline of these burrowing mayflies, even though far from their home waters, because of the implications. “It highlights the growing problem of insect decline and the cascading effects that has on ecosystems around the world,” according to the Virginia Tech researchers.

Ancient species

Fossils of mayflies have been found dating back more than 300 million years. They were apparently abundant about 250 million years ago.

In recent decades Montana has been playing catchup when it comes to documenting mayfly species in the state’s waters. Entomologist W.P. McCafferty of Purdue University noted in his research that Montana is the third largest state in size but has been slow to document its mayfly populations.

Thankfully, that’s slowly changing, with the count of mayfly species now up to 133. That tally has also been boosted by several state scientists.

In 2006, Stagliano and biologist Coburn Currier surveyed the Powder River in Eastern Montana and discovered three species highly specialized to survive in the shifting sands and gravel of prairie rivers.

In 2009, University of Montana scientist Robert Newell wrote a study that surveyed 355 waters in Glacier National Park in search of mayflies.

“Our survey was the first extensive survey of wetland invertebrates in Glacier NP and only the second that we are aware of in western Montana,” he wrote.

In 2007, McCafferty published details of a similar survey in Yellowstone National Park, the first such sampling there. The same year he also wrote about a similar analysis completed in western Montana that increased the number of known mayfly species from 49 to 87.

Such surveys are important, Newell wrote, because “changes to precipitation, snowmelt patterns, and seasonal temperatures” can affect the “characteristics of wetlands and other aquatic environments.”

By documenting mayfly populations in a quantifiable way, scientists can use them to identify environmental damage, Stagliano said. Unfortunately, such data wasn’t available in 2011 when a pipeline broke at Laurel spilling oil into the Yellowstone River, but researchers like Stagliano are now building up baseline data on streams across the state.

Fly fishing

Imitation mayfly nymphs and spinners are a common choice of fly anglers.

Researchers aren’t the only ones who track mayflies. Fly anglers closely follow the timing of mayfly hatches as a potential opportunity to catch fish, sometimes on a dry fly.

Depending on the region, two of the earliest mayfly hatches in Montana are the March brown and gray drake, which can show up in April and May. In higher elevation waters like those near Yellowstone National Park, the pale morning dun is typically the first mayfly to appear, usually in early June or July.

In late June and early July, burrowing mayflies hatch in the Seeley Lake and Swan Lake area of northwestern Montana. They are similar to the species found in Lake Erie.

The big bugs, almost 2 inches long, can bring large fish to the surface. The only problem is that the hatches occur in the evening, meaning anglers need a sixth sense or moonlight to catch fish in the dark.

Some anglers tie patterns with luminescent materials to help them see night strikes, said Walker Scarborough of the Missoulian Angler fly shop. Using a headlamp while fishing, he said, is not a good idea.

The end of the summer and transition to fall in Montana is marked by hatches of the blue-winged olive, a mayfly that prefers cloudy and cool days.

Abundant bug life, wild fish and beautiful waters haunt many fly anglers’ dreams, often providing comfort through the cold months of winter. Yet it is important to realize that insects like mayflies are about more than matching a hatch to catch fish. They are key indicators of the quality of our waters, the lifeblood of our planet.

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