Mississippi Mayfly : Bugs Bode Revival of a Great River

Times Staff Writer


Bugs on the sidewalks. Bugs on the street. Bugs on the lamp posts, the bridges, the boats, the cars. The landscape wrapped in a blanket of squishy, buggy fur.

Piles of bugs. Sometimes it seems like miles of bugs. Bugs on your toes and your nose. Bugs on your thighs. Here’s bugs in your eyes. Bugs in your mouth, too, if you are not careful. Oops! Crunch.

The mayflies are back, and for many folks along the Upper Mississippi River, there is no escape. “It’s like something out of Alfred Hitchcock’s ‘The Birds,’ ” said Sally Sullivan, who has seen her riverfront restaurant here engulfed in a thick blizzard of the pests. A few weeks ago, she had to wade through a foot-deep pile of dead, stinky mayfly carcasses just to reach the front door.


Swarms a Common Sight

Biblical-sized swarms of the insects are as common a summer sight as barges and steamboats along much of these busy waters. Lately, they have descended on towns up and down the river with a vengeance and in numbers not seen in years.

Several communities have had to call out snow crews this summer to plow infested bridges as cars skidded and slid on slicks of greasy bug glop. In late June, a squadron of mating mayflies coated a bridge in South St. Paul, causing an eight-car pileup and a two-hour shutdown of Interstate 494 while crews scraped and sanded the pavement.

Despite the annoyances, however, many river watchers can barely contain their glee over the upsurge in mayfly activity.

“It means we’re cleaning up our act,” said Calvin Fremling, a biologist at Winona State University in Minnesota who specializes in the study of Mississippi aquatic life. “The river is getting cleaner.”

Ecologists see the mayfly resurgence as the surest sign yet that pollution controls are transforming hard-hit sections of the upper Mississippi from what had become little more than a sewer back into a thriving river.

River of Effluent

The June bridge infestation occurred along a lengthy stretch below Minneapolis and St. Paul, where, not too many years ago, the water bubbled from a witches’ brew of noxious gases, chemicals and effluent dumped from the Twin Cities. The mayflies began reappearing in the area three years ago, after a 30-year absence from some places.


This summer, the Interior Department’s Fish and Wildlife Service has turned to mayflies to gauge the river’s health. Using a program designed by Fremling, the agency has enlisted volunteers to scoop samples of dead mayflies from the piles they characteristically die in for hundreds of miles along the river banks. The tiny corpses will be autopsied to check levels of cancer-causing polychlorinated biphenyls and other chemicals and pollutants they have ingested. Similar tests in subsequent years will provide an annual report card on the river’s recovery.

Scientists find the mayfly a particularly handy little pest for such a project. In a yearlong nymphal stage, they suck in contaminants that accumulate in the burrows they dig in the river muck. Then, in synchronized waves spaced six to 11 days apart through the summer, they leap from the water for a frenzied, one-day mating orgy that leaves them lying dead in heaps and ready to be scooped up for lab analysis.

“The flies come out and provide us with a banquet,” said one fisheries analyst. “It’s almost like the organism is doubling over backwards to come and help us.”

Bug With Many Names

Black, with tender, translucent wings, the inch-long Hexagenia bilineata or mayfly (alias june bug, shad fly, Canadian soldier, American soldier, Green Bay fly, sand bar fly, spinner or, as grizzled towboat captains usually refer to them, “those big black bastards”) are, by themselves, quite unthreatening.

The nymphs convert algae and organic debris into some of the yummiest and most nutritious fish food in the deep. Once out of the water they molt, like caterpillars turning into butterflies, and fly off to hover around some nearby tree or bush. They do not bite or sting, in fact cannot even eat because they have no functional mouth parts. About all they do in their 24-hour lives on land is mate and die. But what a way to go.

Just before dusk, huge mating swarms form. Their duty done, the males drop dead. The females take off by the millions, sometimes billions, to dump their eggs in the river. Often buffeted off course by the wind, they plaster themselves to whatever gets in their way--boats, bridges, cars, people. “They’re all over you when you’re out there locking a boat,” complained Ed Helmueller, lock master at Lock and Dam No. 5 north of Winona. “They’re in your mouth. I used to smoke a pipe and they’d go in there.”


Like many a bug, the mayfly is attracted to white light. That makes cars with headlamps, boats with navigational lights, street lights and neon signs favorite targets. As a result, many river towns now have switched to amber, sodium-vapor street lights. Others pull the plug on municipal lights entirely when a mayfly attack is in the wind, in the hope that the bugs will pass them by and go bother someplace else.

Many a night Little League or softball game has been called on account of the mayflies. Swarms frequently darken riverfront carnivals and festivals. Dog races at the track down in Dubuque, Iowa, have been called off. Early this month, a country music concert on the waterfront in LaCrosse, Wis., was performed in the dark after the mayflies hit.

“It’s just like a snowstorm, but instead of snow coming out of the sky these bugs come out of the sky,” said Milt Yeiter, street and bridge superintendent for Burlington, Iowa

In the last few weeks, Burlington has suffered two major mayfly infestations. One forced a shutdown of the McArther Bridge over the Mississippi to Illinois. “We had them at least 6 inches thick under the lights and on the pavement we’ve had probably an inch, inch and a half, Yeiter said. “But after the cars run over them it gets greased. . . . It becomes so slippery it’s like riding on ice.”

Mayflies are not unique to this region, but thrived in unusually vast numbers after Army engineers built 27 locks and dams to aid navigation from the Twin Cities down to St. Louis. The dams created silty pools that provided the perfect growing environment for mayfly nymphs.

Once Bred in Lakes

Not too many decades ago, Lake Erie, Green Bay in Lake Michigan and parts of the Illinois River also had abundant mayfly populations. Then, accumulated filth choked oxygen from the water bottoms and the mayflies began to disappear from many places. Much--but not all--of the fast-flowing upper Mississippi stayed just clean enough to keep the mayflies hatching.


The biggest threat came from the Twin Cities, where poorly treated sewage and industrial wastes were belched into the river in huge quantities. A $350-million upgrading of the main Twin Cities waste water plant, which treats half of all industrial and residential effluent in Minnesota--has significantly decreased the dumping of pollutants into the river. The Metropolitan Waste Control Commission says that lead in treated water has been cut 77%, chromium is down 78%, cyanide down 50% and raw sewage overflows have also been trimmed back considerably.

Officials say it is still not safe to drink untreated Mississippi River water, but the mayflies seem to be taking to it just fine. Towns safe from mayflies since the 1950s have in the last few years been hit. “We do feel good about it,” said MWCC chief administrator Lou Breimhurst.

Sign of Cleaner Water

“It tells that we have done a good job improving the water quality of the Mississippi River. We would hope people put up with a minor inconvenience for the improved water quality.”

The mayfly is a primary link in the underwater food chain, so biologists think its revival should presage the return of many long-scarce game fish species.

Howard Krosch, supervisor of fish and wildlife monitoring at the Minnesota Department of Natural Resources, said that only scavenger species such as carp and gizzard shad could be found during the 1960s in a heavily polluted 30-mile stretch of the river, from St. Paul to Hastings, Minn. Now, he said, the same section teems with walleyed pike, sauger, smallmouth bass and northern pike. “Mayflies are valuable for fish food,” he said. “With the increased amount of food available and the general improvement in water quality, the fish populations have rebounded real fast.”

The fisherman’s delight, however, is a navigator’s plight. “They cause the ship captains the most grief,” said Winona State’s Fremling.


“They blot out their radars, their ship lights and they make decks real slippery. Once I was on a towboat and we plotted a swarm of females coming up the river by the billions. They stayed with the boat all night.” Federal safety rules require ship captains, as well as lock masters, to keep navigational lights on even in the face of a massive bug assault.

The same restrictions do not apply down at Sullivan’s restaurant, however.

“If we see those things coming, we all yell, ‘Fish flies!’ and everybody runs to turn off the lights,” said Sally Sullivan. “We turn off all our security lights. We don’t even open our back security door. We batten down the hatches.”

Times researcher Wendy Leopold also contributed to this story.