In the midst of high waters over the 4th of July along with near record-breaking amounts of rain, one might be forgiven for not missing a usual summertime visitor. Normally early in July the city of Hastings has to work to accommodate our newfound residents, at least during the few hours they are alive.
Mayflies are a hallmark of the Mississippi river town experience. They blacken the skies for a few weeks every summer and then blacken the waters afterward. While mayflies are certainly around this July, they haven’t arrived in the near-apocalyptic numbers that they usually do.
So where then, are the mayflies?

The basics
Mayflies, or shadflies, are a species of insect in the order Ephemeroptera, near relatives of dragonflies and damselflies. The term Ephemeroptera aptly comes from the Greek root ephemeros meaning “daily,” or “for the day,” which is the same root for the English work Ephemeral. While mayflies can actually live for years in their nymph stage, they are mostly known for their tiny adult lifespan which can last as short as a few minutes.
The majority of mayflies’ lives are as nymphs. Nymphs are gilled aquatic larvae with six legs and a stoat abdomen. Nymphs can live for years, molting several times before they become the adult mayflies that are widely recognized.
Nymphs molt two last times before fully developing. Once to climb out of the water and a final time to fully take flight as an adult. Adult, or imago mayflies, live anywhere from a few hours to a few days, up to a month, though generally much shorter than that. Adult mayflies do not eat because they cannot eat. They lack both a functioning digestive tract and mouth. Their main purpose in their ephemeral adult life is reproduction.
Mayflies reproduce while flying, with females flying through swarms of males, then returning to the surface of the water to lay eggs. Female mayflies can lay anywhere from hundreds to thousands of eggs which often hatch all at once producing sky-darkening effect Hastings is long used to. These swarms are so large and dense they can be tracked by weather radar and are catalogued by the National Weather Service.
While mayflies and the order Ephemeroptera aren’t the only species of insect with short adult lifespans, they are “on the radical end of that scale,” said Director of University of Wisconsin Insect Diagnostic Lab Patrick Liesch.
Mayflies are a key example of a species that is “r selected,” according to Liesch. The term r selected refers to organisms that have high reproductive capability but short lifespans. Many insects and even mammals like mice showcase this style of reproduction.
“Their strategy which has been successful for hundreds of thousands of years is to not live for too long,” said Liesch.
On the other hand is “K selected” species that have relatively low reproductive capability but longer lifespans. Humans, whales, and elephants are all examples of K selected organisms.

The hatch
Due to how mayflies reproduce aerially in large swarms, hatches of eggs are meant to occur at one time to produce the up to millions of mayflies that would compose a swarm. Factors that affect when a hatch occurs include water temperature, air temperature, silt quantity, and water acidity among many others.
Mayflies have evolved to all be triggered by these environmental variables at the same time so that they reach adulthood concurrently and are therefore able to reproduce.
One clue to as to why mayflies haven’t yet hatched or been as numerous as one might expect might be due to the unusually high amount of rain. Rain, especially in the amounts that caused the flooding seen throughout Minnesota earlier in July, deeply affect many of the variables for when a hatch can occur.
“Anytime you have a situation where environmental variables change, that can have cascading effects,” which could have impacts on the size and timing of a mayfly hatch, said Liesch.

Mayflies and pollution
Mayflies are incredibly sensitive to pollutants and are a great telltale for water quality. Due to the dumping of raw sewage into the Mississippi up until the early 1980s, large hatches did not occur.
“The primary cause of the collapse of mayfly populations during the late 1960’s, 1970’s and early 1980’s was sewage. Little or no treatment of sewage was occurring during this time. We were literally flushing our toilets into the river in many parts of the state,” according to the Department of Natural Resources (DNR).
The sewage itself was not what was killing the mayflies, however. As sewage was broken down, large quantities of dissolved oxygen in the water was used up by bacteria, leaving little for insects and fish. This process is similar to algal blooms from runoff nitrates that still occur in lakes and rivers and even in the Gulf of Mexico to this day.
“Today because of modern sewage treatment facilities and regulations on the disposal of toxic chemicals, mayflies have returned to most of our waterways,” according to the DNR.

Hastings Mayfly Plan
Every year Hastings Public Works watches and plans for large hatches. While there is not an exact science to predicting a hatch, the public works watch the hydro plant every day to check for early members of a hatch. If mayflies start appearing there, a large hatch might be just about to occur.
“We don’t try to get too exact,” said Hastings Public Works Director Ryan Stempski.
Normally early into June, the city of Hastings shuts off lights downtown and along the river to stop mayflies from swarming the lights and dying in piles and puddles on streets and sidewalks beneath them. “As summer gets hot and sticky, we make the call to shut off the lights because we can never guess the hatch,” said Stempski.
According to Stempski, amidst the high waters, an initial hatch occurred on July 11-12, although this hatch wasn’t large enough to be recorded by the National Weather Service. Fortunately, the lights were already turned off, and the hatch wasn’t a particularly large one. This year, “hatches look to be coming in waves,” said Stempski.
Depending on the size of the hatch, Hastings’ cleanup process can be anything from blowers to street sweepers and water trucks. Particularly immense hatches are treated by the city “as a snow event,” with snowplows brought out to push the tens of millions of dead mayflies off of roads so cars can get through.
For more information on mayflies, visit the DNR website at https://www.dnr.state.mn.us/minnaqua/speciesprofile/mayfly.html