National Weather Service United States Department of Commerce
 
Welcome! This site is a living document of known emergences of the Mayfly along the stretches of the Upper Mississippi River Valley from Davenport, Iowa through St. Paul, Minnesota. An observational and scientific interdisciplinary group in the region, including the National Weather Service, the U.S. Fish and Wildlife Service, the U.S. Geological Survey, the Metropolitan Council, and the Wisconsin and Minnesota DNR, track the Mayfly activity and emergences. Many volunteer observers and photographers also contribute to the site!

 

If you want to become a mayfly observer, or learn more, go to the U.S. Fish and Wildlife site.

 

Bug these guys for more info: Dan Baumgardt (National Weather Service), Mark Steingraeber (U.S. Fish and Wildlife Service)

Last Update: July 24, 2014

The life cycle of a mayfly starts out as a larva that resides on river or lake bottoms.  After 3 months to 2 years, depending on the species, they emerge as winged adults and fly in swarms to mate. Once they emerge, they only live for a few minutes to a few days (again, species dependent). 

Along upper portions of the Mississippi River, one of the largest of the mayfly species (the Hexagenia genus) generally emerge as winged adults in late June to early July. 

The adults are often attracted by lights and can fly several miles inland, commonly creating a slippery mess as they pile up and die on the pavement below the lights. At times they have been so thick on bridges that they had to be cleared with snowplows.

Mayflies are sensitive to gross organic pollution and their presence is good news ecologically because it means that pollution such as sewage is not present in large amounts. 

mayfly
Mayfly

Doppler Radar Captures Massive Mayfly Emergence!
The Evening of June 23, 2012

On Saturday evening, June 23 2012, a massive mayfly emergence occurred along the Mississippi River beginning just after 9 pm. By late evening, mayflies were swarming in La Crosse, La Crescent, and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season, this particular event was one of the best seen on radar yet. In the radar time lapse loop from 9 pm to just after 1030 pm (below), the yellows and oranges indicate a large magnitude of airborne mayflies.

Also very evident was the northward track to the mayfly radar 'echo'. This movement was due to the south wind direction over the area at emergence time. The radar would indicate the bugs were carried north, off of the river, into Blair and Taylor Wisconsin. The radar beam over these locations are detecting mayflies at over 3000 feet above the ground! Their existence was confirmed on the ground north into Trempealeau county near Galesville.

Further south along the Mississippi River, the NWS radar in Davenport, Iowa detected the emergence as well. The mayflies were seen on radar along the river from north of Davenport to about Bellevue, Illinois. Because the beam rises above the ground as it travels further from the radar, the mayflies can only be detected closer to the radar. For example, the Davenport, Iowa radar beam is at about 5000 feet above ground by the time it reaches Dubuque. While observers did note that mayflies emerged in Dubuque Saturday evening, no radar echo was seen in the area - probably because they "flew under the radar". 

Radar Loop Radar Loop

 

Pictures From The Following Day

Picture #1

Picture #2

From the Stoddard-Ferryville, Wisconsin area

 

Picture #2

Picture #6 Picture #4

Picture #2

From the Prairie du Chien, Wisconsin area

 
July 3, 2012

Radar Loop

July 15, 2012

On the evening of June 15, 2012, a larger mayfly emergence took place from near Red Wing, Minnesota through the Twin Cities. Parts of the Minnesota River valley also produced mayflies. This one caused problems in Hastings with plowable mayfly depths!

Radar Loop

July 28, 2012

Radar Loop

August 11, 2012

 

Radar Loop

June 15,16, 2013

On the evening of June 15-16, 2013, mayfly activity was noted in the area. On radar, the Black River, north of La Crosse, was noticeably active with biological targets. The radar loops are from 9-1030 pm each evening. (h.limbata)

June 15, 2013

June 16, 2013

Radar Loop Radar Loop

 

 

June 18, 2013

A "larger" detection was seen the evening of June 18 2013 along the Mississippi River in southwest-western Wisconsin. The interesting observation from radar was the length of time the biological echo was seen along the river (7.5 hours!) and the higher radar return (~35 dBZ). Most river biological echoes seem to last about 2 hours. This loop is from 9pm June 18 - 430am June 19. Ground truth was seen in downtown La Crosse during this June 15-18 period in the form of h.limbata mayflies.

 

Radar Loop

 

June 25,26 2013

Surface observers on June 25 reported a "moderate" emergence on the Mississippi River, pool 16, near Davenport, Iowa. Radar data seemed a bit more active further north up the Mississippi River, and relatively light near the radar along the river. There is much to see in these loops, including showers and thunderstorms to the north-northwest. Concentrate on the river itself to see the biological echo. These loops are from 9-11 pm each night.

June 25, 2013

June 26, 2013

Radar Loop Radar Loop

July 14,2013

A warm, muggy evening prevailed as a noticeable mayfly emergence appeared on the La Crosse, WI radar. This radar loop goes from around 9 until 11 pm. It suggests that the emergence was intially higher south of La Crosse and then spread northward, as the prevailing wind flow was out of the south and east. The image on the right shows a cluster of mayflies as captured by WKBT Meteorologist Bill Graul.

Radar Loop

 

WKBT Mayfly

 
Another Massive Mayfly Emergence!
The Evening of July 20, 2014

 

On an evening very similar to the massive mayfly event of June 23 2012, the Mississippi River produced a massive radar echo as mayflies emerged from the water and became airborne. The mayflies were detectable on radar around 845 pm and reports in the towns and cities began rolling in of the swarming and piles of mayflies. Numerous videos and pictures were circulating on social media, some of which are posted below as well.

The radar detected the flies about 845 pm, emanating from the river (the source) with echo values similar to that of light-moderate rain (35-40 dBZ). With a general south-to-north wind flow above the surface, the mayflies quickly moved north once in the air. As the flies dispersed moving north-northeast, they also gained altitude with some of the echo being detected as far north as Black River Falls and as high as 2500 feet above ground.

By late evening, mayflies were swarming in La Crosse, La Crescent, Stoddard and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season depending on the species, this particular emergence was that of the larger black/brown Bilineata species. The radar loop below shows the reflected radar energy (reflectivity) from 835 pm to just after midnight. The higher the values (greens to yellows) indicate greater concentrations of flies. Note how the swarm is carried northward over time. 

 

Radar Loop

 

Radar Images at 954 pm July 24, 2014

Radar 1 Radar 2

Radar 3

Black background Terrain background Mississippi River shown with 
black arrows. States shown.

 

Pictures from the La Crosse, La Crescent, to Holmen Area 

Picture #1   Picture #2 Picture #2
Credit: Tim Halbach   Credit: Ryan Hollis Credit: WKBT News 8
Picture #4   Picture #5 Picture #6
 Credit: Tim Halbach   Credit: Tim Halbach   Credit: Elle Marie
Picture #15   Picture #16 Picture #17
Credit: WKBT News 8   Credit: WKBT News 8 Credit: WKBT News 8
Picture #10   Picture #13 Picture #20
Credit: WKBT News 8   Credit: Tory Stoffregen Credit: WKBT News 8
       
Pictures from the Minnesota City and Trempealeau Area
Picture #10   Picture #11 Picture #12

Credit: Twitter
@paulmhuddleston

  Credit: Joshua Hanson Credit: Eric Hunter
Picture #13 Picture #15
Credit: Kelly Gardner Credit: Joey Hulett

Minor Mayfly Emergence
The Evening of July 24, 2014

Four nights after the massive emergence of mayflies along the Mississippi River near La Crosse, a minor mayfly emergence was seen on radar. This was verified by a ground report near Onalaska of many seen flying the following morning (Friday). The radar loop below is from 845pm to 1111pm July 24, 2014.

 

Radar Loop

Large Mayfly Emergence
The Evening of July 4, 2015

Mayflies celebrated the 4th of July holiday with their own brand of fireworks - ala a big emergence. Those down at Riverside park in La Crosse had to contend with the mayflies just as the fireworks were starting. Any lit surface quickly became a landing area for the mayflies.

 

 

 

On 3rd St, south of downtown La Crosse, WI.
(Courtesy Dan Baumgardt)

Free mayflies with every fill-up! Kwik Trip southside La Crosse, WI.
(Courtesy Dan Baumgardt)

Mayfly blizzard? Dairy Queen southside La Crosse, WI.
(Courtesy Dan Baumgardt)

Aftermath: Riverside Park, La Crosse, WI, July 5 2015.(Courtesy John Sullivan)

 

   
  Dairy Queen southside La Crosse, WI.
(Courtesy Dan Baumgardt)
Aftermath: Riverside Park, La Crosse, WI, July 5 2015.
(Courtesy John Sullivan)

Light Mayfly Emergence
The Evening of July 23, 2015

 

 

Minor Mayfly Emergence
The Evening of July 8, 2016

On the evening of July 8, 2016, a minor mayfly emergence was noted on radar just after 9 pm. The mayfly swarming lasted through about 10:30 pm on radar.

 

 

Light Mayfly Emergence
The Evening of July 26, 2016

On the evening of July 26, 2016, a light mayfly emergence was noted on radar just after 9 pm. The mayfly swarming lasted through about 10:30 pm on radar.

 

 

At the NWS Office just east of La Crosse.
(Courtesy Dan Baumgardt)

Light to Moderate Mayfly Emergence
The Evening of July 27, 2016

On the evening of July 27, 2016, a light mayfly emergence was noted on radar around 9 pm. Something notable with this emergence was an outflow boundary from earlier storms that forced a portion of the swarm to move south. An outflow boundary is the leading edge of a cool pool of air ejected from a storm. This is annotated on the second radar animation below. The radar was able to detect swarms as far north as Wabasha, MN to as far south as Lynxville, WI. Swarms were also seen in Lake City, MN, though it is difficult to determine if the radar returns in that area were due to the mayflies or showers in the area. The mayfly emergence in Lake City was more of a moderate to heavy emergence on the Johnson Mayfly Emergence Scale.

Radar loop from approximately 8:45 pm to 10:45 pm. Outflow boundary forcing mayfly swarm south.
 

 

  Mayfly swarms in Lake City, MN
(All photos courtesy of Manda Hart Baldwin)
 

Light Mayfly Emergence
The Evening of July 30, 2016

On the evening of July 30, 2016, a light mayfly emergence was noted on radar beginning just before 9 pm near Stoddard, WI. Swarming was detected from Fountain City, WI, to Lynxville, WI. The mayfly swarming lasted through about 10:30 pm on radar.