Willow Flies on the Tennessee River

August 26, 2021  |  Permalink

I opened my eyes at first light and could see the sky tinged with pink clouds through the tent screen. I sat up, got dressed, and unzipped the tent to step outside into the quiet, still August morning. The Tennessee River stretched before me as my feet crunched along the shoreline. Suddenly, it wasn’t just the rosy-golden sunrise that took my breath away. I was at once aware of the drifting clouds of mayflies, numbering in the tens of thousands, silently silhouetted against the morning sky.

It just so happened that our night’s stay at the Nathan Bedford State Park Lakefront campground coincided with a huge mayfly hatch.

Locals call them “willow flies,” and their massive hatches along big rivers are well known to fishermen who often take advantage of the ensuing fish-feeding frenzy. They have multiple hatches throughout the summer months, sometimes emerging in such numbers that they cloak vehicles and buildings, even to the point of causing closures and accidents. Large emergences can even be detected on weather radar!

Mayflies are the very definition of ephemeral, completing their life cycle in the space of about 24 hours. As a matter of fact, the scientific order to which they belong is known as Ephemeroptera. These insects spend the vast majority of their lives in an underwater world, breathing with gills and feeding on microscopic organisms.  While typically associated with clear, fast-flowing streams, there are species that like the muddy, slower-moving waters of west Tennessee; in particular, those in the genus Hexagenia, also known as the Burrowing Mayflies.

Their larvae, known as naiads, construct a u-shaped burrow into the muddy river bottom, harnessing the river current and funneling food through their abode. It takes 11 months for them to mature, and when they are ready, the naiads swim to the water’s surface, split their skin and fly upwards.

They have miraculously metamorphosed from a mini water monster to a graceful, flighted insect, now taking to the night sky and lighting on the nearest tree among millions of other Mayflies.

The first beast to emerge is known as a subimago (or “dun”). After about 8 hours they molt one last time into their sexually mature phase, or imago (aka “spinner”).

What we were witnessing in the early morning hours was the mass emergence of the subimago stage. After filling the skies for about a half-hour, they all came to light on the trees, the branches bending under their weight.

They rested there until the late afternoon when we began to notice a strange precipitation falling on our campsite. Drifting on the breeze were thousands of shed skins from the subimago mayflies.

They covered everything, including ourselves (in fact, we are still finding them in our gear a week later!). The willow flies had now completed their last molt, and as the sun began to set, they took flight once again, this time to fulfill their one purpose – passing the torch to the next generation. After mating, their spent corpses came to rest on the shore and the water’s surface, where hungry predators eagerly gobbled them up. We observed turtles, dragonflies, fish, ants, birds, and even daddy-long-legs partaking in the protein bounty. Late summer is a lean time for many creatures, and this easy food source is manna from heaven.

By sunrise the next morning, all activity had ceased, and except for the millions of carcasses lining the shore, it was as if the phenomenon had never happened.

I realize that many people do not like the thought of experiencing a mass hatch of insects that cover them and everything else around them. I suspect this would be the stuff of nightmares for a select few individuals!

 However, once you get past the “gross” factor, insects are fascinating and beautiful wildlife that form the cornerstone of our ecosystem. These ancient cycles of emergence are incredibly important for local wildlife that depend on them for food. It is crucial that we monitor pollution and other human activities that could have a negative impact on our precious waterways, so that our grandchildren get to experience the magic of the willow flies.

About the author

Holly Taylor got her start with Tennessee State Parks as a Seasonal Interpretive Ranger in 2006, working six years at Edgar Evins State Park before working as a wildlife presenter for the Natural History Education Company of the Mid-South. She took time off to stay at home with her son before returning to TN State Parks as the assistant to State Naturalist Randy Hedgepath in 2018. She graduated from TTU in 2009 with a B.S. in Conservation Biology and also serves as the chapter coordinator for the Cumberland Mountain Chapter of the Tennessee Naturalist Program.