Why you should never smash lake flies and other facts about the annual Lake Winnebago insect hatch

Editor's note: This story originally was published in 2020. 

It's that time of year again.

Lake flies are swarming neighborhoods along Lake Winnebago shorelines, making outdoor surfaces look fuzzy as they cling to the sides of houses, cars and telephone polls, forcing people to lock themselves inside while the flies buzz overhead by the thousands.

While the flies may be an annoyance to those who live along the lakeshore, they're harmless to humans and play an important role in the Lake Winnebago ecosystem, Wisconsin Department of Natural Resources water management specialist Scott Koehnke said.

"They are definitely a nuisance and I get the calls every year about 'how do we get rid of them?' And there isn't a way that we could, let alone would want to get rid of them," Koehnke said. "They're here to stay and most of the people they know it's coming. They're ready for it."

RELATED: Lake flies inundate neighborhoods near the shores of Lake Winnebago, drive some residents away 

They are extremely difficult to clean up after

Squishing a lake fly is not a good idea unless you're ready to bring out the power washer to clean up the mess.

Killing a lake fly is not like killing a mosquito or another bug. They're not very tough, Koehnke said, so if you try to squash one or whack it away, a greenish brown blob of lake fly guts will squish out.

"It's otherworldly," Koehnke said of squishing a lake fly. "It is the worst thing you could possibly come up with. I don't know exactly what it is, but that's the biggest complaint, obviously other than the smell."

Koehnke often hears complaints from people that the flies smell like the bottom of Lake Winnebago — which makes sense, because that's where they live, he said.

People often have to use power washers to get the stains of dead lake flies off their houses, cars and other outdoor property, he said.

There's nothing people can really do to get the flies to stay away from their homes.

"You just deal with them for the week or two, the 10 days that they're there and then clean up after they're gone," Koehnke said.

Some people are allergic to lake flies, Koehnke said, but they are not harmful to humans because they can't bite or transmit diseases.

They typically hatch around Mother's Day

The lake flies are already out and about this year, and began hatching on May 5, Koehnke said.

The flies usually hatch sometime around Mother's Day, depending on the weather conditions.

That's the major hatch, according to Koehnke.

There's a second hatch that takes place in late July and August. The flies that emerge later in the summer are likely a different subspecies of lake flies, he said.

Wind patterns determine how bad the outbreak is in a particular location

Despite having the word "fly" in their name, lake flies are not actually very good at flying.

"They're very weak fliers and wind direction and wind intensity when they emerge is really critical for where you will see them," Koehnke said.

The worst swarms of the flies are typically found in High Cliff State Park and Menasha due to the prevailing south and southwest winds blowing the flies across Lake Winnebago, he said.

Koehnke said he went out Tuesday night and found a big emergence of the flies just south of Neenah, by Rainbow Beach and Fresh Air Park. He didn't observe many flies in the Menasha area at Jefferson Park or in the city of Neenah. Oshkosh hasn't seen many of the flies yet either, Koehnke said.

They play an essential role in the lake ecosystem

Lake flies live on Lake Winnebago's mucky bottom. Like their homes, they're also at the bottom of the food chain, Koehnke said. As the primary source of food for sturgeon and other fish, bats, birds and other animals in the lake's ecosystem, around 95% of lake flies end up getting eaten at some point in their life cycle.

A University of Wisconsin Oshkosh professor once did a study and found that around 60% of the lake sturgeon's diet is made up of lake flies, Koehnke said. Without lake flies, Lake Winnebago would not have the thriving sturgeon population — or sturgeon spearing tradition — that the lake is known for.

In the 50s and 60s, Lake Winnebago had twice the number of lake flies as it does today. That's because there were fewer adult sturgeon in the lake at that time, Koehnke said. Once the DNR started managing the sturgeon population, the number of sturgeon increased and the amount of lake flies decreased.

"You're not going to find a lake sturgeon population like Lake Winnebago anywhere on the earth and without the lake flies that we have here, that population wouldn't exist," Koehnke said.

Their life is short

Depending on moisture and temperature, lake flies can live between seven and 21 days. Their short lifespan is due to the fact that they don't have mouths and can't eat.

"Their only goal as adults is to reproduce and lay eggs," Koehnke said.

Lake flies mate when a female flies through a swarm of male flies, usually around dusk when they are most active, Koehnke said. After mating, the female will fly out to the lake and deposit her eggs. Female lake flies can lay between 5,000 and 10,000 eggs, but most do not survive. Then, eight to 20 days later, the eggs will hatch.

Female lake flies die after laying eggs and the males can mate multiple times before they die, Koehnke said.

Contact Natalie Brophy at (715) 216-5452 or nbrophy@gannett.com. Follow her on Twitter @brophy_natalie. 

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This article originally appeared on Appleton Post-Crescent: Lake Winnebago flies: Things to know about the annual hatch