Just when you thought you could find relief outdoors amid a global pandemic, swarms of aggressive black flies have emerged to ravage the Twin Cities.

“They’re just awful,” said Audra Jechorek, who was swarmed and bitten within minutes of stepping outside her Plymouth home, where, like most Minnesotans, she’s hunkered down during the state stay-at-home orders to fight COVID-19.

“I’m stuck at home, and now I’m stuck in my home,” she said Friday.

Jechorek said she’s had many black fly encounters since moving to Minnesota 30 years ago, but these particular black flies, also called biting gnats, are different, she said. They’re aggressive, draw blood and leave large, red welts like none she’s experienced.

Some neighborhood social media sites like hers are blowing up with laments about black fly injuries, including photos of big, red scabs and blood streaming down necks. People complain they can’t go for morning runs or afternoon walks without being attacked.

Complaints are also piling up at the Metropolitan Mosquito Control District, which typically receives 20 to 30 calls and e-mails a day, said district spokesman Alex Carlson. Since Wednesday morning, the district has received about triple the number of complaints.

The culprit is most likely a species of black fly called simulium tuberosum, one of about 30 black fly species in Minnesota, according to John Walz, a district black fly specialist. About three years ago, the species began showing up in larger numbers in the Twin Cities, he said.

“In 2018, it was bad,” he said. “2019 wasn’t so bad. And in 2020, they’re terrible.”

Walz has his theories about the explosion of complaints. Maybe it’s because people, now stuck at home, are traipsing outdoors more in the middle of the day when the flies are abundant. Or maybe the species has proliferated because waterways have gotten cleaner over the years, improving the black flies’ breeding grounds.

Or, maybe it’s because the district has done a good job tamping down the population of another aggressive black fly — the simulium venustum — leaving more breeding opportunities for the other fierce biter.

With complaints pouring in, Walz and his team are trekking to waterways to collect samples where gnats tend to breed. Their investigation could provide enough information to seek Department of Natural Resources permission to treat hot spots next year before the black flies emerge from the streams.

It’s too late to attack the population this year because they’ve already hatched and the females are spread out in search of blood to lay their eggs for next year’s invasion.

It’s easier to target mosquito populations, Walz said, because they’re weak fliers, sticking to within about a 2-mile radius. Gnats are stronger fliers, heartier, and can ride the winds.

“They can go miles and miles, so they get around pretty good,” Walz said. “They hang high in the tree canopy and watch for dark blobby things on the ground to go get a blood meal from.”

When they find a target, they descend in a swarm and bite. It’s not pretty.

Unlike a mosquito, which punctures the skin and sucks blood, the black fly slices into the skin to get a pool of blood that it can lap up, Walz said. Victims are sometimes surprised when blood trickles down their face or ugly red bumps appear.

Artificial scents and sometimes even a person’s natural chemistry can attract black flies. “So not everyone gets bit,” Walz said.

Jechorek isn’t one of the lucky ones. Her husband and 17-year-old son appear not to be as attractive to black flies as she is.

“I was out for two minutes and then had to sequester myself in my bathroom,” she said. The gnats had burrowed in her hair.

“I brushed and brushed my hair and there were about 10 of them flying above my head,” she said.

Back working in her home office, she later discovered one under her shirt. She squished it, but not before it left a bloody wound.

She’s followed the experts’ recommendations to cover herself in light-colored clothing and she’s tried numerous repellents to fend off the suckers — from eucalyptus oil to products with DEET.

“I’m literally a prisoner in my own home at this point,” Jechorek said. “They’re just so aggressive. It’s a nightmare.”

But relief may be in sight. If the experts are right about which species is causing the current havoc, they will soon die off because they produce only one generation.

“We should be seeing a lot fewer very soon,” Walz said. “We need to get to mosquito season so we can forget about the black flies.”