Peak season: By fall, yellow jacket colonies' populations have peaked, swelling to more than 10,000 in one nest that can be as big as a basketball.
An aggressive time: As cooler weather sets in, their food sources have become scarce and they scavenge more aggressively.
In defense mode: The wasps are also fiercely defending new reproductive queens and males as they develop.
Indian summer: The colonies will not die out until the first deep freeze, and the Indian summer in Chattanooga has only made them more active.
Sources: Tom Stebbins, agent with Hamilton County's University of Tennessee Agricultural Extension Service; University of Idaho Extension
The runners knew something was amiss about halfway through the race, when they heard shrieks and yells farther up the trail.
Kiersten Vradenburgh was feeling good about her pace during Saturday's StumpJump on Signal Mountain -- until she saw a woman sprinting in the opposite direction.
"She's yelling and she's ripping her shirt off," said Vradenburgh. "And I run up to her and ask, 'Are you OK?'"
That's when she and others spotted the spinning swarm of yellow jackets.
Vradenburgh and the runners dove into the thorny thickets along the trail to escape the swarm. But the wasps remained in hot pursuit, latching onto clothes and skin.
Runners expect to do battle against the elements during the StumpJump, a 50-kilometer and 11-mile mountain trail race.
But this year, yellow jackets were all the buzz.
"Of all the people I've talked to it seemed like everyone in the race got stung," said Randy Whorton, director of Wild Trails, which directs the StumpJump. "And it's the same thing at trail races all over the South."
Wasps are par for the course on regional trail runs, he said. But not like Saturday.
His group has discovered and killed nest after nest on trails this summer, but runners had to dodge three surprise nests on Saturday.
On the StumpJump Facebook page, the moderator posted a photo of a yellow jacket Monday and wrote, "The game today is how many stings did you get?"
More than 70 runners responded, with answers like: "Stopped counting at 12," and: "Eight stings. But hey, the pain distracted me from my aching body."
The devilish critters haven't targeted only runners this fall. They fought both Confederate and Union soldiers at the Battle of Chickamauga reenactment last month.
Walker County Fire Chief Randy Camp said firefighters sprayed more than a dozen nests during the reenactment that concluded the weekend of Sept. 21-22.
"We were expecting it to a degree, since everyone was out in a big field and the woods," Camp said. "But we kept having to run out with that spray, again and again."
Local exterminators say wasps are far worse this year than in seasons past. Ola Phipps, owner of Chattanooga-based Lady Bug Exterminating Co., said her company gets two to three calls about yellow jackets each day.
That compares to one or two per month last summer.
"It's more than we've ever had in the past, and other exterminators are reporting the same thing," said Phipps, who has owned the company for 31 years.
The wasps aren't the only pest the company has done battle with this summer, she said. Spiders have been "tremendously bad," she said, as have stinkbugs.
Tom Stebbins, an agent with Hamilton County's University of Tennessee Agricultural Extension Service, said the rainy weather earlier this summer is chiefly to blame, with pest populations thriving in thicker underbrush and standing water.
Early autumn is prime yellow jacket stinging season, he said.
"Weeds are high, nests are big, people are starting to do their fall cleanup," he said.
Yellow jackets typically nest in the ground. Since they are communal, they will attack as a group when they perceive a threat to their nest -- like scores of pounding feet.
Several StumpJump runners had to walk or drop out of the race because of the stings, like Vradenburgh -- who had to start walking when her breathing grew more labored and her throat tightened.
Veteran StumpJump runner Michael Green said the yellow jackets were "the most talked-about thing of the race."
He was stung close to a dozen times when swarmed by about 20 wasps.
"I had on these four-inch black socks and there were 10 stuck right to them," he said Monday. "I'm still itching."
Whorton said no one he has talked to needed serious medical treatment because of the stings, and he doesn't think it will hurt next year's race. Stump Jump runners have pretty thick skins.
One woman was stung 20 times and "was really cool about it," he said. "Ninety percent of our runners would say the stings weren't that big a deal," he said.
Vradenburgh already is itching to defeat the yellow jackets in next year's race. She just plans to carry some medicine with her.
Contact staff writer Kate Harrison at kharrison @timesfreepress.com or 423-757-6673.