Spread of the Joro: Non-native spider may not be cause for concern
An adult female Joro shares a web with the much smaller adult male. Carly Mirabile/University of Georgia/Bugwood.org photo
The sun was still high in the sky on a perfect October day last fall when I finished setting up my campsite in the Chattahoochee National Forest outside Helen, Georgia. Wandering through the woods to explore my new surroundings, I came to a sudden halt at the sight of an enormous spider, perched in the center of a giant web stretched across my path.
Roughly three inches across, its legs were banded yellow and black, those same colors intertwining in an intricate pattern atop its bulbous body. A swatch of bright red splashed across it. I’d never seen anything like it before.
Later, I would discover that I’d found my first Joro spider.
March of the joro
Native to Asia, the Joro spider was first spotted in the United States in 2013, between Athens and Atlanta in northeastern Georgia. Researchers at the University of Georgia investigated those sightings and found that they’d occurred very close to interstate highways.
“We sort of inferred that these spiders hitched a ride on a shipping container, or something to that effect,” said Andy Davis, assistant research scientist at University of Georgia’s Odum School of Ecology.
Since then, the spiders have become a common sight in northern Georgia, with some yards, especially in the Atlanta area, housing hundreds of the creatures. Eventually, Joro spiders will probably inhabit most of the country, said Davis.
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“They’re great hitchhikers, really,” he said. “They’ll get on your car, in your wheel well. I’ve had one on my driver’s side, rearview mirror.”
Davis attributes the spider’s hitchhiking capabilities to sightings recorded in places far away from northern Georgia — Tulsa, Oklahoma; Baltimore, Maryland; Chapel Hill, North Carolina.
The spiders can travel significant distances even without help from people. When Joro eggs hatch in the spring, the young spiders use their silks to ride the wind to someplace new, traveling much farther than they could using their legs alone — a behavior called “ballooning.” Users of the species observation app iNaturalist have logged Joro spider observations throughout the portion of Georgia north of Macon, stretching east to Greenville, South Carolina, with a smattering of observations in the Chattanooga, Tennessee, area too.
Last fall, Western North Carolina saw its first documented Joro spider observations. On Sept. 18, 2021, iNaturalist user flathat_wanderer found one at Gorges State Park in Transylvania County and a week later on Sept. 26, 2021, a Joro spider was found at A-B Tech in Asheville. A few days later, there was a sighting at Chimney Rock State Park, and a Joro spider was spotted along an old logging road in the Tuckasegee area of Jackson County on Oct. 12, 2021. So far this fall, there have been sightings in Macon County and downtown Weaverville.
“Whenever you see a sighting on iNaturalist, usually that means that there’s probably 100 more that haven’t been seen,” said Davis. “So it’s very likely that there’s populations being established right now, as we speak, in North Carolina.”
Continued spread likely
Recent research suggests that the spider could ultimately thrive well beyond the confines of the Southeastern U.S.
The Joro spider is not the only member of its genus to find a home in North America. The closely related golden silk spider, also from Asia, has been present in the Southeast for 160 years, integrating itself to the ecology such that most people don’t even realize it’s not native. However, cold weather has always precluded the golden silk spider, which hails from the tropics, from spreading beyond the Southeast.
A paper that Davis and undergraduate researcher Benjamin Frick published this February in the Royal Entomological Society’s research journal Physiological Entomology suggests that won’t stop the Joro.
By measuring the physiological traits of captured female spiders, which are much larger and showier than the males, Davis and Frick found that the Joro spider has a shorter reproductive season than the golden silk spider, meaning that it can complete its lifecycle in regions where the summer season is short. Its metabolism is double that of the golden silk spider and its heart rate is 77% higher when exposed to low temperatures. In a brief freeze, 74% of Joro spiders survive compared to 50% of golden silk spiders.
“These findings suggest the Joro spider can exist in a colder climatic region than the southeastern USA,” the paper reads.
“This species is going to spread through most of the country because its physiology seems perfectly suited to live here, and also because it's a really good hitchhiker,” said Davis. “I mean, there's no reason that it's not going to spread.”
The question is what that spread will mean for the rest of the organisms that call North America home. At this point, Americans are used to hearing dire warnings about new invasives threatening to destroy the balance of life as we know it. The hemlock wooly adelgid is killing the once-mighty hemlocks, the fungus behind white-nose syndrome is decimating bat populations, and Burmese pythons are destroying wildlife in the Florida Everglades.
“That’s the million-dollar question, is what is this thing going to do to our native fauna,” he said. “The short answer is, we don’t really know yet.”
Researchers are studying that question, but it will take years to find the answer. However, the early news on Joros is pretty good, Davis said.
“It’s not really destroying anything,” he said. “It builds webs. The webs just blow away at the end of the season, so it's not really destroying the trees that it makes webs on. At most, it's eating some of our native bugs. But on the other hand, it's also eating some of the non-native bugs too, which is kind of a good thing.”
Davis contrasted the Joro’s observed impact with that of the spotted lanternfly, another invasive species native to Asia. The spotted lanternfly appeared in September 2014, around the same time as the Joro. Now it’s rampant throughout the Northeast, defoliating trees and “literally destroying the ecosystem,” Davis said. It was first detected in North Carolina last year.
“The Joro isn’t really in that category,” he said.
An unfolding story
Up north, governments are urging citizens to kill spotted lanternflies on sight. But Davis asks the opposite of those who encounter Joros. Killing one or even 10 won’t make any difference at the population level — egg sacs, laid mid-October through November, contain 400-500 eggs apiece. Instead of squishing them, he said, people should use them as a tool to overcome their fear of spiders.
“Don't kill it, and maybe even get to know it,” he said. “Give it a name, because you're going to be seeing it for the next three months. Use it to teach your kids about what spiders do. They're really gentle, and they’re not going to hurt you, and they’re striking to look at.”
Despite their size, Joro spiders have small fangs that don’t easily pierce human skin. According to PennState Extension, the spiders themselves are reluctant biters, and it can be hard to make them bite even when handling them. Their venom is weak, so when bites do occur they are less painful than a bee sting and don’t require medical attention.
Due to trade, travel and similar climates between the two countries, many of the invasive species currently thriving in the United States are native to China, as is the Joro — meaning that its new home is populated not only with species native to North America but also with an increasing list originating from Asia. There’s not yet any evidence that the Joro will harm native species, but there are indications it could help control invasive ones.
The brown marmorated stinkbug, for instance — a universally detested invasive native to Asia.
“No other spiders will eat that thing, but the Joro will,” said Davis. “And it’s probably because they’re from the same region originally.”
The spotted lanternfly is also present in the Joro’s native range, said Davis, and it will be interesting to see how those species interact should their introduced range someday overlap. But only time will tell.
“Non-native species have a way of integrating themselves into the ecosystem after some time, and we don't really know how long that would take,” he said.
