Female invasive joro spiders may dabble in cannibalism

National Geographic

3 days ago

Invasive joro spiders have been building their enormous, golden webs across the eastern United States since 2014. Sometimes the webs stand alone, but other times 10 or 15 females dwell in a mosaic of interconnected webs stretching all the way up a tree—an arachnid apartment complex of sorts. New research finds that these stereotypically tolerant spiders can engage in unneighborly behavior, attacking and even cannibalizing each other.
Joro spiders are native to east Asia. Females look like a yellow and black grape with toothpicks sticking out of it—big enough to fill the palm of a hand, says Andy Davis, an ecologist at the University of Georgia in Athens. 'For an arachnophobe, this thing is the stuff of nightmares.' Despite their frightful looks, joro spiders don't harm people or pets, he says. And Davis and colleagues have found that these creatures are actually rather timid. Blow a puff of air in a joro's face and it freezes for over an hour while other species hold still for a few minutes. 'We kind of labeled them as one of the world's shyest spiders,' Davis says.
(Read more about joro spider's timid tendencies.)
So it was curious when Davis and his team started seeing signs of cannibalism among joro females. First it was a female clutching another female in a web. Then it was an attack in an aquarium into which two females had been placed. And finally, a fight to the death when the researchers released two joro spiders at the same time. While female joro spiders occasionally munch on mates, fighting amongst themselves has never been seen before. Why do joro spiders fight?
Davis's students captured joro spiders (Trichonephila clavata) and staged meetings to look for hints about what spurs aggression. When 25 pairs of similarly sized females met in a plastic food storage container, fights broke out 40 percent of the time, the team reported July 10 in Arthropoda. In some bouts, the spiders ripped each other's legs off, while in others, the winner sunk its fangs into the loser, Davis says. Usually larger spiders have an advantage in combat, so Davis's team was surprised to see that, in 27 meetings of differently sized females, cage fights happened only 18 percent of the time, and larger spiders didn't always instigate the conflict,.
But Tupperware isn't the joro's usual stomping ground, and that might influence the spiders' behavior. Joro spiders have long legs adapted for darting around a web, says ecologist Erin Grabarczyk of Valdosta State University in Georgia who wasn't part of the work. 'Put them on the ground and they look like a baby deer—they sort of wobble around.' It's possible the food storage container stressed the spiders, she says. Joro spiders are originally from southeast Asia and were first detected in Georgia in 2014. Photograph By Alex Sanz/AP
So Davis's team took their tests to a more natural environment—empty webs in a field. They introduced 14 pairs, each time placing the two females into a web that belonged to neither one. This time, only one encounter ended in a fight and the winner wrapped up her victim in silk, as if preparing for a meal.
'It's not surprising that they walked away,' Grabarczyk says. Those webs are likely laden with chemicals and cues attesting to the ownership of that web. And joro spiders may have other tactics to keep the peace between neighbors. For instance, they may use vibrations to communicate, Davis says. 'Somehow they have a way to not eat each other when they live communally.'
The field studies may not resemble real run-ins between joro females, says ecologist Robert Pemberton an independent researcher in Atlanta who wasn't involved with the study. These spiders aren't typically wandering around; they sit and wait, he says. Future experiments with webs containing resident females could better test how they react to intruders on their turf.
(This spider web is strong enough for a bird to sit on.)
Davis and others are exploring joros' behavior and biology to learn about their effects as they spread. So far, the picture of ecological impact isn't clear, Davis says.
In April, Pemberton published a spider census of Atlanta forests for 2022 to 2025; numbers of the joro spider had shot up while native orb weavers declined each year. Meanwhile another study based on citizen scientists' reports suggested that joro spiders' presence didn't greatly change the makeup of spider communities in several states. It's uncertain whether aggression or cannibalism might shape the joros' spread.
The spiders may be finding a place in local food webs. Pemberton found that cardinals will attack joros and may be eating them. Joros may prey on other invasives, including the spotted lanternfly, which destroys trees, Davis says.
'This is a new species that is definitely changing the ecosystem,' Grabarczyk says. 'Understanding that from an ecological perspective is so important.'