Black Widow Spiders Are Being Killed Off by Non-Native Brown Widows

With iconic red hourglass markings, shiny black bodies and a reputation as the deadliest spiders in North America, black widows have captured the public’s attention and imagination. But despite their infamy for being dangerous predators, new research reveals the arachnids have become prime prey for their less lethal cousins: brown widow spiders. 

“Brown widows will aggressively go after black widows, chase them down,” Louis Coticchio, a biologist at the University of South Florida who led the study, tells the New York Times’ Asher Elbein. “They don’t play well with being neighbors.”

Black widow spiders are generally shy insect- and arachnid-eaters that often live in dark, dry spaces near humans, such as in wood piles, sheds or porch furniture. While their bites can cause pain, human fatalities are rare, and victims can be prescribed antivenom if they are at high risk. Despite the species’ name—and its reputation for females eating their mates after copulation—sexual cannibalism is uncommon (though it does happen). Three species in the U.S. fall under the common name black widow: the western species, Latrodectus hesperus; the northern species, L. variolus; and the southern species, L. mactans, per Smithsonian magazine’s Theresa Machemer. 

Their cousins, brown widows, originated in either Africa or South America but have managed to colonize every continent but Antarctica. They were first detected in the United States in 1935 in Florida and have since expanded across the South and parts of the West. But as the numbers of these non-native creatures increased, black widows seemed to disappear, writes Live Science’s Harry Baker. At first, entomologists thought brown widows were outcompeting the native spiders for resources, but with ample food and habitat in Florida, and with only black widows displaced, Coticchio wondered if something else was afoot.

To gain more insight into the interactions between the widow spiders, Coticchio and his colleagues placed a brown widow into a tank with either a southern black widow, a red house spider or a triangulate cobweb spider and recorded the outcome. 

The results were dramatic: Brown widows were 6.6 times more likely to attack black widows than the other spiders. Young brown widows were particularly aggressive toward their cousins, killing and eating young black widows 80 percent of the time. In pairings of adults, black widows were eaten in 40 percent of the trials, while they defensively killed brown widows 30 percent of the time. The research team published the results this month in Annals of the Entomological Society of America.

“We didn’t expect to find such a dramatic and consistent difference in the personalities of the brown widow and the black widow,” co-author Deby Cassill, an ecologist at the University of South Florida, says in a statement. “Brown widows are boldly aggressive and will immediately investigate a neighbor and attack if there is no resistance from the neighbor. The black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider.”

Why these species have such different responses to each other is unclear, but the research team plans to study their interactions in different parts of the world, such as Africa, per Live Science.

“We have found similarly high levels of aggression and activity in invasive brown widows in Israel,” Monica Mowery, a spider biologist at Ben-Gurion University of the Negev in Israel who was not involved in the new study, tells the Times. “One key remaining question is whether brown widows are outcompeting local species [elsewhere].”

Beyond predation, brown widows might also be displacing their native cousins because of their biology: They can lay more eggs and reproduce earlier in their lives than black widows do, leading to them have more offspring, per Mashable’s Mark Kaufman.

As of now, brown widows are not considered invasive, Coticchio tells Gizmodo’s Isaac Schultz, but he says he “would love to see the attitude toward them changed” if they turn out to be the main drivers behind black widow population declines.

Margaret Osborne | | READ MORE

Margaret Osborne is a freelance journalist based in the southwestern U.S. Her work has appeared in the Sag Harbor Express and has aired on WSHU Public Radio.