Few spiders in the United States have a more fearsome reputation than black widows. But across the South, bulbous arachnids with red hourglasses on their bellies are engaged in deadly competition with the brown widow, a relative from abroad, and they’re losing.
This is not the case of one species overtaking another for food or habitat. In published research Monday in the Annals of the Entomological Society of America, Biologists found that young brown widow spiders have a dramatic tendency to seek out and kill their American cousins.
“Brown widows will aggressively go after black widows, go after them,” said Louis Coticchio, a science tutor at St. Petersburg College in Florida and an author on the paper. “They don’t play well with being neighbors.”
Three species of black widows are native to North America, including the southern black widow, Latrodectus mactans. Extremely shy insect hunters, black widows like to live in tight spaces, woodpiles, and sheds. This fondness for human habitation occasionally leads to people being bitten. 1,004 cases in 2021, according to the US Poison Centers — but deaths are extremely rare. “Black widows usually don’t bite when harassed,” preferring to run, play dead or shake cobwebs on a pricking finger, Coticchio said. “Just pinching them is what will bite you.”
Brown widows, a closely related species, arrived in Florida around 1935, probably from South Africa. A single mother produces multiple egg sacs and potentially thousands of young. Just like black widows, brown widows like to live around people. Brown widows are less venomous than their native cousins and are not shy at all.
Mr. Cottichio spent several years working as a venomous keeper at a California zoo, hunting for spiders in his spare time. While searching for western black widows in suburban Los Angeles, he noticed that wherever they overlapped with brown widows, the black widows eventually disappeared. Starting his undergraduate degree at the University of South Florida, he found the same thing. “Every time I went back to a site, there were fewer and fewer until there were none left.”
To find out why, Mr Coticchio and his colleagues looked at the mathematical population model of the two widow species, which showed that black and brown widows were more likely to be eaten than starved, suggesting that they are not competing for scarcity. prey.
When they paired brown and black widows in container habitats, along with other related species such as spider mites and triangular web spiders, the researchers found that brown widows were 6.6 times more likely to kill black widows than other species. Young brown widows, in particular, made a beeline for their native cousins, eating them 80 percent of the time.
The researchers found that adult brown widows were less belligerent and significantly less likely to successfully kill a black widow hiding in an established network. But they still instigated the observed attacks. “Southern black widows were never the aggressors and always the prey,” Cottichio said.
What drives such predation attempts? One possibility lies in the spiders’ respective temperaments, the researchers suggest: Brown widows tend to be bold, investigating nearby webs and attacking spiders that don’t resist. House and web spiders challenge them, and brown widows often coexist peacefully with them. Shy and withdrawn black widows usually tried to escape, fighting back only as a last resort.
“We have found similarly high levels of aggression and activity in invasive brown widows in Israel,” said Monica Mowery, a spider biologist at Ben-Gurion University of the Negev who was not involved in the new study. “A key question that remains is whether brown widows are outpacing local species” in other parts of the world that they have invaded, she said.
In the southern and western United States, the outlook for urban black widows is not good. But the species has an alternative: Black widows like deserts and forests as much as suburbs, Cottichio said, while brown widows prefer urban and suburban areas. The ongoing contest could end with black widows being kicked out of basements and attics in favor of the wild, where their aggressive cousins won’t follow.
In the meantime, Cottichio said, if you find yourself on property with black widows, don’t rush to kill them: They’re already having a hard time.
“If you have something on your property,” he said, “do a good deed and move it outside somewhere.”