Brown widow spiders kill black widows in the US South

In parts of the southern US, black widows are being supplanted by their more aggressive arachnid relatives: brown widows. However, residents of the region can be comforted by the fact that brown widows are less venomous to humans than black widows – symptoms after a bite are usually limited to mild irritation, as with a bite from an ordinary house spider.

In recent decades, the number of southern black widows (Latrodectus mactans) — infamous for the red hourglass shape on their belly — have disappeared from Florida while brown widows (Latrodectus geometricus) were on the rise. Initially, scientists wondered if the decline in black widows was due to competition for food and space with brown widows, a species native to South Africa and Madagascar that arrived in the US in the 1990s.

With enough space and food for both arachnids in Florida, entomologists wondered if something else was going on. It is possible that black widows were sought out and hunted, such as brown widows.

To test this idea in the lab, Richard Vetter of the University of California, Riverside and colleagues had brown widows choose a buffet of arachnids: red house spiders. (Nestikod rufipes)triangular web spiders (triangular steatoda) or southern black widows. They found that brown widows were 6.6 times more likely to kill southern black widows in their enclosure than any other proposed arachnid. Hunting for creatures of the same genus – collections of closely related species – is rare for spiders.

When the researchers watched brown and southern black widows come face to face, they saw both species engage in rapid bouts of “slapping” each other’s legs. They also noted that female brown widows aggressively pursued, caught, and ate southern black widows at all stages of development.

Brown widows acted more aggressively than southern black widows towards other proposed spider species and did not spare the young. “[Brown widows] destroyed them before they even had time to act,” says Vetter. “They are very opportunistic.”

Southern black widows have never been aggressors towards brown widows, but have defended themselves against attackers. “Sometimes a defensive black widow could lure an aggressive brown widow into its web and inject venom,” the researchers write. Brown widows were also bolder and often entered the web of southern black widows.

When the researchers compared the two species, they found that female brown widows were superior to southern black widows in both size and reproductive capacity, which could play a role in the dynamics happening in Florida. Young female brown widows were 9.5% larger than southern black widows, and mature females could produce multiple egg sacs at a time, while southern black widows only produced one.

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