Brown widow spiders are killing off black widows in the southern US

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 take some comfort in knowing that brown widows are less venomous towards humans than black widows – symptoms after a bite are usually limited to mild irritation, like the bite of a common house spider.

In recent decades, the number of southern black widows (Latrodectus mactans) – notoriously identifiable by the red hourglass shape on their abdomen – have been vanishing from Florida, while brown widows (Latrodectus geometricus) have been on the rise. Initially, scientists wondered if the falling number of black widows was linked to competition for food and space with brown widows, a species native to southern Africa and Madagascar that arrived in the US in the 1990s.

Because Florida appeared to have ample space and food for both arachnids, entomologists wondered if something else was afoot. Perhaps black widows were being sought out and hunted by brown widows, for instance.

To test the idea in the lab, Richard Vetter at the University of California, Riverside, and his colleagues gave brown widows their choice of an arachnid buffet: red house spiders (Nesticodes rufipes), triangulate cobweb spiders (Steatoda triangulosa) or southern black widows. They found brown widows were 6.6 times more likely to kill southern black widows in their enclosure than any other arachnid offered. Preying on creatures within the same genus – a collection of closely related species – is rare for spiders.

When the researchers observed the brown and southern black widows meeting face-to-face, they saw both species engage in rapid bouts of “slapping” each other’s legs. They also noted that brown widow females aggressively stalked, captured and consumed southern black widows at all stages of development.

Brown widows acted more aggressively than southern black widows towards the other spider species offered and didn’t spare the young. “[Brown widows] were wiping them out before they even got a chance to get going,” says Vetter. “They’re very opportunistic.”

Southern black widows were never the aggressors towards brown widows, but did defend themselves against attackers. “Occasionally, a defending black widow was able to entrap the aggressive brown widow in a sheet of webbing and inject venom,” the researchers write. Brown widows were also bolder, frequently venturing into the webs of southern black widows.

When researchers compared the two species, they found that female brown widows outperformed southern black widows in both size and reproductive ability, which could play a role in the dynamics playing out in Florida. Young female brown widows were 9.5 per cent larger than southern black widows and mature females could produce multiple egg sacs at a time, whereas southern black widows produce just one.

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