Brown widow spiders are replacing black widow spiders in parts of the southern United States. Brown widows are less venomous towards humans, with symptoms after a bite usually resulting in mild irritation. In recent decades, the number of southern black widows have been falling including the red hourglass shape on their abdomen. Initial research into black widow displacement revealed that brown widows were more aggressive than their counterparts, freely hunting and killing all those within the same genus.
Subsequent lab testing confirmed that brown widows were 6.6 times more likely to kill southern black widows in their enclosure than any other arachnid offered. When researchers observed the brown and southern black widows meeting face-to-face, they noted that brown widow females aggressively stalked, captured and consumed southern black widows at all stages of development. Brown widows were also bolder, frequently venturing into the webs of black widows but the defending black widow was able to entrap the brown widow in a sheet of webbing and inject venom. Preying on creatures within the same genus – a collection of closely related species – is rare for spiders.
Comparing the two species, researchers 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 the southern US. Young female brown widows were 9.5 percent larger than southern black widows and mature females could produce multiple egg sacs at a time, whereas southern black widows produce only one. Although the reason for the fall in southern black widows is unknown, it cannot be fully attributed to competition from brown widows. Researchers openly wonder whether this is a direct result of a prey-specific predator such as the Argentine ant, an invasive species, which makes foraging harder for spiders.
Despite being less venomous, it is important to remember that brown widows still pose harm to humans. It is important to be aware of the spiders that inhabit homes and gardens, taking precautions to eliminate potential hiding places both inside and outside. Techniques such as shaking out shoes before putting them on, wearing gloves when gardening, and moving stacked materials from place to place can all help to reduce the risk of encountering these spiders. Further research hopefully will determine the exact reasons why the southern black widow numbers are declining and whether the rise of the brown widow is directly responsible.