Black widow spiders are notorious for their venomous bites that can cause severe pain and muscle spasms in humans. But these fearsome arachnids have a new enemy in town: brown widow spiders. These invaders from South Africa have been spreading across the southern US since the 1930s, and they have a striking tendency to seek out and kill their black cousins.
Brown widow spiders are more aggressive and fertile than black widows. They can produce thousands of spiderlings from multiple egg sacs, while black widows only produce one egg sac at a time. Brown widows also reach reproductive maturity faster and grow larger than black widows.
“Brown widows will aggressively go after black widows, chase them down,” said Louis Coticchio, a science tutor at St. Petersburg College in Florida and an author of a study published in the Annals of the Entomological Society of America. “They don’t play well with being neighbors.”
In experiments pairing brown widow spiders with related cobweb spider species, the brown widows were 6.6 times more likely to kill southern black widows than other related species. They also killed black widows at all stages of development, from eggs to adults.
“ [Brown widows] were wiping them out before they even got a chance to get going,” says Richard Vetter at the University of California, Riverside.
Black widows, on the other hand, were never the aggressors towards brown widows. They preferred to run away, play dead or flick webbing at their attackers. But sometimes they fought back and managed to trap and inject venom into their rivals.
“Occasionally, a defending black widow was able to entrap the aggressive brown widow in a sheet of webbing and inject venom,” the researchers write.
But this was not enough to stop the brown widow invasion. The researchers found that brown widow spiders were displacing black widows from their preferred habitats, such as crawl spaces, woodpiles and sheds. This could have ecological consequences for the native spider community.
“We have established brown widow behavior as being highly aggressive towards the southern black widows, yet much more tolerant of other spiders within the same family,” Louis Coticchio said in a statement.
The good news for humans is that brown widow spiders are less venomous than black widows and cause mild irritation to humans. Their bites are comparable to those of common house spiders.
“Black widows generally don’t bite when harassed,” preferring to run, play dead or flick webbing at a poking finger, Mr. Coticchio said. “It’s only pinching them that’ll get you bit.”
But that doesn’t mean people should be careless around these spiders. The researchers advise people to wear gloves and long sleeves when working around potential spider habitats, and to seek medical attention if bitten by any spider.