Black widow spiders being wiped out by their lesser-known cousin

By Mark Waghorn via SWNS

Notorious black widow spiders are being wiped out by their lesser-known cousin, according to new research.

They are being preyed upon by equally venomous brown widow spiders, scientists said.

Although slightly smaller than the creepy crawlies, they are just as deadly.

Lighter in color with stripes on their legs, they can be identified by spiky looking eggs sacs.

Lead author Louis Coticchio, a Ph.D. student at the University of South Florida, said: "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."

A black widow's bite is said to be15 times more potent than that of a rattlesnake.

In the past couple of decades, it has been disappearing from parts of the U.S. as their close relative spreads - being outnumbered 20 to one in some areas.

It was believed they were being outcompeted for food or habitat. Now a more sinister reason has been identified.

In experiments, pairing brown widows in containers found they were 6.6 times more likely to kill southern black widows than other related species.

Coticchio said: "I had a sneaking suspicion that Florida in particular provided plenty of food and habitat for both the brown and black widow, and that there was possibly some other area such as behavioral differences that were playing a role.

"My observations in the field showed that brown widows appeared to be much more tolerant of other species outside of their genus, and so if resources were the main factor, then we should have seen the same behavior with other spiders competing for the same resources, but that did not seem to be that case."

Computer simulations also showed black and brown widows are far more likely to die from predation than by starvation - ruling out chasing scarce resources.

When young females of both species were placed together, the brown widows killed and consumed the black widows in 80 percent of cases.

In pairings of adults, black widows were killed in 40 percent of trials - and defensively killed brown widows in 30 percent.

Throughout the experiments, brown widow spiders regularly ventured into black widow webs, the researchers say.

Red house spiders and triangulate cobweb spiders also showed such "bold" behavior, but black widows were never observed as aggressors.

Co-author Professor Deby Cassill said: "We didn't expect to find such a dramatic and consistent difference in the personalities of the brown widow and the black widow.

"Brown widows are boldly aggressive and will immediately investigate a neighbor and attack if there is no resistance from the neighbor.

"For two bold spiders, the initial attack is often resolved by both individuals going to separate corners and eventually being OK with having a nearby neighbor.

"The black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider."

The brown widow was first discovered in downtown San Diego in 2003. Since then it has multiplied exponentially and taken over the black widow’s habitat – from the coast to the inland valleys.

Brown widows don’t have any known predators to interfere with their reproductive success. The invasive creature could have a negative impact on the ecosystem.

The brown widow only injects about half the amount of venom - and is also less aggressive.

The other good news for people is that brown widows tend to favor outside plastic patio furniture and aren't found very often inside homes.

Mr. Cottichio said: "They are very shy when harassed by humans or larger animals that are not considered prey.

"They will run or roll up into a ball and play dead when being attacked or harassed by most other animals outside of their prey range."

Brown widow venom causes less severe reactions to humans than black widows, and bites in people are very rare.

Cottichio said: "One question I would love to answer is how brown widows interact with other species of spiders, more specifically black widows in Africa, where brown widows are believed to have originated.

"I would love to see if their behavior and displacement of black widows is something that they have adapted here in North America or if this behavior is something they exhibit naturally even in areas where they have coevolved with black widows for much longer periods of time."

The study is in Annals of the Entomological Society of America.