#Black #widows #killed #nonnative #brown #widows #-
Bill Kearney | South Florida Sun-Sentinel
Black widow spiders can stir primordial fear — they lurk in dark corners and their venom can cause excruciating pain, and in extremely rare occasions, death. But they seem to be no match for their aggressive non-native cousin, the brown widow.
Scientists at the University of South Florida have recently discovered that brown widows ruthlessly target and attack black widows, and usually win those battles. But researchers can’t quite figure out why.
As a result of this spider vendetta, the non-native brown widow has displaced the Southern black widow in many areas across the southern U.S., as far north as Kansas and as far west as California.
Not only do brown widows hone in on black widows, but they go relatively light on other spiders that share the same shadowy haunts.
The study, led by biologist Louis Coticchio, found that compared to other cobweb spiders, brown widows were six times more likely to kill and eat Southern black widows. They were also more likely to peacefully live next door to other cobweb spiders, such as red house spiders.
Brown widows first showed up in the U.S. in 1935, in Southern Florida. Experts suspect they hitched rides out of Southern Africa on construction materials and in crates of produce and plants. They now live in warm climes on every continent except Antarctica.
Cobweb spiders such as the black and brown widows thrive in urban and suburban areas because attics, crawl spaces, sheds, closets and garages offer ideal dark, dry and sheltered microhabitats. Florida’s building boom has meant more territory.
In addition to living in the dark, cobweb spiders don’t have very good vision, Coticchio said. How do they tell who’s who under a backyard shed? He and other scientists suspect that they have hairs on their bodies that they use for sensing wind, vibration and chemicals, or pheromones.
“Cobweb spiders can access knowledge about neighbors by touching their neighbor’s web or waving their appendages in the air,” said the study.
Through pheromones on webs, they can sense that there are black widows nearby, and they can differentiate between a black widow and another type of spider.
The young eat the young
To attempt to understand how and why the brown widows are bullying the black widows, Coticchio and his team studied how the species reacted to each other, and other species, when they put them in a habitat together.
In each experiment, they matched spiders by size and age. They found that when they put an adult brown widow in the same cage as an adult black widow, the brown widows were much more aggressive, and the black widows basically retreated to a corner and were shy.
“We observed Brown Widows boldly ‘testing’ their neighbor’s web during forays beyond their web matrix, a behavior that we did not observe in the Southern Black Widow or the cobweb spiders,” he wrote in the paper. When face-to-face, “they engaged in rapid bouts of ‘slapping’ each other’s forelegs and tarsae.”
Coticchio found that when an adult brown widow and black widow faced off, success was determined more by home field — or home web — advantage, and the black widows could hold their own.
“But when they were younger,” he said. “It didn’t matter who built the webbing first, the black widows were almost always the losers.”
Cannibalization is common among spiders — they eat their siblings if they don’t disperse on silk parachutes. But when Coticchio and his team put dozens of brown and black hatchlings of the same age and size together in a tank over a period of weeks, the brown widows ate the black widows first. Once they were all gone, they turned on their siblings.
In the real world, such as under a shed, the two species would compete for space, and the younger brown widows would dominate and eat the hatchling black widow.
Other factors in the real-world eradication of black widows in urban and suburban areas include the fact that brown widows have twice the fertility potential as Southern black widows. The young also mature more quickly, making them physically dominant.
Spiders vs. humans
Coticchio, who’s handled hundreds of widow spiders over the years, said that though brown widows are aggressive toward black widows, “they’re big chickens when it comes to people.”
“Widow spiders really only bite [people] when they’re pressed on or pinched or squeezed,” he said. “I’ve also noticed that when you poke or harass them, black widows will flick their webbing at you or run. Brown widows will usually drop and play dead.”
Coticchio said he was accidentally bitten by a black widow once, while cleaning out a tank. The branch that the spider had its webbing on toppled over and pinned the spider against the top of his hand. “In typical spider biologist fashion, I was looking at it as it was biting me, and I was like, ‘Oh cool, it’s biting me.’ Then, ‘Oh wait, it’s biting me!’”
He said he had localized pain and swelling, and for two weeks had what he describes as a “pretty bad throbbing pain in my hand.” He classifies this as a mild reaction. Some of his peers have had more severe bites, which can include muscle cramping, muscle spasms, muscle rigidity, abdominal pain and lower back pain, high heart rate and high blood pressure.
Coticchio said there’s some disagreement about levels of toxicity in each species, but that brown widows inject a smaller dose of venom per bite. In general, the brown widow is not more dangerous to humans, and their bites are less severe.
Statistically, bees and wasps are more likely to kill you. According to the Centers for Disease Control and Prevention, an average of 62 Americans died each year from stinging insect from 2000 to 2017. Bites from all venomous spiders combined killed an average of six people per year in the U.S. from 2008-2015, according to the University of Mississippi.
Black widows are not faced with total eradication. Brown widows love human structure but don’t seem to do well in the wilderness, according to Coticchio. Black widows, on the other hand, utilize both environments, so they find a bit of relief from their aggressors by parachuting to wild areas.
There are a couple vexing questions that remain for Coticchio: Is the targeting of black widows a behavior the brown widows have in Africa, too, or is it something they’ve developed in the U.S.? And why target black widows but not other cobweb spiders in their niche, such as red house spiders and triangulate cobweb spiders? “That’s going to take a lot more research to come up with a good hypothesis,” he said.
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