'They Don't Play Well': Black Widow Spiders Being Deliberately Killed Off by Invasive Cousins

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American researchers have conducted a series of experiments with related species of arachnids, ultimately finding that brown widows target and kill their US cousins and do so more often than with other spider species.
Native black widows in the US are being displaced by brown widows imported from Africa in the 20th century, new findings have recently revealed.

To understand the reasons for the success of brown widow invasions in the Americas, Louis Coticchio, of the University of South Florida, and colleagues conducted a variety of field studies and lab experiments.

Among the experiments, scientists compared the developmental rates of brown widows and southern black widows and evaluated brown widow tolerance to related species, as well as assessing the likelihood of brown and black widow offspring dying as a result of predation or competition for limited resources.

From June 2019 to August 2020, scientists surveyed urban and park structures in Florida and counted spiders: it was important to understand how many breeding brown and black widows live in different cities across the state.
In total, scientists found 223 brown widows and only 10 southern black widows. Moreover, black widows were found only in park buildings - there were none in houses. Black widows produced one egg sac at a time, while brown widows produced several. By final count, female brown widows in Florida were twice as fecund as female southern black widows.
Scientists also conducted three experiments in which spiders of different species were paired together for long periods of time to see how they would cohabit and interact.

"Brown widows will aggressively go after black widows, chase them down," said Louis Coticchio, a science tutor at St. Petersburg College in Florida and a co-author of the paper. "They don’t play well with being neighbors."

The research group first put 10 brown widow and southern black widow spiders in the same container, an experiment that lasted about four weeks - until only one spider remained. Because the experiment endured for a longer period of time, hunger could cause the arachnid to kill and eat other spiders of their own or a related species.
In this experiment, brown widows preferred to catch and kill black widows rather than their siblings. The southern black widow spiders, when the brown widows approached them, would freeze or run away, but would not attack themselves. Only if cornered would they try to counterattack.
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In the second experiment, juvenile female brown widow spiders were paired with spiders of three species - juvenile female southern black widow, adult female steatode and adult female red house spider. These pairings lasted from one to seven days, depending on the aggression of the individuals.
Steatode females cohabitated with brown widow females 80% of the time and killed them 10% of the time. Females of the red house spider cohabitated with brown widows about half of the time, and killed brown widows at about a 40% rate. Incidents where female black widows cohabited with brown widows accounted for only 10% of the time, whereas brown widows stalked and killed black widows in 80% of situations.
In the third experiment, scientists put adult females of brown and black widow in the same container, either one or four of each species. Researchers noted it was important for the brown or black widow to be the first to build a web for survival during the third trial. Southern black widows did so first 75% of the time.
During the final segment of the trials, brown widows killed their American counterparts at an occurrence rate of 40%, with the second test trial seeing a rate of 30%. Surprisingly, for one-third of the same trial, the studied arachnids got along without issue.
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Scientists calculated that brown widows killed black widows six times more often than individuals of other related species - and not at all because of a lack of resources.

According to the maternal risk management model, competition for scarce resources is not a serious cause of death for either species: the risk of starvation for black and brown widow spiders is less than half a percent, but from predators is close to a hundred.

"Southern black widows were never the aggressor and always the prey," Mr. Cottichio said.
Perhaps brown widows are afraid of not having enough shelter - indirect evidence the authors found in the fact that black widows have built their web before. Researchers cannot yet explain why invasive spiders behave this way toward native black widows, but they cite brown widow predation as a major factor in the local black widow extinction in Florida.
The article was published in Annals of the Entomological Society of America.
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