The black widow disappears from the southern United States due to the aggressiveness of the brown widow

The fearsome black widow spider has gained a bad reputation due to its venomous bite. However, in parts of the southern US, these spiders have reason to fear their very close relatives, the brown widow spiders (Latrodectus geometricus), which apparently dislike their company.

So much so that, in the last twenty years, scientists have observed that the native spider has been replaced by the brown widow spider, paradoxically, a species of the same genus Latrodectus.

An investigation published in the journal Annals of the Entomological Society of Americasuggests that it is not simply the latter gaining ground competing for food or habitat, but rather that brown widow spiders have been shown to be prone to seeking out and killing nearby black widows.

Results showed that brown widow spiders are 6.6 times more likely to kill southern black widows than other related species.

The work, carried out by researchers from the University of South Florida (USA), was based on experiments in which these species cohabited with other relatives in controlled habitats. Results showed that brown widow spiders are 6.6 times more likely to kill southern black widows than other similar species.

“We found that the brown widow’s behavior is very aggressive towards southern black widows, but it is much more tolerant of other spiders in the same family,” says Louis Coticchio, lead author of the study.

The brown widow spider is native to Africa, although it has been introduced to all continents except Antarctica. The black widow spider, in turn, is native to North America and comprises two closely related species, the western black widow (Latrodectus hesperus) and the southern black widow (Latrodectus mactans).

The brown widow spider is gaining ground

As an expert on venomous animals, Coticchio, collecting wild spiders in Florida during his research projects, observed that brown widows replaced black widows, but not other related species. This was one of the ideas that inspired him to carry out this work.

“I suspected that Florida, in particular, provided abundant food and habitat for both species, and that there was likely some other factor that could play a role, such as differences in behavior,” he explains.

The scientific team tried to figure out the reasons for this combative behavior by considering factors such as behavior or growth and fertility rates.

“According to previous observations, the brown ones seemed to be much more tolerant with other species belonging to their genus, so if the resources could be the main reason, then we would see the same behavior with other spiders that compete for it, but not this the case”.

So the scientific team, which in addition to Coticchio also included Deby Cassill, an associate professor in the Department of Integrative Biology at the University of South Florida, and Richard Vetter, an expert on spiders at the University of California at Riverside, agreed to look into the possible causes that would lead brown widow spiders to exterminate black widows.

On the other hand, mathematical models were also applied to the survival risk factors faced by both invertebrates. In this sense, the researchers found that the two species are much more likely to die from hunting than from starvation.

In other words, as the researchers state, “competition for scarce resources is not a significant cause of mortality among young spiders of either species.”

When brown widow spiders are near black widow spiders, the former capture and kill the latter in 80% of cases.

In addition, the scientific team also took into account other factors such as the growth rates of each of these arachnids. In this regard, the researchers found that young brown widows are 9.5% larger than black widows, while adult brown widows are 25% smaller than adult black widows.

On the other hand, with regard to fertility rates, the study states that adult brown widows reach reproductive maturity 16% earlier and, in parallel, adult brown widows reach this stage 21% earlier than their black widow relatives. Likewise, female brown widow spiders were about twice as fertile as black widow spiders.

However, the most revealing data were obtained by putting insects and other spider species together. Specifically, what drew the attention of specialists was to verify that, when brown widows approach black widows, the former capture and annihilate the latter in 80% of cases.

Another important data that can be extracted from this work is that when the adult specimens of both species they had to live together, the black widows died in 40% of the trials, while killing in their own defense the brown widows in 30% of the cases. Both insects were only able to cohabit peacefully on 30% of occasions.

And even more, according to the researchers, during the experiments, brown widow spiders often got into the webs of black widows.

an enigmatic behavior

In the words of one of the members of the scientific team, Deby Cassill, “We did not expect to find such a drastic and consistent difference in the personalities of both animals. Brown Widows are boldly aggressive, quickly scanning those around them and will attack if they don’t resist.”

In general, a battle between two fearless spiders is usually resolved when both go to separate corners to confront each other. However, “black widows are extremely elusive and will only react if it is to defend themselves against an aggressive spider,” in the words of the study authors.

Black widow spiders are very elusive and will only fight back if defending against an aggressive spider.

The characterization of the brown widow spider as aggressive is a relative term, according to the researchers, as it reflects its stance towards the black widow spider but not towards humans.

While widow spiders are synanthropic, meaning they’re usually found around man-made structures like barns, garages and sheds, “they flee when harassed by humans or larger animals that don’t consider themselves prey,” details Coticchio.

“They usually run away or curl up in a ball and play dead when attacked or harassed by other animals,” adds the researcher. However, brown widow venom causes less severe reactions in humans than black widow venom, and bites on people are rare.

They usually run away or curl up in a ball and play dead when attacked or harassed by other animals.

Louis Coticchio

Now, the obvious hostility of the brown widow spider towards the black widow spider has led scientists to ask several questions, such as, for example, what makes this insect behave like this with a close species?

As the researchers point out, invasive species tend to outperform native ones due to advantages in factors such as fertility, growth, dispersal or defenses against predators. But direct predation by an invasive species on its native relative, across the animal kingdom, is not very common.

“We would like to know how brown widows interact with other species of spiders, more specifically with African black widows, where they are believed to be the first”, admits Coticchio.

At the same time, “we want to see if their behavior and the movement of black widows is something they’ve adapted to here in North America, or if it’s something they exhibit naturally even in areas where they’ve lived with black widows for a long time. longer periods of time.


Louis A Coticchio et al. al: “Predation by the introduced brown widow spider (Araneae: Theridiidae) may explain local extinctions of native black widows in urban habitats”, Annals of the Entomological Society of America2023

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