Who is killing the black widow spider? An unlikely foe

Matthew Cimitile and Joe Rominiecki

In a nondescript research lab on campus, Louis Coticchio began placing different species of spiders together in artificial habitats. Red house and triangulate cobweb spiders. Brown widow and black widow spiders.

He was interested to see how they would interact. More specifically, he wanted to know if brown widows were playing a part in the dramatic population decline of their more famous black relatives.

Originally from Florida, Coticchio spent years as a zookeeper specializing in venomous animals in California. When he returned to the state to earn a biology degree at USF St. Petersburg, he started to collect wild spiders in Florida for a research project. 

In doing so, Coticchio noticed what other researchers have documented in recent decades: black widow spiders were becoming harder to find. In areas once populated with black widows, the non-native brown widows were becoming increasingly more abundant.

“I had a sneaking suspicion that Florida in particular provided plenty of food and habitat for both the brown and black widow to thrive. So, there might be another reason such as behavioral differences that was playing a role in this outcome,” Coticchio said. 

His experiments using artificial habitats sought to answer the cause of this displacement. What he uncovered would lead
to international headlines.

A misunderstood spider

With the exception of Antarctica, widow spiders are found throughout the globe. 

“Widow spiders typically build permanent homes in structures like garages, sheds, fences and the overhang of houses, so they are a highly visible spider species,” said Deby Cassill, a USF associate professor in the department of Integrative Biology who was Coticchio’s research advisor. “They also have dramatic coloration, with this black shiny body and bright red hourglass. They look like jewelry hanging there. So, for us there is an artistic attraction and, of course, a fear factor.”

Louis Coticchio working in the biology lab for his research into brown and black widow spiders.

Tradition has it that widow spiders received their name via reports of females killing and eating their mate. Though according to the University of Florida’s Institute of Food and Agricultural Sciences (IFAS), this evidence was based on spiders in crowded lab settings. 

Tradition also holds that the black widow is a highly venomous and even deadly spider to humans. This too has been overstated, according to IFAS. Their bite can be painful, causing symptoms such as intense pain, muscle cramping, malaise and vomiting, but human deaths from black widow bites are extremely rare.

While black widow spiders are native to North America and comprise two closely related species - the northern black widow and the southern black widow, which calls Florida home – the brown widow is believed to be native to Africa. They have been introduced on all continents, with the first documented appearance in Florida in 1935. Though sharing a name associated with aggression and venom, reports of brown widows biting humans are extremely rare. When they do occur, their venom causes less severe reactions to humans than black widows. 

The experiment

Coticchio partnered with Cassill and spider expert Richard Vetter of the University of California, Riverside. They devised a study to explore if brown widows were leading to the population decline of black widows, and if so, how. 

“Dr. Cassill gave me the opportunity and free range to explore my research interests and do it my way,” Coticchio said. “She really allowed me to lead this research, while providing oversight to ensure I was doing it right. I owe her much.” 

Coticchio conducted fieldwork in the spiders’ natural habitat and in parks with human-made structures where they are commonly found, such as restrooms, bridges and boardwalks. He surveyed breeding sites, counting the number of viable egg sacs per female, and found brown widows have twice the fertility potential as black widows.

He then collected various spider species found in black widow environments, placing them in a number of different combinations in container habitats to observe behavior. In some containers, brown widows were put in close proximity with black widows. 

The results were startling. 

Rather than cohabitating, the brown widow females aggressively stalked, killed and consumed the black widows. For example,
when sub-adult females were paired, the
brown widows killed and consumed the black widows in 80 percent of pairings. In contrast, brown widow females – both juveniles and adults – largely cohabitated with the two other spider species.

Brown widow spiders were seeking out and killing their black widow relatives, a surprising twist in the natural relationship between species of spiders known for their venomous bites. 

“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,” Coticchio said.

The reaction

In these experiments, brown widows were nearly seven times more likely to attack black widows than other spider species. The aggressive behavior is likely a driver of the black widow spider’s population decline.

“We didn’t expect to find such a dramatic and consistent difference in the personalities of the brown widow and the black widow,” Cassill explained. “Brown widows are boldly aggressive and will immediately investigate a neighbor and attack if there is no resistance from the neighbor. The black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider.”

This new research suggests this isn’t a simple case of one species winning the competition for food or habitat. It’s a case of predation. 

Their findings were reported in the March 2023 issue of the Annals of the Entomological Society of America. Shortly thereafter, the discovery of brown widows hunting black widows exploded across news outlets. The New York Times, NPR Living on Earth, National Geographic and the Jerusalem Post were some of the many outlets that covered the story, reporting on the grisly fate of black widows.

“I never expected it to gain the attention that it did,” Coticchio said. “This was just an undergraduate study I did while working on my bachelor’s degree. The fact that what we
found was broadcasted everywhere, I was just blown away.”

Coticchio, who is now a senior instructional support specialist at St. Petersburg College and in graduate school to become a science educator, hopes future researchers will add to his findings. Specifically, he is interested to know what drives such aggression between these closely related spider species.

“How do brown widows interact with other species of spiders, including black widows, in Africa, where brown widows are believed to have originated?” Coticchio asked. “I want to see if their behavior and displacement of black widows is something that they have adapted to 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.”