Black widows are being slaughtered by their brown widow cousins, and we don't know why

Brown widow spiders (Latrodectus geometricus) have caused population declines among several black widow species in the U.S. (Image credit: Shutterstock)

Black widow spiders in the U.S. are being killed off by an unexpected rival: their invasive relatives, but the motivation behind the highly aggressive attacks is not yet clear, a new study finds. 

The perpetrators, brown widow spiders (Latrodectus geometricus), likely originated in Africa or South America but have since spread to every continent on Earth apart from Antarctica. Brown widows are from the same genus as black widows, of which there are five species, including three that are native to North America: southern black widows (Latrodectus mactans), western black widows (Latrodectus hesperus) and northern black widows (Latrodectus variolus). But unlike black widows, which can all inflict extremely painful and occasionally lethal bites on humans, brown widow bites rarely cause significant harm to people, likely because they inject less venom into their bites despite having venom that is "drop-for-drop" just as toxic, according to the Center for Invasive Species Research (CISR) at the University of California, Riverside. 

In the U.S., brown widow spiders were first spotted in 1935 in Florida, and have subsequently spread across the southern states and into California, according to CISR. Since the invasive species was introduced, southern and western black widow numbers have plummeted, particularly in Florida, where southern black widows have gone "locally extinct" in certain areas.

However, scientists are unsure exactly why this is happening: Other spider species have not been affected by the brown widow's arrival, and there does not appear to be any competition for resources that would force the two widow species to fight one another.

Related: False widow spider preys on baby bat in never-before-seen encounter

In a new study, published Monday (March 13) in the journal Annals of the Entomological Society of America, researchers put solitary brown widows into laboratory tanks with one of three individuals from another spider species — a southern black widow, a red house spider (Nesticodes rufipes) or a triangulate cobweb spider (Steatoda triangulosa), which all overlap with brown widows in the wild — to see how the brown widow reacted to cohabiting with each of the species. 

When paired with the non-widow spiders, the brown widows peacefully cohabited with their tankmates in 50% to 80% of the tests. The rest of the time, one spider would kill and eat the other, but there was little difference between which species would end up victorious.

The new study shows that black widows are more shy than their invasive counterparts. (Image credit: Shutterstock)

But when adult brown widows were paired with adult black widows, the invasive species killed and consumed the black widows 40% of the time, the pair peacefully cohabitated together 30% of the time, and in the remaining trials the black widows ended up victorious — but only after defending themselves from an initial brown widow attack. However, when sub-adult individuals of both species were mixed, the brown widows killed and ate their counterparts 80% of the time. Overall, brown widows were six times more likely to kill black widows than the other two spider species. 

In separate experiments, the team also showed that brown widows produce more offspring than black widows and that those offspring begin to reach maturity faster than black widows. This could explain why sub-adult individuals were so adept at killing younger black widows, which in turn would explain why black widow populations are collapsing in areas where brown widows have invaded, the researchers wrote. However, the researchers were surprised at the stark behavioral differences between brown and black widows.

Related: 11 deadliest spiders

"Brown widows are boldly aggressive and will immediately investigate a neighbor and attack if there is no resistance from the neighbor," study co-author Deby Cassill, an ecologist at the University of South Florida (USF), said in a statement. "But the black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider."

The researchers are unsure why the closely related species react so differently to one another and plan to study brown and black widows in other parts of the world, such as Africa, to see if the same trends apply.

"I would love to see if their [brown widows'] 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," study lead author Louis Coticchio, a doctoral student of conservation biology at USF, said in the statement.

Harry Baker
Staff Writer

Harry is a U.K.-based staff writer at Live Science. He studied Marine Biology at the University of Exeter (Penryn campus) and after graduating started his own blog site "Marine Madness," which he continues to run with other ocean enthusiasts. He is also interested in evolution, climate change, robots, space exploration, environmental conservation and anything that's been fossilized. When not at work he can be found watching sci-fi films, playing old Pokemon games or running (probably slower than he'd like). 

  • leefi1
    When we lived in Las Vegas, we lived a block from the strip in a glorious California style bungalow surrounded by high adobe walls. Working in my garden, I found a black widow, then more and more of them, the rough walls were home to endless numbers. They were everywhere. I called an exterminator who solved the problem. He told us that if I found 35 of them, that there were probably ten thousand. Since we began watering the desert black widows had exploded in numbers all across the desert Southwest. Since the bite of a black widow can result in nausea, vomiting, muscle rigidity and other assorted ghastliness, I am very happy about this invasive species! They can replace black widows in their biological niche, catch and eat insects and never send anyone to the hospital...
    I was been bitten by a venomous snake at age ten. I bear no ill will in general about natural defenses used in times of perceived danger. But the vast numbers of black widows in the desert SW is not in any way normal, nor is our living there!