California is home to at least two species of false black widow spiders, and they are among the most common spiders found in homes throughout the state. The classic false black widow (Steatoda grossa) has been established for decades, while a larger relative, Steatoda nobilis (the noble false widow), was first documented in Southern California in 2011, marking the first confirmed population in the Western Hemisphere. Both are European natives, both build messy webs in and around buildings, and both get mistaken for the real thing.
Species Found in California
The spider most Californians call the “false black widow” is Steatoda grossa. It’s slightly smaller than a mature true black widow, with a body that ranges from chocolate brown to a deep plum color. Some individuals have a small white or tan band across the front of the abdomen, but they lack the bright red hourglass marking that defines a true black widow. Their overall shape is similar, bulbous and round, which is the main reason people confuse the two.
The noble false widow (Steatoda nobilis) is a more recent arrival. Discovered in Southern California in 2011 by researchers at UC Riverside, it’s noticeably bigger. Mature females can reach about 5/8 inch in body length (around 14 mm), which makes them significantly larger than Steatoda grossa and closer in size to a true black widow. Before this species showed up, Steatoda grossa was the largest member of its genus in the state. Both species belong to the same family as true black widows (Theridiidae), which explains the strong physical resemblance.
How to Tell Them Apart From True Black Widows
The fastest way to distinguish a false black widow from a true one is color and markings. True black widows (Latrodectus hesperus, the species native to California) are jet black with a glossy sheen and that unmistakable red or orange hourglass on the underside of the abdomen. False black widows are brown, not black. The coloring tends toward dark chocolate, plum, or purplish brown, and while they may have faint markings, nothing resembles the vivid hourglass.
Size is less helpful for identification because the noble false widow overlaps with true black widows in body length. Web structure offers another clue: both false and true widows build tangled, irregular cobwebs rather than the organized orb webs you see in garden spiders, so web type alone won’t settle the question. If you can get a clear look at the spider’s body color without getting too close, brown versus black is your most reliable indicator.
Where They Live
False black widows are indoor spiders more often than not. You’ll find them in garages, sheds, basements, closets, window frames, and behind furniture. They build their irregular, sticky webs in corners and undisturbed spaces where they can ambush small insects. All life stages, from juveniles to adults, can be found in human-made structures year-round, so they don’t disappear seasonally the way some outdoor spiders do.
Outdoors, they favor low-growing plants, bark crevices, rock walls, and the undersides of bridges. They tend to stay close to the ground and prefer sheltered spots with minimal wind. In California’s mild climate, both species thrive without needing to overwinter indoors, though they clearly prefer the stable conditions that buildings provide. One notable behavior: false black widows are reported to capture and prey upon other spiders, including true black widows.
How Dangerous Is the Bite?
False black widow bites are far less serious than true black widow bites. A true black widow injects a neurotoxin that can cause severe muscle cramping, abdominal pain, elevated blood pressure, and systemic symptoms that sometimes require medical intervention. False black widows produce venom too, but it’s considerably less potent. A bite typically causes localized pain, redness, and swelling that resolves within a few days, similar to a bee sting in severity for most people.
The noble false widow has earned a somewhat worse reputation than its smaller cousin, and there are documented cases of bites causing more prolonged pain, mild nausea, or localized swelling that lasts up to a week. Secondary bacterial infections from the bite wound are a more realistic concern than the venom itself. Keeping the bite clean matters more than worrying about the toxin.
Neither species is aggressive. Bites almost always happen when a spider is accidentally pressed against skin, inside a shoe, behind stored items, or in bedding that hasn’t been moved in a while.
What to Do if You’re Bitten
If you suspect a false black widow bite, wash the area thoroughly with soap and water. Apply an ice pack wrapped in a thin cloth (not directly on skin) to reduce swelling and pain. Elevating the bite site, if it’s on a hand or arm, helps limit swelling. Over-the-counter pain relievers can manage discomfort.
The main reason to seek medical attention is if you’re unsure whether the spider was a false or true black widow. If the spider was dark black with a glossy body or you didn’t get a good look, it’s worth being evaluated, because true black widow bites benefit from early treatment. If you’re confident the spider was brown and lacked an hourglass, home care is usually sufficient. Watch for signs of infection over the following days: increasing redness, warmth, or pus at the bite site.
Managing Them in Your Home
Because false black widows eat other spiders, including true black widows, some people choose to leave them alone. If you’d rather not share your space with them, reducing clutter is the most effective long-term strategy. They rely on undisturbed corners and rarely-moved objects to anchor their webs. Regular vacuuming of corners, behind furniture, and along baseboards removes both webs and egg sacs.
Sealing gaps around windows, doors, and utility penetrations limits entry points. Outdoor lighting attracts the insects that false black widows feed on, so switching to yellow or sodium vapor bulbs near doorways can indirectly reduce spider populations. Sticky traps placed along walls and in garages are effective for monitoring and reducing numbers without pesticides. In California’s climate, these spiders are a year-round presence, so periodic maintenance works better than a one-time cleanup.