Below, you will learn about four types of poisonous spiders found in California that are DANGEROUS to people.
Fortunately, the species below only bite when disturbed or provoked. It’s important to remember that spiders would rather run away from you than bite. If you encounter one of the spiders listed below, please LEAVE THEM ALONE!
Lastly, I think it’s interesting to know that, technically, almost ALL spiders are venomous. This is because they have poison glands that release venom to help subdue their prey, typically small insects. Luckily, most species’ venom does not affect humans.
For today’s post, I found the types of spiders that are known to be very dangerous to people when they do bite, or at least “medically significant.”
Venomous Spiders in California:
#1. Black Widow
- Females are 6 to 19 mm long—shiny black with a distinctive red hourglass-shaped mark. A row of red spots is sometimes visible above.
- Males are half the size of females.
- Bristles on their hind legs, which they use to cover their prey with silk once it has been trapped.
Black Widows are the most venomous spiders in California!
In addition, they are probably the most popular and recognizable spiders in the world. Almost everyone can recognize the red-shaped hourglass mark that appears on the females.
But even though they have highly toxic venom, 15 times stronger than a rattlesnake, they are not aggressive. Black Widows only bite when in danger or if their web is disturbed. They RARELY bite humans.
But, if you are one of the few unlucky people who are bitten, you should go to the hospital immediately. The venom is dangerous, as it affects your nervous system.
Deaths to healthy adults are VERY rare, but the neurotoxic venom can be fatal to small children if untreated.
Only adult females have chelicerae (mouthparts) long enough to deliver venom, as males are too small to pierce our skin.
The best places to look for these venomous spiders in California include overhanging ledges, woodpiles, under benches or stones, near entrances to abandoned rodent burrows, or around outbuildings. Inside your home, they can be found in dimly lit locations, such as dark corners, closets, or other cluttered areas. They don’t like moisture and prefer dry areas.
Black Widows have a bizarre mating ritual. It begins with the male coming to the female’s web and destroying it. This reduces the female’s pheromones and makes the web less attractive to other males.
The females do not seem to mind the destruction of their property, and it actually seems to make them more receptive to the male. Even so, sometimes, the female kills and eats the male after mating. This gruesome behavior is what gave these poisonous spiders their name.
#2. Brown Widow
- Latrodectus geometricus
- Females are .6 to 16 mm in length and are light brown. The abdomen has an orangish hourglass-shaped mark.
- Males are roughly half the size of females. Look for an orange spot on the abdomen that may be broken into two spots.
- The legs of both sexes are light brown with dark brown near the joints.
- Also known as the Grey Widow
These venomous spiders are not native to California.
Brown Widows are believed to have originated in either Africa or South America. But now, they are found across the world, typically near buildings in warmer areas.
In areas with both species, researchers show that Brown Widows are outcompeting and potentially displacing Black Widows. Brown Widows employ a neurotoxic venom, causing pain, muscle rigidity, vomiting, and sweating. However, while deadly to their prey, the bites of the Brown Widow are typically much less harmful to humans than the infamous Black Widow.
In addition, they are timid and rarely bite. In the rare case that a bite does occur, the symptoms include high blood pressure, sweating, vomiting, respiratory muscle weakness, and possible seizures. Sometimes you can develop a very stiff abdomen that may mimic appendicitis. The worst pain is generally in the first 8-12 hours.
One of the easiest ways to identify these dangerous spiders in California is to look for their egg sacs. They have pointy protrusions and are frequently referred to as “fluffy” or “spiky” in appearance.
#3. Black-Footed Yellow Sac Spider
- Cheiracanthium inclusum
- Pale yellow-beige color with dark brown markings on its palps, jaws, and the ends of its feet.
- An orange-brown stripe often runs down the top center of its abdomen.
- Also known as the American Yellow Sac Spider. As you can see, it gets its name from its appearance.
Yellow Sac Spiders are venomous and capable of biting humans.
Their bite begins with a bit of pain followed by itching and swelling. Symptoms from the bite typically are gone within 7-10 days. On the bright side, Yellow Sac Spiders rarely bite!
The severity of one of their bites is not nearly as bad as a Black Widow. A study of twenty confirmed Yellow Sac Spider bites found that none resulted in necrosis (decaying of tissue due to cells dying). Instead, it seems like most bites just caused swelling and localized pain.
These venomous spiders are most often found in California on the foliage of trees and shrubs. But it’s not out of the question to find one in your house or other manufactured structures. Honestly, it’s pretty uncommon to actually see a Yellow Sac Spider unless you’re actively searching for one. The reason is that they are nocturnal and only come out at night to feed and mate.
Lastly, in 2011, Black-footed Yellow Sac Spiders caused quite a big problem for Mazda. The automobile company had to recall 65,000 Mazda 6 vehicles after discovering that webs from this species were clogging fuel system ventilation tubes. It seems that the spiders were attracted to the smell of gasoline, but it’s unclear why ONLY this particular model was affected!
#4. Desert Recluse
- Loxosceles deserta
- Adults are between 6 – 13 mm long.
- They are tan to dark brown. Look for a darker fiddle-shaped marking on the top.
- They only have six eyes, as opposed to eight on most other species, arranged in three groups of two.
Look for this venomous spider in the deserts of California.
Desert Recluses are commonly mistaken for the more dangerous Brown Recluse, even though these two species do not share the same range or habitat.
Desert Recluse Range Map
The venom from a Desert Recluse is necrotic. Their bites cause lesions and ulcers on the skin that can take several weeks to heal. Make sure to head to the hospital if you suspect you were bitten by this dangerous spider.
Luckily, Desert Recluses are rarely found indoors and avoid urban areas in the desert. Instead, the best places to find them are in the wild, far away from people, near native vegetation, or in packrat dens.
*WARNING – If you are bitten in California and think the spider is venomous, please go to the hospital!
If you have an allergic reaction, do not use this article to try to identify the spider. If possible, it is beneficial to try to capture the spider that has bitten you so they can determine the correct treatment.
Did you know that spiders are considered venomous, NOT poisonous?
There’s a common misconception between poisonous and venomous; one is ingested, and the other is injected. So, for example, If you eat something that makes you sick, it is considered “poisonous.” But if an animal, like a spider, delivers its toxins when it bites, then it’s considered “venomous.”
Check out these other guides to animals found in California!
Which of these venomous spiders have you seen in California?
Leave a comment below!