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How to identify it
- Mature females are about 1/2 inch (13 mm) in body length, shiny jet black with a red hourglass under the abdomen; very well-fed females can look brownish. Sizes differ by source: UC gives about 1/2 inch body length, while Colorado State says mature females usually reach 1/4 to 1/3 inch. Colorado State adds that the hourglass varies and may be two separate spots, a rough rectangle or barely visible.[1][3]
- Young spiders are tan and white with stripes, darkening to black as they molt; males keep the striped juvenile pattern and are much smaller. Colorado State says males are one-half to one-third the size of females, about 1/8 to 1/4 inch, light brown or gray and banded, with large knob-like palps when mature.[1][3]
- Egg sacs are yellowish, teardrop or pear shaped and very tough; Colorado State describes them as light whitish yellow to brown.[1][3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Brown widow | Adult females look like immature western black widows; the brown widow's spiky, pollen-grain-like egg sac is the surest way to tell them apart. In urban southern California, brown widows now greatly outnumber western black widows around homes and parks, while black widows still predominate in dry, natural habitats.[1][6] |
| False black widow (Steatoda) | Chocolate brown with no red marking. Colorado State adds that Steatoda cobweb spiders often have white markings on top of the abdomen and a white band around its front edge.[1][3] |
Where it lives
From British Columbia and Alberta to Mexico and east to the western Great Plains; in California it lives in deserts and also in mountains above 5,000 feet. Around homes, females may be found every few feet in cracks, clutter and rock walls.[1]
Biology and life cycle
A female typically lays about 300 eggs per sac and can make more than 10 sacs from a single mating. Colorado State gives about 200 eggs per sac and says females may make several sacs in a lifetime. Eggs hatch in about two weeks, and spiderlings molt once inside the sac before leaving.[1][3]
Webs are tangled silk with strong vertical support lines; silk of a mature female rips audibly when you run a finger through it. Females usually stay in the web, which is usually near the ground, often within three feet of it, and come out at dusk to wait for prey.[1][3]
Females typically mature in four to six months; males develop faster and usually live only a few weeks, while widows can live two years or more under good conditions. Colorado State says many move into homes by autumn.[3]
Despite the name, Colorado State says females eating males is not the norm, especially when the female is well fed.[3]
Damage and signs
- In garages they build webs by doors and vents where insects pass; bites happen mostly when people clean out or pick up items in such places. Colorado State says most bites happen when someone presses on a spider resting under a rock or a piece of wood, and that widows rarely, if ever, bite away from the web; a female guarding an egg sac is more likely to bite.[1][2][3]
Health risk
Bites may feel like a pinprick or go unnoticed. MedlinePlus says dull muscle pain spreads from the bite within 15 minutes to an hour, felt mostly in the chest for upper-body bites and in the abdomen for lower-body bites, and severe cramping can be mistaken for appendicitis or a heart attack; NIOSH also lists sweating, nausea, headache and high blood pressure. Call 911 or Poison Help (1-800-222-1222) right away. Severe symptoms usually improve in 2 to 3 days, and death in a healthy person is very rare. Antivenin may be given to children, pregnant women and older people, but it can cause serious allergic reactions.[4][5]
Control
Prevention
Wear gloves when handling stored items and wood, and store garage items in sealed bags or taped boxes off the floor and away from walls. Colorado State suggests heavy leather gloves and a long-sleeved shirt when working where widows live. UC adds checking picnic tables, trash can grips, lawn chairs, plant pots with curled lips and molded plastic toys.[1][3]
Repellents such as mock orange or garlic and ultrasonic devices don't work on spiders, according to Colorado State.[3]
Keep them out
Seal cracks in the foundation and gaps around windows and doors, keep screens in good repair, and point outdoor lights away from doors and windows so they attract fewer insects.[2][1]
Sanitation
Remove clutter, woodpiles, leaf litter and heavy vegetation near the foundation, and vacuum or sweep webs and spiders regularly; vacuumed spiders usually don't survive. Colorado State says webs and spiders can be crushed or vacuumed, and egg sacs crushed or frozen for several weeks.[2][1][3]
Traps and physical control
Search with a flashlight at night, when widows sit in their webs, and remove them; repeat through the year because spiderlings balloon in on the wind.[1]
Pesticides
Insecticides don't give long-term spider control, and UC says they generally shouldn't be used outdoors. Colorado State agrees that physical removal works best but says a foundation spray before cold weather can briefly reduce spiders moving indoors, and that insecticides do little against egg sacs.[2][1][3]
If someone is bitten
MedlinePlus says to seek medical help right away and call Poison Help for guidance. Until help arrives, wash the bite with soap and water, put ice wrapped in a cloth on it for 10 minutes on and 10 minutes off, keep the bitten limb still, and remove rings and tight jewelry. If you can, bring the spider in a secure container.[4]
Common questions
- Are all black spiders black widows?
- No. The false black widow is dark brown with no red, and young black widows aren't black at all. Colorado State notes that young widows and males have red, orange or yellow spots and stripes on the abdomen.[1][3]
- Do male black widows bite?
- Almost never. Colorado State says bites are nearly always from females, and that males, with smaller fangs and venom glands, are unlikely to get through adult human skin.[3][1]
- What does a black widow bite feel like?
- Often like a pinprick, and some people don't feel it. Within an hour, muscle pain can spread from the bite to the chest or abdomen. Get medical help and call Poison Help at 1-800-222-1222.[4]
Sources
- University of California Statewide IPM Program, “Pest Notes: Widow Spiders and Their Relatives, Publication 74149 (revised March 2017).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnblackwidow.pdf (accessed 2026-11-11)
- University of California Statewide IPM Program, “Pest Notes: Spiders, Publication 7442.” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnspiders.pdf (accessed 2026-11-11)
- Colorado State University Extension, “Western Widow Spider.” extension.colostate.edu/resource/western-widow-spider/ (accessed 2026-11-11)
- MedlinePlus, U.S. National Library of Medicine, “Black widow spider (Medical Encyclopedia).” medlineplus.gov/ency/article/002858.htm (accessed 2026-11-11)
- CDC, National Institute for Occupational Safety and Health (NIOSH), “Venomous Spiders at Work.” www.cdc.gov/niosh/outdoor-workers/about/venomous-spiders.html (accessed 2026-11-11)
- Journal of Medical Entomology, via PubMed, “Vetter et al. (2012), The prevalence of brown widow and black widow spiders in urban southern California, J Med Entomol 49(4):947-951.” pubmed.ncbi.nlm.nih.gov/22897057/ (accessed 2026-11-11)
- iNaturalist, “Latrodectus hesperus taxon page (order and family).” www.inaturalist.org/taxa/47382 (accessed 2026-11-11)