Brown widow spiders have not established populations in Colorado. The state’s primary widow spider is the western widow, a close relative of the more familiar black widow. If you’ve spotted a brown-colored spider with an hourglass marking in Colorado, you’re most likely looking at an immature western widow or a different species entirely. Here’s how to tell the difference and what to know about both.
Why Brown Widows Are Unlikely in Colorado
Brown widows are a tropical and subtropical species that thrive in warm climates. They’ve spread across the southern United States, particularly in states like Florida, Texas, California, and along the Gulf Coast. Colorado’s cold winters and high elevation make it inhospitable for brown widows, which cannot survive sustained freezing temperatures. There are no confirmed established populations in the state.
That said, individual brown widows occasionally turn up outside their normal range by hitchhiking in shipped goods, furniture, or potted plants. A single spider arriving in a box from southern California is possible but very different from an established breeding population. If you think you’ve found one, the egg sac is the easiest way to confirm: brown widow egg sacs are tan, roughly oval, and covered in small spiky tufts of silk that make them look like a dried thistle or a tiny sea urchin. No other spider in Colorado produces anything like it.
What You’re Probably Seeing Instead
Colorado is home to the western widow spider, and younger females are commonly mistaken for brown widows. Immature western widows are gray or pale brown with banding patterns on their legs and abdomen. They darken as they mature, eventually becoming the shiny black most people associate with widow spiders. If you’ve found a brownish spider with faint hourglass markings, leg bands, and lighter spots on its body, it’s very likely a young western widow going through its normal color progression.
The distinguishing feature of all widow spiders is the hourglass pattern on the underside of the abdomen. On western widows, this marking is highly variable. It can appear as two disconnected spots, a roughly rectangular shape, or be so faint it’s barely visible. On brown widows, the hourglass is typically orange or yellow rather than red. Color alone isn’t always reliable for identification, which is why the egg sac shape is the most definitive clue.
How Brown Widow Bites Compare to Black Widows
Even if a brown widow did show up in Colorado, the medical risk is far lower than with a western or black widow. Drop for drop, brown widow venom is similarly toxic to other widow species. But brown widows inject significantly less venom per bite, likely because of their smaller size and smaller venom glands.
A study of 15 verified brown widow bites in Africa found that none of the victims developed the classic symptoms of a widow spider bite, such as spreading muscle pain, cramping, and chest tightness. The two main symptoms were pain at the moment of the bite and a red mark afterward, which is roughly what you’d expect from any common household spider. One case report does describe a more severe reaction that required hospitalization, so more serious bites are possible but appear to be rare.
Western widows, by contrast, produce a neurotoxin that causes genuine systemic symptoms. Muscle pain, chest tightness, and cramping are the most common reactions. These bites are rarely fatal in healthy adults but can be quite painful and may require medical treatment.
Managing Widow Spiders in Colorado
Whether you’re dealing with western widows or a suspected brown widow, the management approach is the same. Physical removal is the most effective method. Widow spiders prefer dark, undisturbed spots near ground level: woodpiles, animal burrows, gaps in foundations, storage areas, and the undersides of outdoor furniture. Wearing heavy leather gloves while clearing these areas protects you during the process. Crushing or vacuuming the spider and its web is straightforward and immediately effective.
Widow spiders rarely bite when they’re not in their web, so the risk comes from accidentally pressing against one while reaching into a dark space. Females are more defensive when guarding an egg sac and more likely to bite in that situation.
If you find egg sacs, how you treat them matters. Research from the National Pest Management Association found that standard water-based insecticide sprays failed to kill developing spiders inside egg sacs. The silk acts as a barrier, and spiderlings emerged from treated egg sacs at the same rate as untreated ones. Oil-based aerosol products, however, penetrated the silk far more effectively, delivering the active ingredient to over 99% of the eggs inside and preventing spiderling emergence in 94 to 100% of cases. If you’re spraying egg sacs rather than physically removing them, an oil-based aerosol product is the only type likely to work.
For longer-term prevention, applying insecticide around the exterior foundation and lower-story windows can reduce spider migration into your home. Reducing clutter in garages, sheds, and crawl spaces eliminates the kind of quiet, dark habitat widows prefer.
How to Identify a Widow Spider in Colorado
Here’s a quick comparison of what you might encounter:
- Western widow (common in Colorado): Mature females are shiny black, about 1/4 to 1/3 inch in body length, with a red or red-orange hourglass on the underside. Immature females are brown or gray with banding. Egg sacs are smooth and rounded.
- Brown widow (not established in Colorado): Adults range from light brown to dark brown, occasionally nearly black. The hourglass is orange or yellow. Legs have brown banding, and the abdomen often has white spots or stripes. Egg sacs are tan and covered in spiky silk tufts.
If you’re unsure what you’ve found, your local Colorado State University Extension office can help with identification. Capturing the spider in a jar (or taking a clear photo of the underside and any egg sacs) gives an entomologist enough to work with.