Brown widow spiders have not established outdoor populations in Colorado, and the state’s cold winters make it unlikely they will any time soon. Colorado sits well outside the brown widow’s current range in the United States, which is concentrated in warmer southern and coastal states like California, Florida, Texas, and the Gulf Coast region. That said, the species has been steadily pushing into higher latitudes over the past two decades, and isolated individuals can hitch rides on shipped goods into virtually any state. Understanding why Colorado remains inhospitable to brown widows, and what you should actually watch for, requires a look at the spider’s biology and the climate barriers it faces.
Why Colorado’s Climate Is a Problem for Brown Widows
Brown widows originated in warmer climates and are considerably less cold-tolerant than the native widow spiders already living in Colorado. Research measuring the thermal limits of different widow species found that the brown widow’s lower lethal threshold sits around 3.2°C (roughly 38°F), while the western black widow, the species you’re most likely to encounter in Colorado, can survive temperatures down to about −1.9°C (around 28°F).1The Journal of Arachnology. Thermal tolerances of different life stages, sexes, and species of widow spiders (Araneae: Theridiidae) That gap matters a great deal when winter nighttime lows in much of Colorado drop far below freezing for months at a stretch. The same study found that the single best predictor of where brown widows actually live is minimum temperature, confirming that cold is the main barrier keeping them out of northern and interior states.
Colorado’s combination of high elevation, dry air, and sustained winter cold creates conditions that would kill exposed brown widows. Even along the Front Range, where cities like Denver and Colorado Springs have relatively mild winters by Rocky Mountain standards, overnight lows regularly dip below 0°F (−18°C) in January. That is far below what a brown widow can tolerate outdoors.
The Urbanization Loophole
Climate alone doesn’t tell the whole story. One of the more interesting findings about brown widow expansion is that urban environments can partially override climate limitations. A study modeling the ecological factors behind brown widow invasion success in North America found that while native widow species are strongly constrained by climate, brown widows are more strongly constrained by the availability of urban habitat. Cities with heated structures, sheltered microhabitats, and artificial lighting create pockets where the effective climate is warmer and more stable than the surrounding landscape.2SHAREOK. Modeling ecological impacts and role of urbanization on invasion success of brown widows (Latrodectus geometricus) in North America
This means a brown widow that arrived in Colorado inside a shipping container or a piece of outdoor furniture could survive for a while in a heated garage, warehouse, or greenhouse. Individual spiders turning up indoors in states well outside their normal range is not unheard of. But surviving indoors as a stowaway is very different from establishing a breeding population. To actually take hold, the spiders would need enough individuals to find mates, consistent access to prey, and shelter from Colorado’s cold that extends beyond a single heated room. So far, no evidence suggests this has happened.
What Colorado Does Have Instead
Colorado is home to the western black widow, which is well adapted to the state’s climate and found in areas ranging from the eastern plains to foothill communities. The western black widow is the spider most Coloradans are thinking of when they worry about venomous spiders around their property. It builds messy, irregular webs in sheltered spots like woodpiles, crawl spaces, meter boxes, and the undersides of outdoor furniture.
If you find a widow spider in Colorado, it is almost certainly a western black widow rather than a brown widow. The two look different enough that you can tell them apart with a little practice. Western black widows are jet black and glossy, with the well-known red hourglass marking on the underside of their abdomen. Brown widows, by contrast, have a mottled tan-to-brown body with darker markings on the legs and an orange or yellowish hourglass rather than a bright red one. Their egg sacs are also distinctive: brown widow egg sacs look like small spiky balls, almost like a sandspur, while black widow egg sacs are smooth and round.
How Brown Widows Have Spread Across the U.S.
Brown widows were first documented in the United States in the early twentieth century, but their spread has accelerated dramatically in recent years. A synthesis of over 5,000 occurrence records from 30 countries found that the vast majority of documented sightings (about 98%) occurred in just the past two decades, and the species has been progressively appearing at higher latitudes over time.3BioOne Complete. Patterns of the introduction, spread, and impact of the brown widow spider, Latrodectus geometricus (Araneae: Theridiidae), in the Americas More than 90% of those records came from citizen-science platforms like iNaturalist, which means regular people photographing spiders in their backyards have been the primary way scientists track the invasion.
The species is now well established in southern California, Florida, the Gulf states, and parts of the Southeast. Research on endosymbiont bacteria in brown widows collected across the U.S. confirmed established populations in South Carolina, Florida, Texas, and California, with genetic signatures suggesting multiple independent introductions rather than a single wave spreading outward from one landing point.4Nature / Scientific Reports. Endosymbiont diversity across native and invasive brown widow spider populations This pattern of repeated introductions through commerce and shipping is what makes predicting the next colonized area tricky. A spider that arrives via a furniture shipment doesn’t care about the route a natural range expansion would take.
What Brown Widows Do to Native Widows Where They Arrive
In states where brown widows have become established, the consequences for native widow species have been severe. Field surveys and laboratory experiments have shown that brown widows have displaced both the southern black widow in Florida and the western black widow in California from urban habitats.5Annals of the Entomological Society of America. Predation by the Introduced Brown Widow Spider May Explain Local Extinctions of Native Black Widows in Urban Habitats The mechanism isn’t just competition for web sites. Brown widows appear to have roughly twice the reproductive output of southern black widows, their young grow faster and mature earlier, and bolder brown widows were found to be six times more likely to kill and consume native black widows than other cobweb spiders sharing the same habitat.6Oxford Academic. Predation by the Introduced Brown Widow Spider (Araneae: Theridiidae) May Explain Local Extinctions of Native Black Widows in Urban Habitats
This is a genuine ecological concern, not just an abstract worry. In parts of urban southern California, western black widows have largely vanished from areas where they were once common, replaced almost entirely by brown widows. The irony is that the brown widow, while venomous, is generally considered less dangerous to humans than the species it displaces.
How Dangerous Is a Brown Widow Bite, Really
Brown widows belong to the same genus as black widows, and their venom contains related neurotoxins. But there is a meaningful difference in how severe their bites tend to be. Brown widow bites are generally milder than black widow bites, typically causing localized pain, some redness, and perhaps mild nausea. The spiders also tend to inject less venom per bite. That said, more serious reactions do occur. A clinical case report documented a brown widow bite that caused a moderate reaction including rash, pain radiating up the limb, and nausea, features the authors noted were similar to what a black widow bite can produce.7PubMed Central. A Case of Brown Widow Envenomation in Central Florida
Research on the molecular evolution of the key toxin in widow venom, alpha-latrotoxin, found that the amino acid sequence can differ by up to 16% among species within the Latrodectus genus.8Molecular Biology and Evolution. Molecular Evolution of α-Latrotoxin, the Exceptionally Potent Vertebrate Neurotoxin in Black Widow Spider Venom Those differences in toxin structure help explain why bites from different widow species don’t all feel the same. Antivenoms developed against one widow species do show cross-reactivity with others, though medical use of antivenom for widow bites remains inconsistent across different countries and clinical settings.9PubMed. Antivenom treatment in arachnidism For a Colorado resident, the practical point is simple: if you’re bitten by any widow spider and develop symptoms beyond mild local pain, seek medical attention. Don’t try to identify the species first.
Where Brown Widows Like to Build Webs
Brown widows have specific preferences when choosing a spot to set up their webs, and knowing these can help you identify their presence if you live in or travel to an area where they’re established. Laboratory testing found that brown widows strongly prefer tight, acute angles (around 30 degrees) over wider openings, deeper cavities over shallow ones, and textured surfaces like corrugated cardboard over smooth ones.10The Journal of Arachnology. Laboratory refugia preferences of the brown widow spider, Latrodectus geometricus (Araneae: Theridiidae) They also showed a preference for spots that had previously housed another brown widow, suggesting chemical cues play a role in site selection.
In practice, this means brown widows in urban areas gravitate toward the joints and angles of patio furniture, the undersides of railings, inside bucket handles, beneath the lips of plant pots, and in the folds of stacked lawn chairs. These are the same kinds of spots where you’d find western black widows in Colorado, which is another reason visual identification matters. If you’re in a state with brown widows, checking these spots before reaching blindly into them is good practice.
Controlling Brown Widows Where They Do Live
For people in states where brown widows are established, pest management turns out to be trickier than you might expect. Standard water-based pesticide sprays, the kind often used in perimeter treatments around homes, are largely ineffective against brown widow egg sacs. Research testing six water-based sprays and four oil-based aerosol products found that almost all of the water-based products failed to prevent baby spiders from hatching and emerging normally. Oil-based aerosols, by contrast, prevented hatching in 94 to 100% of treated egg sacs.11PubMed. Efficacy of Several Pesticide Products on Brown Widow Spider (Araneae: Theridiidae) Egg Sacs and Their Penetration Through the Egg Sac Silk The reason is physical: oil-based formulations penetrate the silk of the egg sac and reach the eggs inside, while water-based sprays bead up on the silk surface and never get through.
This finding has real implications for pest control strategy. If you hire an exterminator who relies on the standard perimeter spray around your house’s foundation, that treatment may kill adult spiders it contacts directly but will do almost nothing to the egg sacs, which can contain dozens to hundreds of eggs each. For brown widow control, physically removing egg sacs and targeting them with oil-based products is more effective than broad spraying. Reducing clutter in garages, sheds, and patio areas also helps by eliminating the tight-angled hiding spots the spiders prefer.
Natural Enemies That Keep Numbers in Check
Brown widows aren’t without predators and parasites. A small wasp called Philolema latrodecti specializes in parasitizing widow spider egg sacs. In French Polynesia, this wasp was found infesting about 31% of brown widow egg sacs that researchers dissected, and it has been used as a biological control agent against black widows in Hawaii.12Journal of Medical Entomology. Establishment of the Brown Widow Spider (Araneae: Theridiidae) and Infestation of Its Egg Sacs by a Parasitoid, Philolema latrodecti (Hymenoptera: Eurytomidae), in French Polynesia and the Cook Islands However, there’s an interesting wrinkle: comparative research found that the wasp develops more successfully in the egg sacs of native widow species than in brown widow egg sacs, producing more and larger offspring from native hosts. This suggests the brown widow may be partially resistant to this natural enemy, giving it a competitive edge over native widows that are more heavily parasitized.13Biological Invasions. Parasitoid development and superparasitism in invasive versus native widow spider host egg sacs
This dynamic is worth noting because it compounds the displacement problem. In areas where brown widows and native widows coexist, the native species faces both direct predation by brown widows and heavier parasitism by the shared natural enemy. The brown widow, meanwhile, reproduces faster, matures sooner, and loses fewer offspring to the wasp. It’s a lopsided contest.
Should Coloradans Worry
The honest assessment is that brown widows are not a current concern for Colorado residents, but they’re worth keeping on your radar for practical reasons. If you order outdoor furniture, planters, or other items shipped from southern California, Florida, or Texas, a brown widow or its egg sac could arrive in the packaging. This is how isolated introductions happen. If you find a tan-to-brown widow spider with an orange hourglass and spiky egg sacs in a recently received shipment, you can remove it without much drama. A single stowaway in a Colorado winter has essentially no chance of establishing a population outdoors.
The species to pay attention to in Colorado remains the western black widow, which is native, well-adapted to the climate, and present across much of the state from spring through fall. The precautions are the same ones that have always applied: wear gloves when reaching into woodpiles, check shoes left in the garage, and be cautious around sheltered spots in sheds and crawl spaces. Colorado’s cold winters are effective pest control all on their own, at least for the time being.
Climate Change and the Long-Term Outlook
The question that lingers in the background is whether warming temperatures could eventually make Colorado viable for brown widows. The species has been marching to higher latitudes for two decades, and that trend shows no sign of slowing. The thermal tolerance research that pegged the brown widow’s lower limit at around 3.2°C was measuring acute cold tolerance, the temperature at which the spider stops functioning.14The Journal of Arachnology. Thermal tolerances of different life stages, sexes, and species of widow spiders (Araneae: Theridiidae) Sustained winter conditions well below that threshold, as Colorado experiences, would remain lethal even with a few degrees of warming over coming decades.
Where the picture gets murkier is in the urban heat island effect combined with long-term warming. Denver’s urban core is measurably warmer than surrounding areas, and as infrastructure expands and winters shorten, the microclimate in certain sheltered urban spots could eventually become borderline survivable. This is speculative rather than imminent, but it follows the same pattern seen in other invasive species that gradually creep into marginal habitat as conditions shift. For now, though, Colorado’s elevation, aridity, and winter severity remain a solid barrier. The brown widow is a spider of warm, humid, coastal and urban lowlands, and the Front Range is none of those things.