Venomous snakes and spiders in Colorado

Colorado's venomous snakes are three rattlesnake species. Together they account for 1,392 licensed records; the prairie rattlesnake alone has 95% of them, and the least recorded, the western massasauga, has 17.

One widow species (Western black widow: 167 records) is on the Colorado list. There is no recluse spider on the Colorado list: no recluse species reaches 10 records in the state. Widow bites cause muscle pain and cramps; recluse bites can cause a slow-healing wound. Both are rare and seldom fatal.

Yes. Two scorpion species are recorded in Colorado, from 109 licensed records: Striped bark scorpion (50 records), Northern scorpion (27 records). Every scorpion has venom; only the Arizona bark scorpion's sting is medically significant, and it is not on the Colorado list.

Beyond the main list, four scorpion species are known from Colorado by only 3 to 9 records each. Some are genuine rarities at the edge of their range; others may be strays, released pets or old misidentifications, so treat them as unconfirmed.

These eight account for 53% of all licensed snake records in Colorado; the most recorded, the western terrestrial garter snake, alone for 24%.

These eight account for 46% of all licensed spider records in Colorado; the most recorded, the bold jumper, alone for 16%.

Observation records of venomous snakes in Colorado run mostly May through September, peaking in May with 25% of them. December through February together hold under 2% of the records, so venomous snakes in Colorado are rarely recorded in winter.

Observation records of spiders in Colorado run mostly April through October, peaking in August with 16% of them. Winter (December to February) holds 2% of the records.

Observation records of scorpions in Colorado run mostly April through September, peaking in August with 36% of them. December through February together hold under 2% of the records, so scorpions in Colorado are rarely recorded in winter.

Weld County has the most records of venomous snakes in Colorado, 850, most of them of the prairie rattlesnake. Moffat County and Baca County follow.

Weld County dominates: 26% of the state's licensed snake records come from it alone, and the top five counties hold 54%. 63 of Colorado's 64 counties have at least one licensed snake record.

Colorado counties with their own page: Boulder County, El Paso County, Larimer County and Weld County.

Yes. Colorado has three venomous snake species on its list, the kind people usually call poisonous: prairie rattlesnake, western rattlesnake and western massasauga. Strictly, snakes are venomous (they inject venom), not poisonous (harmful to eat or touch).

Yes. Three species are on the Colorado list: the prairie rattlesnake, the western rattlesnake and the western massasauga, with 1,392 licensed records between them. Most licensed records of the prairie rattlesnake come from Weld County, Moffat County and Baca County.

No. GBIF holds no copperhead record for Colorado under any license.

No. GBIF holds no cottonmouth record for Colorado under any license.

No. GBIF holds no coral snake record for Colorado under any license.

Colorado has 1,328 licensed records of the prairie rattlesnake, which ranks the state 1st of the 14 states where it reaches our threshold. Its observation records in Colorado run mostly May through September, peaking in May; most licensed records come from Weld County, Moffat County and Baca County. Range in other states and photos: Prairie rattlesnake profile.

Colorado has 47 licensed records of the western rattlesnake, which ranks the state 8th of the 9 states where it reaches our threshold. Most of its licensed records in Colorado come from Rio Blanco County, Moffat County and Mesa County. Range in other states and photos: Western rattlesnake profile.

Colorado has 17 licensed records of the western massasauga, which ranks the state 7th of the 8 states where it reaches our threshold. Range in other states and photos: Western massasauga profile.

Many snakes and spiders taken for venomous ones are harmless species with a similar look. These are the usual confusions, with each species' licensed records in Colorado.

No copperheads reach the Colorado list, and GBIF holds no copperhead record for Colorado under any license, so a snake taken for one here is most likely a harmless lookalike. Those recorded in the state: northern water snake (42), central plains milksnake (29) and eastern milksnake (8).

No cottonmouths reach the Colorado list, and GBIF holds no cottonmouth record for Colorado under any license, so a snake taken for one here is most likely a harmless lookalike. Those recorded in the state: northern water snake (42).

Rattlesnakes: 1,392 licensed records in Colorado. Harmless species often taken for them are also on the state list: gopher snake (365), plains hog-nosed snake (45) and Great Plains rat snake (5). Together they have 415 licensed records, and here the venomous species are the more recorded side, 3.4 to 1.

No coral snakes reach the Colorado list, and GBIF holds no coral snake record for Colorado under any license, so a snake taken for one here is most likely a harmless lookalike. Those recorded in the state: central plains milksnake (29), eastern milksnake (8) and long-nosed snake (3).

Widow spiders: 167 licensed records in Colorado. Harmless species often taken for them are also on the state list: triangulate bud spider (49), twospotted cobweb spider (7), boreal combfoot (1) and false black widow (1). Together they have 58 licensed records, and here the venomous species are the more recorded side, 2.9 to 1.

Recluse spiders are not on the Colorado list, but they are recorded in Colorado 20 times (records of any license: brown recluse 18, Apache recluse 2): treat any spider like this as venomous until identified. Harmless species often taken for them in the state: hobo spider (29), longbodied cellar spider (5), spitting spider (3) and Pennsylvania grass spider (1).

By count of venomous snake species, Colorado ranks 20th of 50 states, level with 8 other states. Nationally, Arizona leads with 14 species; five list none.

Of the seven states bordering Colorado, Arizona, New Mexico, Oklahoma, Kansas, Utah and Nebraska list more venomous snake species (Colorado: 3 species; Arizona 14, New Mexico 10, Oklahoma 8, Kansas 6, Utah 5, Nebraska 4, Wyoming 2).

Venomous snakes listed next door but not in Colorado: the eastern copperhead (Kansas, Nebraska and Oklahoma); the Mohave rattlesnake (Arizona, New Mexico and Utah); the timber rattlesnake (Kansas, Nebraska and Oklahoma); the western diamondback rattlesnake (Arizona, New Mexico and Oklahoma); the Arizona black rattlesnake (Arizona and New Mexico); the black-tailed rattlesnake (Arizona and New Mexico). The most widely shared is the prairie rattlesnake, listed in all seven bordering states.

Yes. One widow species is on the Colorado list: the western black widow (167 records).

Not on our list: GBIF holds 18 brown recluse records of any license in Colorado (brown recluse 18), but they are filed under names that here stand for another species or lie outside the species’ known range, so they are not counted (see the methodology); such records can be genuine rarities, strays, transported animals or misidentifications. Treat any spider that looks like a recluse as venomous.

Colorado has 167 licensed records of the western black widow, which ranks the state 3rd of the 18 states where it reaches our threshold. Its observation records in Colorado run mostly May through November, peaking in August; most licensed records come from El Paso County, Denver County and Boulder County. Range in other states and photos: Western black widow profile.