Venomous Animals in the Park

Black Widow spider featuring red hourglass
Western Black Widow Spider

NPS- Montezuma Castle NHS

There are many species of spiders in Idaho and only a few are venomous (where their bite would be a danger to humans). Most spiders are very beneficial and are favored diets of birds and other animals we enjoy watching. Spiders, like all animals in the park, are protected by law. Please watch for them and leave them alone.

Hobo Spiders
NPS fact sheet on hobo spiders explains what to look for and what to do if you get bitten. These species were originally from Europe. It is believed that they were transported to the US via shipping lanes and ended up in Seattle, WA in the late 1920s. They have since expanded slowly throughout the Northwestern US and Western Canada. Their bites have often been confused with the brown recluse; therefore, public awareness of the hobo spider is low.

Black Widow Spiders
The black widow spider (Latrodectus spp.) is notorious for its neurotoxic venom. Adult females are shiny black with a red hourglass marking on the bottom of their abdomens. Males are half the size of the female, usually dark brown and have no hourglass mark. (Black Widow spiders and general pest managment of spiders is a concern at many National Park Service areas.)

Brown Recluse Spiders
Known primarily for the flesh-eating properties of its venom, the brown recluse is a uniform brown color and can be identified by the dark brown, violin-shaped pattern violin that surrounds its six eyes. Fear of the brown recluse bite often leads people to wrongly identify other benign spiders as brown recluse. The brown recluse spider is often encountered hiding in cool dark places during the day -such as inside folded clothes or under logs- but is most active at night. This spider is not aggressive and usually only bites when accidentally pressed against human skin. (thank you to Amistad and their Field Guide to Spiders)

Yellow Sac Spiders
Cheiracanthium inclusum is very common in most of the United States. The yellow sac spider is a cause of many bites in the U.S., and a lot of house spiders are crushed on suspicion of being yellow sac spiders. Its bite correspond in pain to a bite from a wasp. Details about this spider and others can be found at
close up photo of western rattlesnake's head with tongue extended
Western Rattlesnake


There are 2 species of snakes in Idaho that are venomous. Luckily they are rattlesnakes so, often, they will give you warning. Rattlesnakes do not have to rattle before striking, watch for them if you are walking around their home.

Western Rattlesnake
Like other rattlesnakes, you can usually identify the Western Rattlesnake Crotalus oreganus by the triangular head and the rattle at the end of their tail. However, the rattles are not always present—they can break off, or a young snake may not have developed their rattles yet. Locally, western rattlesnakes are usually light brown with darker brown blotches down the middle of their back. However, their colors can vary over a range of shades, and they usually blend in well with their surroundings. (Thank you to Zion National Park for this description of the snake. Bandelier National Monument also has a Fact Sheet on this snake- PDF.)

Prairie Rattlesnake
Crotalis viridis viridis, the Prairie Rattlesnake, an important component of the park’s ecosystem, helps control rodent populations. It is not an aggressive animal, and would prefer to get away from you if given a chance. Yellowstone National Park has an interesting page on this snake, which is one of their park residents as well.

Last updated: June 18, 2017

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