Rocky Mountain wood tick (Dermacentor andersoni)
The Rocky Mountain wood tick (Dermacentor andersoni) is a hard tick of the mountainous western United States and southwestern Canada. It is a patterned Dermacentor tick, brown with a silver-gray shield. It is the main tick people meet at mid-elevations across the Rocky Mountain West. It can spread Rocky Mountain spotted fever, Colorado tick fever, and tularemia. A female that stays attached a long time can also cause tick paralysis. It lives in a very different region from the Lone Star tick and carries different diseases. It does not spread Lyme disease. This page is for learning and is not medical advice.
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How to identify it
- Adults are brown to reddish-brown with a marbled silver-gray pattern on the scutum, the hard shield behind the head.
- Short, stubby mouthparts and a roughly rectangular mouthpart base (basis capitulum), typical of patterned Dermacentor ticks. The Lone Star tick has long mouthparts.
- Adult female: the silver-gray shield covers only the front third of the back, and the rest is reddish-brown. When fully fed she balloons and turns slate-gray, while the small shield stays the same.
- Adult male: the marbled silver-gray and white pattern covers the whole back, with no bare reddish rear. Males look evenly mottled.
- Festoons (small rectangular notches) run along the rear edge of the body.
- Telling it for sure from the near-identical American dog tick takes a microscope. Experts count tiny structures called goblets on the breathing plates (spiracular plates). D. andersoni has roughly 100–200 larger goblets, and D. variabilis has several hundred small ones. Nothing you can see in the field will tell them apart.
Size by life stage
Larvae are pinhead-sized (~0.5 mm) with six legs. Nymphs are about a poppy seed (~1–1.5 mm) with eight legs. Unfed adult females are roughly 1/8 inch (~3–5 mm), and males are a little smaller. A fully fed female swells to about 1/2–5/8 inch (~1.3–1.6 cm) and turns a grayish slate color.
Where it lives and when
It lives in the Rocky Mountain region of the western US and southwestern Canada. It is usually found at about 4,000–10,500 feet. In the US it is found across Montana, Idaho, Wyoming, Colorado, Utah, and Nevada. It also lives in eastern Oregon and Washington, western Nebraska and the Dakotas, and higher elevations of northern Arizona and New Mexico. It is not found in the Southeast. It is also not found in the eastern and south-central range of the Lone Star tick.
Season
Adults are active from spring into midsummer, roughly March through July (snowmelt through mid-July). They peak in mid-spring, when most human bites and disease spread happen. Larvae and nymphs feed mainly on small rodents. Adults become less active as the summer dries out.
Where it waits
- Sagebrush, shrublands, and open grasslands at mid-elevations
- Lightly wooded foothills and brushy areas along trails
- Low grass, brush, and plants from ground level up to about 3 feet, where adults wait for passing hosts
- Areas where deer, elk, cattle, and other large mammals travel
Diseases it can carry
Tick presence is not the same as personal disease risk. Not every bite transmits a pathogen.
Look-alikes
It is almost identical to the American dog tick (Dermacentor variabilis), which has the same brown body and silver-gray patterned shield. Their ranges overlap along the eastern edge of the Rockies. Telling the species apart reliably takes a microscope. Region is the best clue in the field: high elevations in the Rocky Mountain West point to the wood tick. Both are easy to tell from the Lone Star tick (a single white dot, long mouthparts). They are also easy to tell from the much smaller, darker blacklegged (deer) tick.
Compare all tick speciesCommon questions
What diseases does the Rocky Mountain wood tick carry?
Rocky Mountain spotted fever (RMSF), Colorado tick fever, and tularemia. A female that stays attached for several days can also cause tick paralysis. This is a weakness that goes away once the tick is fully removed. It does not spread Lyme disease. RMSF is the most dangerous. Symptoms usually start about 2–14 days after a bite, and fast treatment with doxycycline is critical. See a doctor quickly for fever and rash after a tick bite.
How do I tell it apart from the American dog tick?
Often you can't by eye. Both are brown with a silver-gray patterned shield. The best clue in the field is location. The Rocky Mountain wood tick lives at higher elevations across the Rocky Mountain West. The American dog tick is common across a much wider area east of the Rockies. A sure ID needs a microscope to compare the breathing plates.
Where in the United States does it live?
In the Rocky Mountain states and foothills, at roughly 4,000–10,500 feet. That includes Montana, Idaho, Wyoming, Colorado, Utah, and Nevada. It also includes eastern Oregon and Washington, western Nebraska and the Dakotas, and higher parts of northern Arizona and New Mexico. It is not found in the Southeast.
What is tick paralysis?
It is muscle weakness caused by a toxin, not an infection. The toxin (a neurotoxin) is in the saliva of an attached female tick. The weakness spreads upward, usually starting in the legs. It usually improves within roughly 24–72 hours after the tick is completely removed. Get medical care for any unexplained weakness after a tick bite.
Sources
- 1.Where Ticks Live (opens in a new tab) · CDC
The Lone Star tick is widely distributed in the Northeast, South, and Midwest U.S.
- 2.Rocky Mountain Spotted Fever (RMSF) (opens in a new tab) · CDC
Spread by American dog, Rocky Mountain wood, and brown dog ticks. Most US cases are in South-Central states; can be fatal without prompt doxycycline.
- 3.
- 4.Tularemia (opens in a new tab) · CDC
Bacterial illness that can be spread by tick bites (including the Lone Star tick) and other routes.
- 5.TickEncounter Resource Center (opens in a new tab) · University of Rhode Island
Tick identification, prevention, and reporting resources.
- 6.
Educational information only, not medical advice. If you were bitten, use the tick bite tool and contact a licensed clinician if symptoms develop. Read our sources and disclaimer.