Rocky Mountain wood tick (Dermacentor andersoni)

How to identify it

• Adults are brown to reddish-brown with an ornate silver-gray (marbled) pattern on the scutum, the hard shield behind the head.
• Short, stubby mouthparts and a roughly rectangular mouthpart base (basis capitulum) — typical of Dermacentor 'ornate' ticks, unlike the long mouthparts of the Lone Star tick.
• Adult female: the silver-gray shield covers only the front third of the back, leaving the rest reddish-brown; when engorged she balloons and turns slate-gray while that small shield stays.
• Adult male: the marbled silver-gray/white pattern covers the entire back (no bare reddish rear), so males look uniformly mottled.
• Festoons — small rectangular notches — run along the rear edge of the body.
• Definitive separation from the near-identical American dog tick relies on microscopic spiracular-plate goblet counts (roughly 100–200 larger goblets in D. andersoni versus several hundred small ones in D. variabilis), not on anything visible in the field.

Size: 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 slightly smaller. An engorged female swells to about 1/2–5/8 inch (~1.3–1.6 cm) and turns a grayish slate color.

Where it lives & when

Rocky Mountain region of the western US and southwestern Canada, generally at elevations of about 4,000–10,500 feet. In the US it is found across Montana, Idaho, Wyoming, Colorado, Utah, and Nevada, plus 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 or the eastern/south-central range of the Lone Star tick.

Season: Adults are active from spring into midsummer — roughly March through July (snowmelt through mid-July) — peaking in mid-spring, which is when most human bites and disease transmission occur. Larvae and nymphs feed mainly on small rodents, and adults become less active as the summer dries out.

• 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 quest for passing hosts
• Areas where deer, elk, cattle, and other large mammals travel

Diseases the rocky mountain wood tick can carry

Tick presence is not the same as personal disease risk. Not every bite transmits a pathogen.

Look-alikes

Almost identical to the American dog tick (Dermacentor variabilis), which shares the same brown body and ornate silver-gray shield; their ranges overlap along the eastern edge of the Rockies, and reliable species-level ID requires a microscope. Region is the best field clue — high-elevation Rocky Mountain West points to the wood tick. Both are easy to tell from the Lone Star tick (a single white dot, long mouthparts) and from the much smaller, darker blacklegged (deer) tick.