Western blacklegged tick (Ixodes pacificus)
The western blacklegged tick (Ixodes pacificus) fills the same role on the Pacific Coast that the blacklegged (deer) tick fills in the East. It lives along the US West Coast and is most common in California's coastal and foothill woodlands. In the far West, it is the tick that spreads Lyme disease and anaplasmosis. Adults are small and dark. They have black legs against a brownish body, and a shield that covers only the front half of the back. Nymphs are the size of a poppy seed, so they are easy to miss. They cause many of the region's human infections. This page is for learning and is not medical advice.
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How to identify it
- Adult female: brownish-black legs that stand out against the body (the 'blacklegged' clue). A dark shield (scutum) covers only the front half of the back. Behind it sits a smooth, reddish-orange, teardrop-shaped belly (abdomen).
- Adult male: smaller, oval, and dark brownish-black all over. The shield covers nearly the whole back, with no reddish-orange area.
- Long mouthparts that point forward. No eyes and no festoons (no boxy scalloping along the rear edge). Like all Ixodes ticks, it has a groove on its underside (anal groove) that curves in front of the anus.
- No white dot, streaks, or silvery mottling anywhere. It is plain-colored, which sets it apart from the Lone Star tick and the patterned Dermacentor ticks.
- Nymph: a dark plate on the back over a pale, see-through belly. It is about the size of a poppy seed and often missed.
Size by life stage
Larvae are pinhead-sized (~0.5 mm) with six legs. Nymphs are poppy-seed-sized (~1–2 mm) with eight legs. Unfed adult females are about 3 mm (roughly 1/8 inch). Unfed males are a little smaller (~2–2.5 mm). A fully fed female swells to about 10 mm (3/8 inch) or larger. She turns gray-tan as she fills with blood.
Where it lives and when
Found only on the US Pacific Coast. This is not the Lone Star tick's southeastern range. CDC describes it as living along the Pacific coast, especially in northern California. Established groups live in California, western Oregon, and western Washington. In California, coastal counties and the Sierra Nevada foothills have especially large numbers. A few scattered records come from Nevada, Utah, and Arizona. It is the West Coast counterpart of the eastern blacklegged (deer) tick. It does not live in the South-Central or Southeastern states.
Season
Its timing is mostly the reverse of the eastern deer tick, which peaks in summer. Adults look for hosts mainly from late fall through spring. In coastal California they are most active in the cool, wet winter months. Nymphs are active from late winter into summer and peak from March through July (mid- to late spring). Because nymphs are tiny and bite in the warmer months, they cause much of the human infection risk.
Where it waits
- Cool, humid coastal and foothill forests (oak woodland, Pacific madrone, and Douglas fir stands)
- Leaf litter and fallen fir needles (duff) on the forest floor, where nymphs are most common
- Grassy trail edges, low brush, and chaparral next to brush or forest. Adults climb plants and logs a few feet off the ground there to wait for hosts
Diseases it can carry
Tick presence is not the same as personal disease risk. Not every bite transmits a pathogen.
Look-alikes
Its closest look-alike is the eastern blacklegged (deer) tick, Ixodes scapularis. The two are nearly identical. The main way to tell them apart is location: the Pacific Coast versus the eastern and midwestern US. Within its own range, it can be confused with the Pacific Coast tick (Dermacentor occidentalis) and the American dog tick (Dermacentor variabilis). But those ticks are larger, with shorter mouthparts, a patterned silvery shield, and festoons. The western blacklegged tick has none of these. Tiny unfed nymphs are the hardest to identify. They are often mistaken for a fleck of dirt or a poppy seed.
Compare all tick speciesCommon questions
What diseases does the western blacklegged tick spread?
Lyme disease and anaplasmosis. In the far-western US, it is the main tick that spreads both. CDC also lists it as a carrier of Borrelia miyamotoi, which causes a hard tick relapsing fever. This is for learning only. See a doctor about any illness after a tick bite.
How is it different from the deer tick people talk about back east?
Mostly by where it lives. It is the West Coast counterpart of the eastern blacklegged (deer) tick (Ixodes scapularis), and the two look almost the same. Ixodes pacificus is found along the Pacific coast, especially in California.
Does the western blacklegged tick carry Rocky Mountain spotted fever?
No. In the West, Rocky Mountain spotted fever and related spotted fevers are tied to Dermacentor wood and dog ticks. In the Southwest, the brown dog tick is also involved. Ixodes pacificus is not.
When am I most likely to be bitten?
In winter, spring, and summer. Adults look for hosts in the cool, wet winter and spring. Nymphs, the size of a poppy seed, bite in spring and summer. Because nymphs are so small and easy to miss, they cause much of the region's human infection risk. Check carefully for ticks after time in oak woodland or on shady trails.
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.Lyme Disease (opens in a new tab) · CDC
Spread by blacklegged (deer) ticks, NOT the Lone Star tick. Early sign is often an expanding erythema migrans rash.
- 3.Anaplasmosis (opens in a new tab) · CDC
Spread by blacklegged and western blacklegged ticks; treatable with prompt antibiotics.
- 4.TickEncounter Resource Center (opens in a new tab) · University of Rhode Island
Tick identification, prevention, and reporting resources.
- 5.
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.