California is home to 48 tick species, but only about six regularly bite humans. If you found a tick on yourself, your child, or your pet and you’re trying to figure out what it is, the three species you’re most likely dealing with are the western blacklegged tick, the Pacific Coast tick, or the American dog tick. Each one looks distinctly different, and knowing which you have matters because they carry different diseases.
Western Blacklegged Tick
This is California’s most widespread and abundant tick, and the only one in the state that transmits Lyme disease. It’s also one of the smallest ticks you’ll encounter, which makes it easy to miss on your skin.
Females have a reddish-brown body with a dark shield-like plate (called a scutum) just behind the head, giving them a two-toned look: dark “shoulders” and a reddish abdomen. Males are smaller than females and appear nearly solid black. Both sexes have noticeably dark legs and narrow, pointed mouthparts. If you’re looking at the tick under good light, those dark legs are one of the quickest ways to distinguish this species from the others.
Nymphs, the immature stage most likely to transmit Lyme disease, are roughly the size of a poppy seed. At that size they’re easy to mistake for a speck of dirt or a small freckle. Adults are closer to the size of a sesame seed when unfed, though females swell considerably after feeding.
Pacific Coast Tick
The Pacific Coast tick is the second species Californians encounter most often. It’s larger than the western blacklegged tick and has a more ornate appearance. Look for silvery or whitish mottled markings on the shield behind its head. These pale markings against a brown body are the fastest visual clue: if the tick has a patterned, almost marbled look on its back, you’re likely looking at a Pacific Coast tick or an American dog tick rather than a western blacklegged tick.
This species is the only known carrier of Pacific Coast tick fever, caused by the bacterium Rickettsia philipii. It can also carry the agents responsible for tularemia and Rocky Mountain spotted fever. Its mouthparts are broader and more rectangular compared to the narrow, pointed mouthparts of the western blacklegged tick.
American Dog Tick
The American dog tick is the largest of the three common species and the one most people recognize from other parts of the country. Adults are brown to reddish-brown with distinctive gray or silver markings on their scutum. Unfed females measure about 5 mm long (roughly a quarter inch), while males are slightly smaller at about 3.6 mm. After a blood meal, females can balloon to 15 mm long and 10 mm wide, becoming engorged and grayish in color.
A key visual difference from the Pacific Coast tick: on the American dog tick, the mouthparts and their base are clearly visible when you look at the tick from above, and the second segment of the mouthpart “feelers” (palps) is roughly as wide as it is long, giving them a blocky appearance. Immature stages lack the white scutum markings adults have and instead show red markings near their eyes. This tick can transmit Rocky Mountain spotted fever and tularemia.
Quick Visual Comparison
- Small, dark legs, plain back: Western blacklegged tick. Reddish body on females, nearly all-black males. No ornate patterning on the shield.
- Medium-sized, mottled silver markings: Pacific Coast tick or American dog tick. Both have patterned shields, but the American dog tick is generally larger with a blockier head when viewed from above.
- Swollen and gray, hard to identify: An engorged female of any species. Once a tick is fully fed, species-level identification by eye becomes much harder. Size can help: an engorged American dog tick female can reach the size of a small grape.
Size at Every Life Stage
Ticks go through three life stages before becoming adults, and they need a blood meal to advance to the next one. Larvae are the size of a grain of sand and have only six legs. Nymphs are about the size of a poppy seed and have eight legs. Adults are roughly the size of an apple seed when unfed. These comparisons hold across most hard tick species, though the western blacklegged tick runs smaller at each stage than the Dermacentor species (Pacific Coast and American dog ticks).
Nymphs are the life stage most likely to transmit disease to humans because they’re small enough to go unnoticed and tend to feed in spring and early summer when people are active outdoors. For Lyme disease specifically, an infected western blacklegged tick must be attached and feeding for at least 24 hours before it can pass the bacteria to you, so finding and removing ticks quickly matters.
Where and When You’ll Find Them
The western blacklegged tick is found throughout the state, from the coastal ranges to the Sierra foothills, especially in areas with leaf litter, tall grass, and oak woodlands. Adults are most active during the cooler, wetter months from late fall through spring. Nymphs peak in spring and early summer.
Pacific Coast ticks and American dog ticks prefer drier, brushier terrain and are more active during the warmer months. You’ll often pick these up along trails in chaparral, grasslands, and areas where deer, cattle, or dogs frequent.
What to Do if You Can’t Identify It
If you’ve removed a tick and want a definitive identification, several California county public health labs will identify and test ticks by mail. The process is straightforward: remove the tick with fine-tipped tweezers by grasping it as close to your skin as possible and pulling straight out without twisting. Place the tick in a clean sealed container or zip-top bag with a damp piece of paper towel. Do not use alcohol, tape, or other preservatives, as these can interfere with pathogen testing.
Write down when and where the tick was found. Some county labs, such as the Napa-Solano-Yolo-Marin County Public Health Laboratory, charge $45 for identification and testing, regardless of species. They’ll identify the tick and, if it’s a western blacklegged tick, test it for the Lyme disease bacterium. For other species, you’ll get an identification that tells you which diseases, if any, that tick could carry.
About one in three Lyme disease cases reported in California actually involve people who were infected while traveling outside the state, so knowing where you picked up the tick is useful information for your healthcare provider.