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Scorpions Spiders Mites and Ticks: Arachnida

Rocky Mountain Wood Tick (dermacentor Andersoni): Species Accounts

Physical characteristics: Adult wood ticks typically measure 0.08 to 0.21 inches (2 to 5.3 millimeters) in length, but females filled with blood look like plump beans and may reach a length of 0.65 inches (16.5 millimeters) and a width of 0.45 inches (11.4 millimeters). Their flat, pear-shaped bodies are covered with a tough outer skeleton.

Adult females are reddish brown with a grayish white shield on the back, near the front of body; the shield turns grayish when it fills with blood. Males are spotted with brown and gray and do not have a distinctive white shield. There are eleven folds along the edge of the abdomen.

Rocky Mountain wood ticks attach themselves to their hosts by secreting a special kind of glue around their mouthparts just before inserting them into the host's flesh. (©Larry West/Photo Researchers, Inc. Reproduced by permission.)

Geographic range: The Rocky Mountain wood tick is widely distributed in North America, primarily throughout Rocky Mountain states and into southwestern Canada.

Habitat: This tick is found in spring and summer in brushy areas of foothills and mountains that are home to small mammals.

Diet: Wood ticks at the adult and larval stages feed as external parasites on the blood of reptiles, birds, and mammals and can survive for more than one year without feeding at all. When they find a host, males feed for about five days without becoming engorged, or completely filled with blood. Then they become sexually mature and ready to mate; after mating they resume feeding. Females feed for up to seven days (until they are fully engorged), during which time they mate. The body weight of a fully engorged female will increase from about 0.000176 ounces (5 milligrams) to more than 0.0247 ounces (700 milligrams).

Behavior and reproduction: Adults infest and live as parasites on large animals, such as bear, sheep, cattle, horses, dogs, and humans. Young ticks are called larvae or nymphs. Larval ticks have only six legs, while nymphs have all eight. The larvae and nymphs feed on smaller mammals, including rabbits and squirrels. Insects at all stages attack jackrabbits and porcupines. Larvae, nymphs, and adults climb grass stems and bushes while searching for a host animal. They use carbon dioxide and other chemicals associated with mammals to detect the presence of potential hosts. Ticks attach themselves to their hosts by secreting, or giving off, a special kind of glue around their mouthparts just before inserting them into the host's flesh. Only immature ticks and adult females become engorged with the host's blood. Engorged larvae, nymphs, and unfed adults normally spend the cold winter months in grasses and leaf litter.

Ticks require a blood meal before they can molt; this is also a necessity for their eggs to develop. Mating usually takes place on the host animal. After she finishes feeding, the mated female leaves the host and looks for a protected place to lay her eggs. She lays up to 7,400 eggs over a period of ten to thirty-three days and then dies. The six-legged larvae hatch and begin to search for chipmunks, mice, voles, and other small rodents. If they do not find a host within a month, they die. They feed on their first host for two to eight days before dropping to the ground and molting. At this stage they can often survive for more than year without feeding again. With their full complement of eight legs, the tick nymphs seek slightly larger hosts, such as rabbits, ground squirrels, marmots, and skunks, and feed on them until they become engorged in three to eleven days. They drop off to molt into adults in about two weeks. Adults can survive nearly two years without feeding. After finding an even larger host, partially fed adults are ready to mate.

Rocky Mountain wood ticks and people: Females may carry and transmit several diseases from small mammals to humans, including Rocky Mountain spotted fever, tularemia, and Colorado tick fever in the United States. This happens only rarely in Canada.

Conservation status: This species is not endangered or threatened. ∎

Additional topics

Animal Life ResourceInsects and SpidersScorpions Spiders Mites and Ticks: Arachnida - Physical Characteristics, Diet, Behavior And Reproduction, Arachnids And People, Hair Follicle (fah-lih-kuhl) Mite (demodex Folliculorum): Species Accounts - GEOGRAPHIC RANGE, HABITAT, CONSERVATION STATUS