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Biotic Inventory: Documenting Diversity at the Katharine Ordway Natural History Study Area
Dermacentor variabilis, the American Dog Tick
Figure 1 [missing] : Dorsal view of collected specimen
Figure 2 [missing]: Ventral view of collected specimen
Species variabilis 2
Common names: Dog Tick, Wood Tick
To classify a small specimen, it is first important to note the number of legs the specimen has. If the specimen has six legs, it can be classified as an insect, whereas having eight legs indicates an arachnid. Since the collected creature has eight legs, it is a member of the class Arachnida. A tick can be distinguished from other arachnids by the shape of the body and the clawed shape of the legs. Once a specimen is known to be a tick, there are several ways to further identify the specific genus and species.
First, the general morphology of the tick can serve as an indicator for the species of the tick. The collected tick has a reddish brown coloration and distinct, lighter markings on the back. These coloration traits are common to the American Dog tick, or Dermacentor variabilis. Furthermore, these ticks as adults measure approximately 1/8 of an inch in length, however females may be significantly larger just following a feeding6. The collected tick was approximately this length.
Ticks are also distinguished by their outer-most dermal layer, which is either a hard shell or a soft outer coat. Ticks with a hard outer shell are referred to as “hard ticks,” while ticks that don’t have this characteristic are referred to as “soft ticks.” The Dermacentor variabilis is a hard tick,5 and the collected specimen has a hard shell, correlating with the classification of Dermacentor variabilis.
Finally, it’s important to note the ecology and geography of the collected tick. This tick was collected at the Ordway Study Area in Minnesota in a brushy area on the ground. The American Dog Tick is common to this habitat, further supporting the classification of this specimen as a Dermacentor variabilis, or the American Dog Tick.
American Dog Ticks commonly inhabit wooded areas. They are likely to be found along roadsides, hiking trails and paths, among brushy woodlands, and in tall grasses, as well as in marshy areas and bordering streams and lakes.6 These habitats make the tick easily unnoticed, and the bare legs of hikers as well as other animal appendages are likely to brush against these plants, making the tick easily transmissible. As more suburban homes are built on the edge of wooded lands, the likelihood of human to tick contact greatly increases, leading to concerns about the spread of certain diseases such as Rocky Mountain Spotted Fever. The Dermacentor variabilis does not, however, transmit Lyme’s disease, a disease commonly transmitted by other tick species. 4,6
All ticks are parasites, needing some sort of host from which to suck their blood and gain nutrients. In the case of the Dermacentor variabilis, the host is always a mammal. In order for this parasitic process to occur, the tick will attach itself to the skin of the host using parts of its mouth, while simultaneously releasing a chemical anesthesia to prevent the host from noticing it’s presence. The major prey for this tick includes mostly small mammals, such as mice, but they also feed on much larger animals such as humans, dogs and horses if available.
The American Dog Tick also has predators, keeping their populations controlled. These predators include salamanders, turkeys, toads, newts, centipedes and skinks.5
The life cycle of the tick is divided into three main phases: larvae, nymph, and adult. First, the mature mother tick, who releases thousands of eggs at a time, drops the larvae to the ground then dies.3 At this stage of life, the tick has only six legs, appearing more like an insect. Once released, the larvae follow the scent as well as the body temperature signature of their prey. They move with outstretched legs so to ensure attachment to the first mammal they come in contact with, without delineating between precisely what kind of animal this is (although the host is often small). The tick larvae will feed on the first host until it is nourished enough to drop off, molt, and start the second phase of life as a nymph.
Once the nymph tick has developed from the larvae, it now has all eight of it’s legs, and will again need to find a host to feed on. Once this host is found, the tick nymph will stay attached for several days before dropping to the ground and molting to form the final and reproductive stage of the life cycle, adulthood.5
Adult ticks attain their permanent dark coloration with lighter spots on their posterior exoskeletons, and also have genital openings, a feature which both the larvae and nymphs lack. These traits help make the life stages distinguishable. The tick life cycle requires at least 54 days to complete, but can take up to two years depending on host availability, host location, and weather.1
Adult American Dog Ticks become active in mid-April and stay active through September. They are most commonly found in the United States east of the Rocky Mountains, and are especially common in the eastern United States. They are also likely to be found in more central states, such as Iowa and Minnesota.1
This specimen was collected at the Katherine Ordway Natural History Study Area in a litter of brush next to a large pond. It was a hot day and the sun was shinning on the leaves, as seen in the provided photograph below.
Figure 3 [ missing ]: Place of collection of Dermacentor variabilis
1. Chan, Wai-Han, and Phillip E. Kaufman. University of Florida. 2008. Featured Creatures.
2. DiTerlizzi, Tony. Iowa State University Entomology. 2005. American Dog Tick.
3. Encyclopædia Britannica. 2010. American dog tick. Encyclopædia Britannica Online.
4. Iowa State University. 2009. http://www.ipm.iastate.edu/ipm/iiin/tamerican.html.
5. Study of Northern Virginia Biology. American Dog Tick. Web.
6. Welch, Kenneth A., and Kirby C. Stafford. The Connecticut Agricultural Experiment Station. 2001. The American Dog Tick.
Compiled by Sadie Bazur-Leidy
Biodiversity & Evolution (BIOL 270) Professr Sarah Boyer. Spring 2010
Specimen collected at Macalester College’s Katharine Ordway Natural History Study Area on April 15, 2010.