American dog ticks (Dermacentor variabilis), commonly known as wood ticks, are arachnids found across North America. Accurate identification is important for personal safety and understanding potential health risks. Recognizing their distinct visual characteristics helps differentiate them from other tick species or insects, allowing for proper precautions and timely action if a bite occurs.
Key Identifying Features
An adult, unengorged American dog tick typically has an oval, flattened, reddish-brown body with distinct white or grayish markings on its back, a feature known as ornamentation. Unfed adult females are about 5 mm (0.2 inches) long, while males are slightly smaller, approximately 3.6 mm (0.14 inches) long.
A defining characteristic is the scutum, or dorsal shield, located just behind the mouthparts. In females, this scutum is smaller, covering only the anterior portion of their back as a creamy-white shield. In adult males, the scutum covers nearly the entire dorsal surface, displaying a mottled pattern of white, cream, or gray markings.
Both sexes have eight legs extending laterally from their bodies. Their mouthparts, known as the capitulum, protrude from the front and include a rectangular base and short palps. American dog ticks also have festoons, small, rectangular areas along the posterior margin of their abdomen.
Appearance Across Life Stages
Wood ticks undergo four life stages: egg, larva, nymph, and adult. Eggs are tiny, round, and dark brown, typically laid in secluded outdoor locations.
Larvae emerge with six legs and are very small, approximately 0.6 mm (0.02 inches) long, often described as pin-head sized. They appear straw-colored or yellowish before feeding and can become gray to black once engorged. After feeding, larvae molt into nymphs.
Nymphs possess eight legs, similar to adults, but are still quite small, measuring about 0.9 mm (0.04 inches) long. Unfed nymphs are typically pale yellowish-brown, becoming slate gray after a blood meal. Both larvae and nymphs lack the distinct white scutum markings seen on adults and may have reddish markings near their eyes.
Engorgement significantly alters the appearance of adult females. An unfed female is about 5 mm long, but after a blood meal, a fully engorged female can swell dramatically, reaching up to 15 mm (0.6 inches) in length and 10 mm (0.4 inches) in width. They resemble a dark pinto bean or raisin and often turn a grayish-green or slate gray color. Males do not change size as much during feeding due to their scutum covering most of their body.
Differentiating from Similar Creatures
Distinguishing the American dog tick from other common tick species requires attention to specific visual cues. The blacklegged tick (Ixodes scapularis), also known as the deer tick, is generally smaller than the American dog tick. Adult blacklegged ticks have a reddish-orange body and a solid black scutum, lacking the white or cream-colored markings of the American dog tick. Their legs are noticeably dark, and their mouthparts are longer.
The lone star tick (Amblyomma americanum) also has distinct features. Adult female lone star ticks are reddish-brown with a single, prominent white spot in the center of their back. Male lone star ticks display a geometric black pattern. While some American dog ticks might have a white dot, it is typically larger and located more towards the front of their back, near the head, compared to the lone star tick’s central spot. Lone star ticks also often possess longer mouthparts.
When differentiating ticks from non-tick arthropods like spiders, a key distinction is the body segmentation and leg arrangement. Ticks have a single, unsegmented body region and eight legs that spread out along the sides in nymphs and adults. Spiders, conversely, have two distinct body segments (cephalothorax and abdomen) and their eight legs typically cluster near the front.