Blacklegged tick (Ixodes scapularis)

How to identify it

• Adult female: brown to reddish-orange body with a solid, dark brown-to-black dorsal shield (scutum) directly behind the mouthparts; the shield is plain and unmarked (inornate) and contrasts sharply with the exposed orange/red rear of the body.
• Adult male: smaller and uniformly dark brown, with the shield covering nearly the entire back and no reddish coloration.
• Dark brown-to-black legs — a helpful field cue.
• No single white dot (rules out the female Lone Star tick) and no pale, mottled/ornate shield pattern (rules out the American dog tick).
• Nymph: minute (about a poppy seed), pale tan with a small dark shield; responsible for most disease transmission to people.

Size: Larvae are under ~1 mm (pinhead), nymphs about a poppy seed (~1.3–1.7 mm), and unfed adult females roughly 3–3.7 mm — under 1/8 inch (males slightly smaller, ~2–2.7 mm). An engorged, fully fed female can swell to roughly 1/2 inch (~10 mm) and turn tan, gray, or dark brown.

Where it lives & when

Established across the eastern half of the United States. Populations are densest in the Northeast and mid-Atlantic (Maine down through Virginia), the upper Midwest and north-central states (notably Minnesota, Wisconsin, and Michigan), and parts of the Southeast, with a documented range expansion north and west. On the Pacific Coast it is replaced by the closely related western blacklegged tick (Ixodes pacificus). This is a different distribution from the Lone Star tick's more southeastern and south-central range.

Season: Active whenever temperatures are above freezing. Adults show two peaks — one in spring (around April–May) and another in fall (into late October), and adults can quest on mild winter days above ~40°F, unlike many other ticks. Nymphs peak in late spring and summer (roughly June–July), which is when most Lyme disease and other infections are acquired. Overall bite risk is greatest in spring, summer, and fall.

• Deciduous and mixed woodlands, especially shaded, humid areas with leaf litter
• Brushy edges where woods meet lawns, fields, or trails
• Overgrown yards, stone walls, and ground cover near wooded property
• Areas frequented by white-tailed deer and small rodents (key hosts)

Diseases the blacklegged tick can carry

Tick presence is not the same as personal disease risk. Not every bite transmits a pathogen.

Look-alikes

The western blacklegged tick (Ixodes pacificus) is nearly identical but occurs on the Pacific Coast, so geography is the main clue. Other small Ixodes species can be hard to separate without magnification. The blacklegged tick is distinguished from the female Lone Star tick by the absence of a white dot, and from the American dog tick and Rocky Mountain wood tick (Dermacentor) by its smaller size, dark legs, and plain dark shield rather than a pale, ornate, mottled shield. Unfed nymphs are the easiest stage to overlook or misidentify, so save any tick you remove for identification.