Dermacentor variabilis

Adults are relatively large hard ticks with ornate dorsal scutum patterns. Males and females exhibit sexual dimorphism in Haller's organ morphology, with females showing distinct structural differences. Spiracular plates have a posterior extension that is longer and narrower compared to the western sister species D. similis. Larvae have six legs; nymphs and adults have eight legs.

Found in highly wooded, shrubby, and long-grass areas. Favors ecotones between forest and meadow, grassy meadows, field-woodland borders, and vegetation along stream banks. Adults show attraction to heat stimuli associated with road edge habitats.

Eastern North America, from Nova Scotia and Massachusetts southward through the eastern United States. Range extends west to approximately the Rocky Mountains; populations formerly considered western D. variabilis have been reclassified as D. similis. Established populations documented in southern Manitoba and undergoing range expansion in the northern Maritimes of Canada.

Activity varies by latitude: April–August in northernmost range (Nova Scotia, Massachusetts) with peak May–June; April–September/October in coastal Virginia with peaks in May and July; late April–September in Ohio with peaks May–June and August–September; late March–August in Georgia with multiple peaks; April–July in Florida where activity is limited by high summer temperatures and low winter humidity.

Obligate blood-feeding parasite. Requires blood meal at every life stage (larva, nymph, adult) for development and reproduction.

Complete four-stage life cycle: egg, six-legged larva, eight-legged nymph, and adult. Unlike most tick species, D. variabilis exhibits a behavioral diapause in larvae after egg emergence, allowing unfed larvae to overwinter and synchronize questing with spring onset. Life cycle may take 2–3 years in cooler climates. Virgin females capable of parthenogenetic reproduction, producing viable offspring without mating.

Exhibits "questing" behavior: climbs vegetation and waves forelegs to detect passing hosts using Haller's organ, a sensory pit on forelegs that detects carbon dioxide, ammonia, pheromones, humidity, and infrared radiation (body heat). Larval behavioral diapause represents an adaptive strategy for northern range colonization.

Vector for multiple human and animal pathogens. Serves as bridge between wildlife reservoirs and human/domestic animal populations.

Primary vector of Rocky Mountain spotted fever (Rickettsia rickettsii), with transmission possible within 30 minutes to 4 hours of attachment—significantly faster than previously believed. Also transmits tularemia (Francisella tularensis) and can induce tick paralysis via neurotoxin. Not a competent vector for Lyme disease (Borrelia burgdorferi). Control methods include habitat modification (grass cutting), permethrin-treated clothing, and prompt removal with fine-point tweezers or tick removal hooks.

• Dermacentor similisFormerly considered western populations of D. variabilis; distinguished by spiracular plate morphology (narrower, shorter posterior extension) and genetic differentiation; west of Rocky Mountains
• Dermacentor andersoniOverlapping range in western Canada; lacks the behavioral diapause observed in D. variabilis larvae under identical environmental conditions
• Ixodes scapularisSmaller blacklegged tick; primary Lyme disease vector; lacks ornate scutum patterns of Dermacentor
• Rhipicephalus sanguineusBrown dog tick; more cosmopolitan distribution; lacks the distinctive ornate dorsal patterning

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
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• Establishment and range expansion of Dermacentor variabilis in the northern Maritimes of Canada: Community participatory science documents establishment of an invasive tick species.
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