Horizontal and vertical movements of host-seeking Ixodes pacificus (Acari: Ixodidae) nymphs in a hardwood forest.

The nymph of the western black-legged tick (Ixodes pacificus) is an important bridging vector of the Lyme disease spirochete (Borrelia burgdorferi) to humans in the far-western United States. The previously unknown dispersal capabilities of this life stage were studied in relation to logs, tree trunks, and adjacent leaf-litter areas in a mixed hardwood forest using mark-release-recapture methods. In two spatially and temporally well-spaced trials involving logs, the estimated mean distances that nymphs dispersed ranged from approximately 0.04 to 0.20 m/day on logs vs 0.11 to 0.72 m/day in litter. Prior to recapture in either trial and within the confines of the sampling grids, the greatest estimated dispersal distances by individual nymphs released on logs, and in litter 0.5 m or 1.5 m from logs, were 2.4, 3.0, and 3.0 m, respectively. Nymphs released on logs or litter tended to remain within the same biotopes in which they were freed while host-seeking. In two simultaneous trials involving trunks spaced close-at-hand, nymphs released at the trunk/litter interface on all four aspects collectively dispersed a mean of 0.353 m/day on trunks vs 0.175 m/day in litter. In either trial, the greatest distances that recaptured nymphs climbed trunks, or dispersed in litter in an encircling 3-m grid, were 1.55 m and 2.97 m, respectively. Nymphs ascending trunks did not exhibit a preference for any one aspect, and the B. burgdorferi-infection prevalences in nymphs that climbed trunks (3.2-4.0%) did not differ significantly from those that moved horizontally into litter (10.5-17.6%). We conclude that I. pacificus nymphs use an ambush host-seeking strategy; that they disperse slowly in all biotopes studied; that they usually continue to host-seek in or on whatever substratum they access initially; and that B. burgdorferi-infected nymphs are as likely to move horizontally as vertically when offered a choice.