Climbing simulated vegetation to heights of ungulate hosts by larvae of Dermacentor albipictus (Acari : Ixodidae)

Larvae of winter ticks, Dermacentor albipictus (Packard), ascend vegetation in autumn and form clumps that attach to passing ungulate hosts. We tested the hypothesis that vegetation height determines the height of clumps. Dur ing the vegetation-to-ungulate transmission period (early September to mid- November), larvae were released. at the base of simulated vegetation (nylon rods 245 cm tall) in outdoor and laboratory trials and in the absence of h ost cues. Rod height exceeded the height of the tallest ungulate host, whic h is the moose, Alces alces (L.). Most larvae stopped climbing and formed c lumps 50-190 cm above ground, which coincided with torso heights of moose; elk, Cervus elaphus L.; and deer, Odocoileus spp. Rafinesque. More clumps f ormed in outdoor trials than in laboratory trials and clump heights tended to increase over. the course of the experiment, but clump number, size, and height did not cell-elate with weather conditions. Winter tick larvae appe ar to determine their height above ground in the absence of external cues, but this mechanism may be modified by external conditions.