Survival rates of immature Ixodes pacificus (Acari: Ixodidae) ticks estimated using field-placed enclosures

Granulocytic anaplasmosis (GA) and Lyme borreliosis are emerging tick-borne diseases caused by infection with Anaplasma phagocytophilum and Borrelia burgdorferi, respectively, and maintained in rodent-Ixodes spp. tick cycles, including I. pacificus in the western U.S. Ixodes pacificus has a multiple-year life cycle and B. burgdorferi and A. phagocytophilum are transstadially, but not transovarially, transmitted within ticks, thus ticks function importantly in maintaining infection in nature. In this study, the survival of larval and nymphal I. pacificus was determined using ticks placed in tubes in leaf litter from June 2005 to September 2006 at two field sites in the California northern coast range mountains and a laboratory control. In all three sites, nymphal and larval survival ranged from 90-400 d, with differences in mean survival among sites. Fewer ticks died in the autumn in the moister field sites compared with the drier incubator control treatment. The first large die-off event in late autumn occurred at all sites shortly before relative humidity increased from 80-100% and temperature declined from approximately 22-15 degrees C. The concurrent die-off in the incubator population, subject to relative humidity and temperature regimes that were invariant, suggests that survival time was dependent on other factors in addition to environmental conditions. These results suggested that many ticks exhausted resources or tolerance for relatively low humidity within six months of questing, and that higher humidity prolonged survival. Based on observed longevity, humans and other animals could acquire A. phagocytophilum infection from adult I. pacificus that were infected up to three years earlier.