Nymphs of the western black-legged tick (Ixodes pacificus) collected from tree trunks in woodland-grass habitat.

Nymphs of the western black-legged tick, Ixodes pacificus, were found on the trunks of trees during spring and summer in northwestern California. In a woodland-grass habitat, large- and medium-sized (> 130 cm and 80-130 cm in circumference, respectively), moss-covered oak (Quercus spp.) trees supported ticks significantly more often than trees without these characteristics. Additionally, trees with basal leaf-litter and lacking shade (at time of sampling) were significantly associated with the presence of ticks. Mean tick-burdens were > 1 for all oaks (1.06), all trees with basal leaf-litter (1.05), and all trees of large-circumference (1.19); 0.79 ticks per tree were collected over the entire study. Moss reduced the surface temperature of trees by a mean of 1.9 degrees C (range of 1.6-5.0 degrees C) and increased relative humidity by up to 2.5% from the ambient. These microclimatic changes, along with the presence of refugia in bark and western fence lizard (Sceloporus occidentalis) hosts on the lower-most surface of trees, likely accounted for ticks questing on the trunks. Although of undetermined epidemiological significance, the presence of host-seeking I. pacificus nymphs on tree trunks may shed light on the relation of abiotic and biotic factors to the life history of this important vector of disease.