Diel activity of nymphal Dermacentor occidentalis and Ixodes pacificus (Acari: Ixodidae) in relation to meteorological factors and host activity periods.

Relation of diel activity and questing behavior of nymphal Dermacentor occidentalis Marx and Ixodes pacificus Cooley & Kohls to meteorological factors was investigated in a shaded versus a sun-exposed outdoor arena. Oak-woodland soil covered partially with leaf litter and small rocks, and 24 vertically oriented grass stems 2.5, 5.0, 10.0, and 20.0 cm tall were provided as substrate and potential questing sites. Tick activity and weather conditions were monitored bihourly during 15 diel (24-h) experiments (D. occidentalis, 8; I. pacificus, 7). In shade, D. occidentalis was active throughout the day, but questing occurred mainly at night and in the morning on grass stems or atop soil when temperatures were cool and relative humidities high. Ticks seemed to prefer to quest at heights between approximately 4 and 10 cm. The time of day and height at which D. occidentalis quested on grass stems coincided with the activity periods and size of its lagomorph and rodent hosts. Low percentages (< or = 15%) of I. pacificus nymphs (n = 100 or 200) were active atop soil or leaf litter at night or sporadically throughout the day, but none ascended grass stems. This finding was reconfirmed by monitoring diurnal behavior of nymphs in an outdoor aquarium having leaf litter as substrate; < or = 4% of 53 ticks were detected on the topmost layer of leaves and, of those, most I. pacificus were situated on the lower versus the upper surfaces of such leaves. Activity of I. pacificus was correlated positively with relative humidity and negatively with soil temperature in one experiment. In the sun-exposed arena, ticks of both species died within 9-11 d as daytime soil-surface temperatures sometimes reached maximums of 73-77 degrees C and relative humidities dropped to 14-24%. In contrast, D. occidentalis and I. pacificus survived for up to 6 and 8 wk, respectively, in the shaded arena. After its introduction into the shaded arena, the western fence lizard (Sceloporus occidentalis Baird & Girard) acquired more I. pacificus nocturnally while asleep in soil than during its diurnal period of activity above ground. Sleeping wild lizards also became infested more often and had significantly greater burdens of I. pacificus subadults, primarily larvae, than diurnally active lizards. Collectively, these findings demonstrate that I. pacificus subadults are capable of locating and attaching to their saurian hosts subterraneanly as well as above ground.