Efficiency of drag sampling for estimating population sizes of Ixodes pacificus (Acari: Ixodidae) nymphs in leaf litter.

Drag sampling is a commonly used method to obtain relative estimates of the density of questing nymphal Ixodes scapularis Say and I pacificus Cooley & Kohls ticks, which are primary vectors of Lyme disease spirochetes to humans in North America. However, the efficiency of drag sampling in determining absolute population densities of questing nymphs has not been evaluated previously. Therefore, we assessed the efficiency of a single drag-sampling occasion to estimate the total population size of questing I pacificus nymphs in a leaf-litter habitat in California. Repeated daily removal sampling was carried out in four areas, each covering 300 m2, on 17 occasions over a 23-d period in the spring of 1999. In total, 573 I. pacificus nymphs were collected, of which 55 (9.6%) were collected on the initial sampling occasion and 20 (3.5%) on the last occasion. The total population size of questing nymphs, i.e., the intersection with the horizontal axis of a linear regression of daily nymphal catch rates on the number of nymphs caught previously, was estimated to be 936. Thus, the efficiency of the initial sampling occasion to estimate the total population size was 5.9% (4.8, 5.0, 5.8, and 9.1%, respectively, for the four individual sampling areas). Further, the overall mean efficiencies of the two, five, and 10 first removal sampling occasions to estimate the absolute nymphal density was 5.2, 4.7, and 4.3%, respectively, and 13 sampling occasions were required to collect 50% of the estimated total nymphal population.