Effects of Environmental Factors on the Abundance of Blacklegged Ticks

The nymphal stage of the blacklegged tick, Ixodes scapularis, is the major vector of Lyme disease, the most common vector-borne disease in North America. Tick abundance has generally been estimated using either flag/drag samples or samples from hosts. However, the biases of these sampling methods have not been adequately studied. We compared samples using both methods from sites in Massachusetts and Wisconsin. Tick abundance was compared with variables related to weather (temperature, relative humidity, and tick adverse moisture events), vegetation (canopy cover, tree density, shrub density, ground vegetation, and leaf litter cover), and host abundance (mice, small mammals, medium sized mammals, and all hosts). A model with relative humidity in the leaf litter and canopy cover variables gave the best prediction of tick numbers per flag/drag sample (R2 = 0.829, p = 0.0006). In contrast, the number of small mammals collected per sample in Sherman traps and pitfall traps gave the best prediction of ticks collected per sample from all hosts (R2 = 0.580, p = 0.0057) and the number of ticks per mouse (R2 = 0.580, p = 0.0057). Therefore, the most significant environmental factors that influence I. scapularis abundance vary, based on the particular location of the tick sampled. Ticks found in leaf litter and ticks found on hosts experience very different environments and the factors that influence their abundance are different, so studies of tick population biology should select sampling methods based on the features of the population under study. These results indicate that risk of encounter with host-seeking ticks is greatest in areas with dense canopy cover and moist leaf litter