Ixodes scapularis density in US temperate forests shaped by deer, earthworms, and disparate factors at two scales

Forest and wildlife management can help regulate the risk of human exposure to tick-borne pathogens by influencing the population density of host-seeking ticks and the pathogen infection prevalence in tick populations. For the blacklegged tick (Ixodes scapularis), also known as the deer tick, the overall density of host-seeking nymphs and the density of nymphs infected with Borrelia burgdorferi sensu stricto (which causes Lyme disease) are often closely and positively correlated. We sought to identify biotic and abiotic factors influencing nymph abundance in a multiscale assessment within and across forested landscapes in northern Wisconsin. On a set of randomly located plots within each of six mesic, hardwood-dominated forests, we collected host-seeking and on-host ticks, observed two common mammal hosts (white-tailed deer and mice), and characterized the structure and composition of the forest plant community. We also surveyed activity of exotic invasive earthworms, which are altering forest structure in the region. At the broader forest-level scale (n = 6), nymphal tick abundance was positively related to deer population density after accounting for the negative influence of forb cover. At the finer plot-level scale (n = 87), nymph abundance was positively correlated with the abundance of coarse woody debris and negatively correlated with the density of earthworm castings, an indicator of Lumbricid (nightcrawler) activity. Thus, we identified a suite of key factors, operating at different spatial scales, that shape the suitability of forest habitat for questing immature ticks. These findings inform our basic understanding of blacklegged tick ecology and also have important implications for managing tick-borne pathogens in forest ecosystems and landscapes.