Lyme borreliosis (LB) is the most prevalent vector-borne disease occurring in north temperate regions worldwide, spread by ticks of the genus Ixodes[1, 2]. To accurately predict disease incidence, and to design effective management programs, it is essential to understand those factors that regulate tick population dynamics. In the case of the blacklegged tick (Ixodes scapularis), the primary vector of Lyme disease, babesiosis, and human anaplasmosis in North America, authors have suggested several factors that influence vector survival and, therefore, abundance. Such factors include the population density of white-tailed deer, the primary adult tick reproductive host [3, 4], and the population density of disease reservoir rodents , which can be influenced by acorn mast production  and weather [7, 8]. Assessing the effects of weather-related variables can be very complex because of the potentially confounding effects of host-related dynamics, the potential for shifting importance of different population regulating factors from year to year , and the difficulty in determining which weather variables actually affect tick survival.
Abundance and LB infection prevalence of questing ticks are important variables in determining disease risk to humans. An entomological index has previously been developed for Rhode Island (RI), USA, providing a measure of infected nymphal I. scapularis encountered per unit of time sampled . This index was strongly predictive of LB risk for the state, supporting the hypothesis that case distribution was largely a function of peridomestic risk. However, Mather et al.  observed extreme variability of tick abundance in similar habitats, suggesting that additional environmental factors may regulate tick population dynamics. Given their diminutive size, I. scapularis nymphs can display sensitivity to conditions of low environmental moisture, with laboratory studies confirming greater nymphal mortality under low humidity conditions [11, 12].
Nymphs of I. scapularis are more susceptible to desiccation when questing for their next blood meal because of their high surface to volume ratio . Indeed, nymphal ticks can desiccate within 48 h if deprived of moisture, even though they are able to imbibe water moisture from partially saturated air [11, 13]. However, field studies on the relationship between environmental moisture, tick populations, and LB incidence, have produced conflicting results. Some investigators have documented associations between climatic variables such as drought and precipitation events with both tick populations  and LB incidence [14, 15]. In contrast, other studies have found only modest or no relationship between precipitation and tick numbers [5, 16]. One plausible explanation for these confounding results is that the weather variables measured did not consistently reflect the environmental conditions experienced by the ticks. Weather station data are generally collected from airport monitoring areas and might not be related in a simple fashion to the microclimatic conditions within the leaf litter habitats where nymphal I. scapularis dwell. Furthermore, tick survival might bear a more complex relationship to environmental moisture than previously documented. Indeed, Rodgers et al.  reported significant nymphal I. scapularis mortality after the ticks were continuously exposed to atmospheric moisture below an 82% relative humidity (RH) threshold for more than 8 hours, even when RH was subsequently increased to favorable levels.
In this study, we present new insight into the mechanism by which environmental moisture regulates blacklegged tick populations and tick encounter risk. This work is the result of a 14-year retrospective analysis of a statewide tick surveillance program and corresponding weather monitoring data in RI between 1997–2010.