Hosts, but not climate, predict blacklegged tick abundance

Background/Question/Methods Many models predict that ongoing climate warming will expand the geographic ranges of arthropod vectors and the diseases they transmit.  However, these models are typically silent on the specific mechanisms by which climate warming is expected to affect vector demography.  Recently, it was hypothesized that the geographic range of blacklegged ticks (Ixodes scapularis), which are the vectors of Lyme disease, anaplasmosis, and babesiosis, is limited by mean annual degree-days >0 C.  These ticks have three host-feeding life stages (larva, nymph, adult), and spend about 98% of their time off host in a quiescent state on the forest floor.  The logic underlying the degree-day (DD) hypothesis is that: (1) daily mortality rates for each tick life stage are constant through time; (2) inter-stage development times decrease linearly with temperature; (3) therefore, lower total mortality occurs under warmer conditions.  The hypothesis predicts a positive linear correlation between DD>0 and tick abundance.  We tested this hypothesis using data on annual abundance of larval, nymphal, and adult blacklegged ticks and on DD>0 between 1993 and 2007 in New York’s Hudson River Valley.  We separately analyzed abundances of ticks that were seeking hosts and those that were attached to rodent hosts.

Results/Conclusions

In no case did we find correlations between DD>0 and tick abundance.  Similarly, inter-stage survival was not affected by DD>0.  The abundance of nymphal ticks (the life stage responsible for most pathogen transmission) was strongly positively correlated with the prior abundance of white-footed mice and eastern chipmunks, two key hosts for the larval stage.  Several studies demonstrate that summer abundance of mice and chipmunks is determined largely by tree seed production (especially acorns) the prior fall.  If climate warming affects the future distribution and abundance of ticks and tick-borne disease, we hypothesize that the effect will be indirect.  All else equal, warming is likely to increase average seed production of temperate trees, potentially leading to long-term increases in average rodent abundance.  Because both the abundance and pathogen-infection prevalence of ticks are positively correlated with rodent abundance, the indirect effect of climate warming might be an increased burden of tick-borne disease.  However, direct effects of climatic variables on tick abundance and distribution remain elusive.