Climate-associated variation in the within-season dynamics of juvenile ticks in California

Changing climate has driven shifts in species phenology, influencing a
range of ecological interactions from plant-pollinator to
consumer-resource. Phenological changes in host-parasite systems have
implications for pathogen transmission dynamics. The seasonal timing, or
phenology, of peak larval and nymphal tick abundance may be an important
driver of tick-borne pathogen prevalence through its effect on
cohort-to-cohort transmission. Tick phenology is tightly linked to
climatic factors such as temperature and humidity. Thus, variation in
climate within and across regions could lead to differences in
phenological patterns. These differences may in turn explain regional
variation in tick-borne pathogen prevalence of the Lyme disease-causing
Borrelia bacteria in vector populations in the United States (US). For
example, one factor thought to contribute to high Lyme disease prevalence
in ticks in the eastern US is the asynchronous phenology of ticks in that
region, where potentially infected nymphal ticks emerge earlier in the
season than uninfected larval ticks. This allows the infected nymphal
ticks to potentially transmit the pathogen to hosts that are subsequently
fed upon by the next generation of larval ticks. In contrast, in the far
western US where Lyme disease prevalence is generally much lower, tick
phenology is thought to be more synchronous with uninfected larvae
emerging slightly before, or at the same time as, potentially infected
nymphs, reducing horizontal transmission potential. Yet sampling larval
and nymphal ticks, and their host-feeding phenology, both across large
spatial gradients and through time, is challenging, which hampers attempts
to conduct detailed studies of phenology to link it with pathogen
prevalence. In this study, we demonstrate through intensive within-season
sampling that the relative abundance and seasonality of larval and nymphal
ticks is highly variable along a latitudinal gradient and likely reflects
the variable climate in the far western US with potential consequences for
pathogen transmission. We find that feeding patterns were variable and
synchronous feeding of juvenile ticks on key blood meal hosts was
associated with mean temperature. By characterizing within-season
phenological patterns of the Lyme disease vector throughout a climatically
heterogeneous region, we can begin to identify areas with high potential
for tick-borne disease risk and underlying mechanisms at a finer scale.