Tracking seasonal activity of the western blacklegged tick across California

Abstract

1. Understanding seasonal patterns of activity, or phenology, of vector species is fundamental to determining seasonality of disease risk and epidemics of vector-borne disease. Spatiotemporal variation in abiotic conditions can influence variation in phenological patterns and life history events, which can dramatically influence the ecological role and human impact of a species. For arthropod vectors of human diseases such as ticks, these phenological patterns determine human exposure risk, yet how abiotic conditions interact to determine suitable conditions for hostseeking of vector species is difficult to disentangle.2. Here, we use MaxEnt to model spatial patterns and differences in host-seeking phenology of the western blacklegged tick (Ixodes pacificus) in California using spatially and temporally refined adult tick occurrence data and similarly refined climate and environmental data. We empirically validate the model using phenological data from field studies conducted at sites across California's latitudinal gradient.3. We find adult tick host-seeking activity varies substantially throughout the year, as well as across the large latitudinal gradient in the state. Suitable conditions for host-seeking are found earlier in fall and later in the spring in northern than in southern California. These seasonal patterns are primarily associated with monthly precipitation, minimum winter temperature, and winter precipitation, with maximum monthly temperature possibly playing a more prominent role in limiting hostseeking activity earlier in the spring in southern than northern California. Synthesis and applications.Modelling the seasonal activity of the western blacklegged tick, we find both a longer window for host-feeding and more protracted risk of human exposure to this vector species in northern than southern California.We further identify key environmental factors associated with these patterns, including precipitation and temperature that are otherwise challenging to elucidate in field and laboratory studies over large spatial scales. Moreover, we illustrate how species distribution models, in combination with temporally refined species occurrence and environmental data, can be used to investigate environmental factors predictive of geographic variation in seasonality or phenology of vector species. This produces not only novel ecological insight, but key information for public | 2563Journal of Applied Ecology MACDONALD et AL.

Projected climate and land use change alter western blacklegged tick phenology, seasonal host‐seeking suitability and human encounter risk in California

“…The MSS value for the seasonal "climate only" model is 0.2844 and the seasonal "climate and land use" model value is 0.2393, which reasonably reflects field observations from sites across California (MacDonald, O'Neill, et al, 2019). Within each ecoregion, we use pixels with values above these thresholds in at least one month of the year to calculate average and standard deviation of predicted suitability.…”

“…Here we use a seasonal species distribution modeling approach (Hereford et al, 2017;MacDonald, O'Neill, et al, 2019) to investigate the response of western blacklegged tick host-seeking activity to projected climate and land use change across California's ecologically diverse ecoregions and latitudinal gradient.…”

“…We focused on adult observations as juvenile stages (larvae and nymphs) had comparatively small sample sizes, and may differ in detectability via flagging between regions of California (Lane et al, 2013; MacDonald & Briggs, 2016). We did not combine occurrence data across life stages as the different life stages vary in seasonal activity and abiotic associations (MacDonald, 2018;MacDonald & Briggs, 2016;MacDonald et al, 2017), and therefore cannot be captured in the same SDM (MacDonald, O'Neill, et al, 2019). (Fitzpatrick & Hargrove, 2009;Wiens, Stralberg, Jongsomjit, Howell, & Snyder, 2009).…”

“…We also identify changes in human encounter and potential pathogen transmission risk associated with land use change and increasing human development in regions projected to be highly suitable for the tick vector. Specifically, we use climate and land use projections for California coupled with seasonal species distribution models (SDMs; Hereford, Schmitt, & Ackerly, 2017;MacDonald, O'Neill, et al, 2019)…”

Impact of prior and projected climate change on US Lyme disease incidence

“…Rainfall and moisture availability also influence host-seeking activity in nonlinear ways. Low humidity exposure substantially increases tick mortality and inhibits host-seeking activity (Stafford 1994, Lane et al 1995, Vail and Smith 1998, Schulze et al 2001, Rodgers et al 2007, Nieto et al 2010, MacDonald et al 2019b. To avoid desiccating conditions, Ixodid ticks often modify their questing behavior to remain closer to the moist vegetative surface, or return frequently to rehydrate, both of which decrease the probability of obtaining a blood meal and thereby limiting survival and reproduction (Randolph and Storey 1999, Prusinski et al 2006, Sonenshine and Roe 2013, Arsnoe et al 2015, McClure and Diuk-Wasser 2019.…”