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Global environmental change is having profound effects on the ecology of infectious disease systems, which are widely anticipated to become more pronounced under future climate and land use change. Arthropod vectors of disease are particularly sensitive to changes in abiotic conditions such as temperature and moisture availability. Recent research has focused on shifting environmental suitability for, and geographic distribution of, vector species under projected climate change scenarios. However, shifts in seasonal activity patterns, or phenology, may also have dramatic consequences for human exposure risk, local vector abundance and pathogen transmission dynamics. Moreover, changes in land use are likely to alter human–vector contact rates in ways that models of changing climate suitability are unlikely to capture. Here we used climate and land use projections for California coupled with seasonal species distribution models to explore the response of the western blacklegged tick (Ixodes pacificus), the primary Lyme disease vector in western North America, to projected climate and land use change. Specifically, we investigated how environmental suitability for tick host‐seeking changes seasonally, how the magnitude and direction of changing seasonal suitability differs regionally across California, and how land use change shifts human tick‐encounter risk across the state. We found vector responses to changing climate and land use vary regionally within California under different future scenarios. Under a hotter, drier scenario and more extreme land use change, the duration and extent of seasonal host‐seeking activity increases in northern California, but declines in the south. In contrast, under a hotter, wetter scenario seasonal host‐seeking declines in northern California, but increases in the south. Notably, regardless of future scenario, projected increases in developed land adjacent to current human population centers substantially increase potential human–vector encounter risk across the state. These results highlight regional variability and potential nonlinearity in the response of disease vectors to environmental change. [ABSTRACT FROM AUTHOR]

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 host‐seeking of vector species is difficult to disentangle.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.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 host‐seeking 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 health practitioners in managing vector‐borne disease transmission and targeting public outreach and interventions. [ABSTRACT FROM AUTHOR]

Background: Tick-borne diseases, particularly Lyme disease, are emerging across the northern hemisphere. In order to manage emerging diseases and predict where emergence will likely occur, it is necessary to understand the factors influencing the distribution, abundance and infection prevalence of vector species. In North America, Lyme disease is the most common vector-borne disease and is transmitted by blacklegged ticks. This study aimed to explore the abiotic and environmental drivers of density and infection prevalence of western blacklegged ticks (Ixodes pacificus) in southern California, an understudied and densely populated region of North America. Results: Over the course of this two-year study, densities of I. pacificus adults were consistently positively associated with host availability for juvenile ticks and dense oak woodland habitat. Densities of nymphal and larval I. pacificus, on the other hand were primarily predicted by host availability for juvenile ticks in the first year of the study, and by habitat characteristics such as dense leaf litter in the second year. Infection with the causative agent of Lyme disease, Borrelia burgdorferi (sensu stricto), and related spirochetes was not predicted by the abiotic conditions promoting I. pacificus populations, but rather by diversity of the tick community, and in particular by the presence of two Ixodes tick species that do not generally feed on humans (Ixodes spinipalpis and Ixodes peromysci). Borrelia spp. infection was not detected in the I. pacificus populations sampled, but was detected in other vector species that may maintain enzootic transmission of the pathogen on the landscape. Conclusions: This study identified dense oak woodlands as high-risk habitats for I. pacificus tick encounter in southern California. The shift in relative importance of host availability to habitat characteristics in predicting juvenile tick abundance occurred as California's historic drought intensified, suggesting that habitat providing suitable microclimates for tick survivorship became centrally important to patterns of abundance in the face of deleterious abiotic conditions. These results underscore the need for further investigation of the effects of climate change on tick-borne disease in California. Finally, despite low risk of human Lyme disease infection posed by I. pacificus in southern California, evidence of infection was found in other tick species, suggesting that enzootic transmission of tick-borne borreliae may be occurring in southern California, and involve parallel enzootic cycles with other tick and host species but not necessarily humans. [ABSTRACT FROM AUTHOR]

• Pesticide threats influenced by use rate and overlap with ecological communities. • Pesticide hotspots exist in agricultural valleys, yet are seasonally transient. • 0–5 % of species richness hotspots overlap annual pesticide use hotspots. • Spatial location and degree of overlap varies monthly and by taxa. The consequences of agricultural pesticide use for ecological and environmental health are partially determined by the overlap of pesticide use, in space and time, with ecologically important regions. Yet, data limitations have largely inhibited understanding of where and when such overlap occurs. Combining detailed pesticide use data from the diverse agricultural regions of California with species richness data for several taxa including birds, mammals, amphibians and reptiles, we evaluate the location and persistence of pesticide use hotspots, and where and when they align with ecologically important areas. Hotspots of pesticide use were generally located in agricultural valleys, as anticipated, yet were surprisingly ephemeral. Between 0–5 % of species richness hotspots intersected annual pesticide use hotspots depending on the focal taxa. The level of overlap also varied over the growing season, peaking for most taxa in May through July. Considering the spatial and temporal heterogeneity in both pesticide use and biodiversity is important to focus monitoring and mitigation efforts to reduce the ecological impacts of pesticide use when and where they occur. [ABSTRACT FROM AUTHOR]

A diversity of Borrelia burgdorferi sensu lato (Johnson, Schmid, Hyde, Steigerwalt & Brenner) (Spirochaetales: Spirochaetaceae) genomospecies, including the Lyme disease agent, Borrelia burgdorferi sensu stricto (s.s.), have been identified in the western United States. However, enzootic transmission of B. burgdorferi s.l. in small mammals and ticks is poorly characterized throughout much of the region. Here we report prevalence of B. burgdorferi s.l. in small mammal and tick communities in the understudied region of southern California. We found B. burgdorferi s.l. in 1.5% of Ixodes species ticks and 3.6% of small mammals. Infection was uncommon (~0.3%) in Ixodes pacificus Cooley and Kohls (Acari: Ixodidae), the primary vector of the Lyme disease agent to humans in western North America, but a diversity of spirochetes-including Borrelia bissettiae, Borrelia californiensis, Borrelia americana, and B. burgdorferi s.s.-were identified circulating in Ixodes species ticks and their small mammal hosts. Infection with B. burgdorferi s.l. is more common in coastal habitats, where a greater diversity of Ixodes species ticks are found feeding on small mammal hosts (four species when compared with only I. pacificus in other sampled habitats). This provides some preliminary evidence that in southern California, wetter coastal areas might be more favorable for enzootic transmission than hotter and drier climates. Infection patterns confirm that human transmission risk of B. burgdorferi s.s. is low in this region. However, given evidence for local maintenance of B. burgdorferi s.l., more studies of enzootic transmission may be warranted, particularly in understudied regions where the tick vector of B. burgdorferi s.s. occurs., (© The Author(s) 2020. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The distribution, abundance and seasonal activity of vector species, such as ticks and mosquitoes, are key determinants of vector-borne disease risk, and are strongly influenced by abiotic and habitat conditions. Despite the numerous species of tick vectors in the heavily populated North American West Coast, all but Ixodes pacificus, the primary vector of the Lyme disease spirochete, is poorly characterized with regard to seasonal activity patterns and fine scale drivers of distribution and abundance, particularly in heavily populated regions of southern California. This lack of knowledge inhibits both scientific understanding and public health efforts to minimize vector exposure and risk of pathogen transmission to humans. Here we address this gap by characterizing the abiotic and habitat drivers of the distribution, abundance, and diversity of the vector tick community using fine scale temporal surveys over two seasons (2014 and 2015) across coastal and inland regions of Santa Barbara County, CA. We also characterize patterns of seasonal activity of the more common vector species to understand seasonality in risk of vector exposure, and specifically focus on human encounter risk using standardized tick drags as our method of collection. Leveraging plot-level habitat and abiotic variables in partial least squares regression analysis, we find the seven different vector species collected in this study have divergent drivers of activity and abundance. For example, I. pacificus is strongly associated with dense forest habitats and cool and moist microclimates, while Dermacentor occidentalis and Dermacentor variabilis, competent vectors of Rocky Mountain Spotted Fever, were found to be more tolerant of higher average temperatures and more open habitats. These results suggest that I. pacificus may be expected to experience reductions in geographic distribution and seasonal activity under projected land cover and climate change in coastal southern California, while D. occidentalis may experience more limited effects. We discuss implications for changing tick-borne disease risk associated with pathogens transmitted by Ixodes as well as Dermacentor species ticks in the western US, and contrast these predictions with eastern North America., Competing Interests: The author has declared that no competing interests exist.

Patterns of seasonal activity and density of host-seeking western blacklegged ticks, Ixodes pacificus, were investigated in central and southern California. Weekly to monthly drag sampling was undertaken at two sites in Santa Barbara County and one site in Los Angeles County over multiple years. Adult I. pacificus became active in the winter (late November) and were rare or absent by late April to early May. Nymphal ticks became active in early to late February, were absent by early May to early June, and were rarely encountered using the drag method throughout their period of peak seasonal activity. Larval ticks became active earlier in the season, or at the same time as nymphs (early to late February) and were absent by early May. These results suggest a highly truncated period of I. pacificus seasonal questing activity, particularly apparent in the juvenile tick stages, in central and southern California relative to observed patterns in Lyme-endemic northwestern California. Notably, the highly truncated period of questing activity of the juvenile stages has important implications for pathogen transmission dynamics in that there exists only a brief window for horizontally transmitted pathogens to be acquired by one tick cohort and subsequently transmitted, through hosts, to the next tick cohort in this system. The broader patterns observed also suggest low human risk of tick-borne disease in central and southern California, and have implications for reduced tick-borne disease risk in the western US more generally under projected climate change., (Copyright © 2015 Elsevier GmbH. All rights reserved.)