1 © 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. Modeling/GIS, Risk Assessment, Economic Impact A Generalized Additive Model Correlating Blacklegged Ticks With White-Tailed Deer Density, Temperature, and Humidity in Maine, USA, 1990–2013 Susan P. Elias, 1,6, Allison M. Gardner, 2 Kirk A. Maasch, 3,4 Sean D. Birkel, 3,4 Norman T. Anderson, 5 Peter W. Rand, 1 Charles B. Lubelczyk, 1 and Robert P. Smith, Jr. 1 1 Maine Medical Center Research Institute, Vector-borne Disease Research Laboratory, 81 Research Drive, Scarborough, ME 04074, 2 School of Biology and Ecology, University of Maine, Orono, ME 04469, 3 School of Earth and Climate Sciences, University of Maine, Orono, ME 04469, 4 Climate Change Institute, University of Maine, Orono, ME 04469, 5 Anderson Environmental Health, Winslow, ME 04901, and 6 Corresponding author, e-mail: susan.elias@maine.edu Subject Editor: Holly Gaff Received 15 June 2020; Editorial decision 24 July 2020 Abstract Geographical range expansions of blacklegged tick [Ixodes scapularis Say (Acari: Ixodidae)] populations over time in the United States have been attributed to a mosaic of factors including 20th century reforesta- tion followed by suburbanization, burgeoning populations of the white-tailed deer [Odocoileus virginianus Zimmerman (Artiodactyla: Cervidae)], and, at the northern edge of I. scapularis’ range, climate change. Maine, a high Lyme disease incidence state, has been experiencing warmer and shorter winter seasons, and rela- tively more so in its northern tier. Maine served as a case study to investigate the interacting impacts of deer and seasonal climatology on the spatial and temporal distribution of I. scapularis. A passive tick surveillance dataset indexed abundance of I. scapularis nymphs for the state, 1990–2013. With Maine’s wildlife management districts as the spatial unit, we used a generalized additive model to assess linear and nonlinear relationships between I. scapularis nymph abundance and predictors. Nymph submission rate increased with increasing deer densities up to ~5 deer/km 2 (13 deer/mi 2 ), but beyond this threshold did not vary with deer density. This corroborated the idea of a saturating relationship between I. scapularis and deer density. Nymphs also were as- sociated with warmer minimum winter temperatures, earlier degree-day accumulation, and higher relative hu- midity. However, nymph abundance only increased with warmer winters and degree-day accumulation where deer density exceeded ~2 deer/km 2 (~6/mi 2 ). Anticipated increases in I. scapularis in the northern tier could be partially mitigated through deer herd management. Key words: blacklegged tick, climate, Ixodes scapularis, Maine, white-tailed deer The blacklegged tick [Ixodes scapularis Say (Acari:Ixodidae)] is the vector of agents causing tick-borne illnesses such as Lyme disease (Spielman et al. 1985), the most common vector-borne illness in the United States (Rosenberg et al. 2018). Within recent decades, geo- graphic expansion of Lyme disease cases in the Midwest and north- east has occurred in all compass directions (Kugeler et al. 2015) and has corresponded with the expanding range of I. scapularis (Eisen and Eisen 2018, Bisanzio et al. 2020). Geographical range expan- sions of invasive I. scapularis populations in the United States have been attributed to a mosaic of factors including 20th century re- forestation followed by suburbanization and burgeoning popula- tions of the white-tailed deer [Odocoileus virginianus Zimmerman (Artiodactyla:Cervidae); Telford 2017]. At the northern edge of its range, northward range expansion of I. scapularis has been at- tributed to climate change (Brownstein et al. 2005a; Ogden et al. 2006a,b; 2008a,b; Leighton et al. 2012; Parham et al. 2015; Eisen et al. 2016; Clow et al. 2017). The spatial extent and abundance of I. scapularis is linked to its associations with bloodmeal hosts, habitat, and climate. Conditions permitting, the life cycle of I. scapularis—eggs, larvae, nymphs, adults—is typically completed in 2–4 yr (Eisen et al. 2016). In the U.S. northeast, I. scapularis larvae and nymphs feed on a range of wildlife hosts, including birds, rodents, and deer (Eisen et al. 2016). Migratory birds are long-distance dispersers of larvae and nymphs (e.g., Smith et al. 1996, Rand et al. 1998, Ogden et al. 2008b), whereas white-tailed deer (Odocoileus virginianus) and rodents Journal of Medical Entomology, XX(X), 2020, 1–14 doi: 10.1093/jme/tjaa180 Research Downloaded from https://academic.oup.com/jme/advance-article/doi/10.1093/jme/tjaa180/5903223 by guest on 12 October 2020