Since the 1970s, the Midwestern USA has experienced an expansion of blacklegged ticks (Ixodes scapularis), the primary vector of Lyme disease caused by Borrelia burgdorferi, leading to increased Lyme disease incidence. Public health surveillance indicates that Northwestern Wisconsin has served as refugia for these ticks, seeding an expansion into neighboring states such as Michigan. However, the process of re-emergence and invasion remains unclear. To improve tick management, we examine whether environmental variables in the Midwestern (eastern North Central) region have constrained tick dispersal and whether connectivity corridors can be identified. By developing fine-scale spatial population genomic data, our analyses reveal genetically diverse populations in Wisconsin, with northern populations contributing to recent expansions within the state. We identify several east-west gene flow corridors facilitating tick dispersal in Wisconsin. An independent dispersal network exists along Wisconsin’s Mississippi River, extending southwards. In contrast, Michigan populations exhibit sharp genetic divergence from Wisconsin and Indiana populations, with low genetic diversity and high in-state gene flow. We also identified high landscape connectivity in the region connecting the Michigan Peninsulas and significant gene flow at the landmass near southern Lake Michigan. Geographical isolation, as well as landscapes with low soil humidity during summer and high human disturbance, were found to limit gene flow across the region, although these effects were minor. Management of blacklegged ticks in the region can be enhanced by recognizing that landscape connectivity has influenced the dispersal of distinct genetic populations, and targeted interventions in seemingly less tick-favorable landscapes may help mitigate the spread.

Since the 1970s, the Midwestern USA has experienced an expansion of blacklegged ticks (Ixodes scapularis), the primary vector of Lyme disease caused by Borrelia burgdorferi, leading to increased Lyme disease incidence. Public health surveillance indicates that Northwestern Wisconsin has served as refugia for these ticks, seeding an expansion into neighboring states such as Michigan. However, the process of re-emergence and invasion remains unclear. To improve tick management, we examine whether environmental variables in the Midwestern (eastern North Central) region have constrained tick dispersal and whether connectivity corridors can be identified. By developing fine-scale spatial population genomic data, our analyses reveal genetically diverse populations in Wisconsin, with northern populations contributing to recent expansions within the state. We identify several east-west gene flow corridors facilitating tick dispersal in Wisconsin. An independent dispersal network exists along Wisconsin’s Mississippi River, extending southwards. In contrast, Michigan populations exhibit sharp genetic divergence from Wisconsin and Indiana populations, with low genetic diversity and high in-state gene flow. We also identified high landscape connectivity in the region connecting the Michigan Peninsulas and significant gene flow at the landmass near southern Lake Michigan. Geographical isolation, as well as landscapes with low soil humidity during summer and high human disturbance, were found to limit gene flow across the region, although these effects were minor. Management of blacklegged ticks in the region can be enhanced by recognizing that landscape connectivity has influenced the dispersal of distinct genetic populations, and targeted interventions in seemingly less tick-favorable landscapes may help mitigate the spread.

For several decades, the Midwestern USA has been impacted by blacklegged tick (Ixodes scapularis) range expansion, which, as the main vector of the Lyme disease-causing bacterium Borrelia burgdorferi, is linked to a regional increase in Lyme disease incidence. Earlier studies of genetic differentiation of blacklegged ticks have not tested detailed hypotheses about range expansion in the Midwest, despite the importance of this topic to public health. We addressed this gap by investigating the origin and environmental factors that influenced blacklegged tick establishment and spread in the Midwestern region. By analyzing fine-scale spatial population genomic data, we find low genetic differentiation consistent with the known recent range expansion. However, within Wisconsin, blacklegged ticks have unique genetic ancestries that differ from other Midwestern regions, suggesting multiple origins. Our data provide evidence for blacklegged tick sources in northern Wisconsin contributing to the recent expansion. In addition, we find a distinctive mixture of ancestry along the Mississippi River in southwestern Wisconsin and in Indiana, which was previously not identified. The most recently invaded populations in Michigan exhibit sharp genetic divergence from Wisconsin and Indiana samples despite their proximity, warranting further examination of their genetic origin and expansion processes. Lastly, landscape factors contribute to significant reductions in gene flow, potentially limiting genetic exchange and disease transmission within Midwestern states. This new knowledge of blacklegged tick range expansion processes can improve vector surveillance, pest management, and public health related to tick-borne disease risks.