Abstract
Landscape structure, which can be manipulated in agricultural landscapes through crop rotation and modification of field edge habitats, can have important effects on connectivity among local populations of insects. Though crop rotation is known to influence the abundance of Colorado potato beetle (CPB; Leptinotarsa decemlineata Say) in potato (Solanum tuberosum L.) fields each year, whether crop rotation and intervening edge habitat also affect genetic variation among populations is unknown. We investigated the role of landscape configuration and composition in shaping patterns of genetic variation in CPB populations in the Columbia Basin of Oregon and Washington, and the Central Sands of Wisconsin, USA. We compared landscape structure and its potential suitability for dispersal, tested for effects of specific land cover types on genetic differentiation among CPB populations, and examined the relationship between crop rotation distances and genetic diversity. We found higher genetic differentiation between populations separated by low potato land cover, and lower genetic diversity in populations occupying areas with greater crop rotation distances. Importantly, these relationships were only observed in the Columbia Basin, and no other land cover types influenced CPB genetic variation. The lack of signal in Wisconsin may arise as a consequence of greater effective population size and less pronounced genetic drift. Our results suggest that the degree to which host plant land cover connectivity affects CPB genetic variation depends on population size and that power to detect landscape effects on genetic differentiation might be reduced in agricultural insect pest systems.
Highlights
Landscape structure can affect population connectivity and popu‐ lation size, influencing the distribution of genetic variation among populations (Manel, Schwartz, Luikart, & Taberlet, 2003; Storfer et al, 2007)
We examined the relationship between landscape structure and patterns of genetic variation among Colorado potato beetle (CPB) populations, ad‐ dressing the questions: Does crop rotation affect patterns of genetic variation across growing regions? Do populations isolated by more non‐suitable habitat have greater genetic differentiation and less ge‐ netic diversity than other populations? We hypothesized that popula‐ tions connected by more potato land cover, shorter rotation distances, and more suitable overwintering habitat would exhibit less genetic differentiation and higher ge‐ netic diversity, whereas populations separated by more grassland, grain crops, water bodies and greater rotational distances among potato fields would exhibit higher genetic differentiation and lower genetic diversity
We hypothesized that CPB genetic differentiation would decrease with increasing potato, shorter rotational distances, and greater abundance of forest land cover and sandy soil between sites
Summary
Landscape structure can affect population connectivity and popu‐ lation size, influencing the distribution of genetic variation among populations (Manel, Schwartz, Luikart, & Taberlet, 2003; Storfer et al, 2007). The spatial distribution of agricultural pest populations can be conceptualized with a metapopulation model (Slatkin, 1977) in which the amount and location of suitable habitat (host plants) changes over time, and only a fraction of pest individuals find and colonize host patches each generation Under this framework, the founding pest population size at a given host patch depends on the effective distance between plant host patches, which can be increased via resistance of the intervening land cover to pest dispersal (McRae, 2006). We hypothesized that popula‐ tions connected by more potato land cover, shorter rotation distances (in space), and more suitable overwintering habitat (forest land cover and sandy soils) would exhibit less genetic differentiation and higher ge‐ netic diversity, whereas populations separated by more grassland, grain crops, water bodies and greater rotational distances among potato fields would exhibit higher genetic differentiation and lower genetic diversity. Milder winters can contribute to a higher proportion of “volunteer” potatoes (plants resulting from unharvested tubers remaining in the field from the previous year), which could act as an important bridge between overwintering and summer habitat
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