Isolation by Distance, Source-Sink Population Dynamics and Dispersal Facilitation by Trade Routes: Impact on Population Genetic Structure of a Stored Grain Pest

Population genetic structure of agricultural pests can be impacted not only by geographic distance and the broader ecological and physical barriers but also by patterns related to where crops are produced and how they are moved after harvest. Stored-product pests, for instance, specialize in exploiting grains such as wheat and rice from on-farm storage through transportation to final processing at often geographically distant locations; therefore human-aided movement may impact their dispersal. Although stored product insects are associated with stored grain, they can also exploit resources in the surrounding environments so different ecological regions where the grain is grown and stored may also influence population structure. Here we used 1,156 SNP markers to investigate how geographic distance, ecological and agricultural variables can impact the genetic structure and gene flow of the stored food pest beetle Rhyzopertha dominica. We found a substantial degree of admixture between weakly structured populations in the US. Ecological regions were more important in explaining R. dominica population structure than crop type, suggesting insect movement between wheat and rice grain distribution channels. We have also found a significant correlation between the genetic and geographical distance (i.e., isolation by distance). However, our modeling approach combining the ecological and management variables has highlighted the importance of the volume of grain received by a location in the dispersal dynamics of the pest. The first-generation migrant analysis offered additional supported to movement over great distances that are likely associated with grain movement. Our data suggest that a multitude of factors play small but significant parts in the movement dynamics of the pest. The beetles can take advantage of the source-sink dynamic of grain movement in the US, but also engage in a high rate of movement at the local scale. Understanding population structure for R. dominica will provide insights into the potential for local processes of adaptation and broader patterns of movement that will impact management programs and the potential for spread of resistance genes.