Genetic evidence of bimodal dispersal distances among adult western corn rootworm (Coleoptera: Chrysomelidae)

A coherent understanding of adult western corn rootworm (Diabrotica virgifera virgifera) movement ecology has remained elusive because of conflicting evidence of short- and long-distance lifetime dispersal, a type of dilemma called Reid's Paradox. Attempts to resolve this paradox using population genetics strategies have been hindered by the lack of gene flow-genetic drift equilibrium in much of North America related to this species' recent range expansion out of the Great Plains across the Corn Belt. We addressed this challenge by studying a longer-established population in northeastern Colorado and western Kansas, where D. v. virgifera has been resident for >175 yr. We assessed population differentiation using 2,036 single-nucleotide polymorphism markers to obtain indirect estimates of dispersal distances. Significant isolation by distance and pairwise FST estimates across 14 locations suggest these populations are at or near gene flow-genetic drift equilibrium. Low FST values and shallow isolation-by-distance slopes suggest gene flow over longer distances (280 km) than supported by many direct measures of dispersal distance, another type of dilemma known as Slatkin's Paradox. Indeed, based on estimates of adult population density and Wright's neighborhood at each location, median estimated lifetime dispersal of ~87% of adults was only 174 m. Together, our genetic evidence and findings from earlier studies suggest that D. v. virgifera populations consist of 2 behavioral phenotypes, migrants that engage in long-distance dispersal and residents that disperse only locally by diffusion. The resulting bimodal dispersal distribution resolves both Reid's and Slatkin's paradoxes.