Genetic evidence of bimodal dispersal distances among adult western corn rootworm

Net per-generation dispersal distance of individuals can be estimated from the radius of Wright's neighborhood area. We report estimates of the genetic neighborhood radius of western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), populations in its presumed ancestral home range of eastern Colorado using a genotyping by sequencing approach. 2,036 SNP loci were employed to calculate pairwise F_ST’s between 14 populations along two transects and revealed significant isolation by distance. Combined estimates of neighborhood size and adult population density were used to calculate neighborhood radii and confidence limits, which were all < 1 km, and the median of means for radius for both transects combined was 0.174 km, or only 174 m. These estimates of average lifetime dispersal radius for western corn rootworm adults seem surprisingly low, on the order of the radius of a typical circle-irrigated cornfield in the region where the study was conducted. This small neighborhood size contrasts with the number of migrants exchanged between populations per generation estimated from pairwise F_ST’s, ranging from 18 to >250, indicating a fairly high rate of movement regardless of distance. However, the estimates for neighborhood radius theoretically reflect diffusion dispersal distances of only about 7/8 (86.5%) of the population. Thus, our results suggest a bimodal pattern of dispersal behavior in a western corn rootworm population, where ~1/8 of the population disperses beyond 0.174 km (i.e., beyond the immediate environs of the natal field), often long distances of at least 275 km, the greatest distance examined.