Impacts of urban versus agricultural landcover on gene flow in the walnut husk fly, Rhagoletis suavis
We are investigating associations between landcover and spatial population dynamics in the walnut husk fly, Rhagoletis suavis. Our study area in Southeast Iowa is a 360 km2 landscape, ~65% monoculture cropland and 25% urban/ suburban landcover. Previous landscape ecological analysis showed that, despite complete habitat occupancy across the landscape, fine grain (30 m) landcover composition explains larval population density in this herbivore at a lower effective spatial scale in urban/ suburban areas than in the rest of the landscape, with local negative spatial autocorrelation in these densities clustered on urban landcover. Fine scale landscape composition has thus been shown to impact local population densities that may in turn, along with other demographic traits (e.g. sex ratios), mitigate dispersal tendencies. We are assessing the extent to which matrix landcover composition and local demographic traits explain patterns of genetic variation in R. suavis at fine spatial scale. We are analyzing microsatellite data for >30 R. suavispopulations using 13 polymorphic loci. We are combining this genetic dataset with a larger, multi-year, landscape ecological dataset to better understand how demography and matrix characteristics may interact to impact insect distributions in human-dominated landscapes.
Preliminary analysis of eight populations shows that natal population density explains a significant proportion of mean population differentiation (R= 0.71, p= 0.16), consistent with the prediction that population densities are important in mitigating dispersal. Further, the positive relationship between density and differentiation implicates negative density-dependent dispersal. Pair-wise FST values show fine scale population structure in both urban and agricultural R. suavis populations, with significant differentiation detected between all populations, even in close proximity (< 1.5 km), and low levels of inbreeding detected in most (> 60%) individuals. Further, counts of over 5,000 R. suavis flies collected over three seasons show significantly female-biased sex ratios. Preliminary comparisons of male versus female FST values indicate that dispersal in R. suavis is also female-biased. These data further underline the potential importance of demographic traits in mediating dispersal in complex landscapes. Our preliminary results suggest that R. suavis is an excellent model for disentangling the complex interplay between demography, ecology, and population genetics in the context of human-dominated landscapes. The insights we generate will contribute to a growing empirical foundation to understand the spatial population dynamics of insects in intensely altered landscapes.