This project explores the respective roles of host (bruchid) species and plant species on the genetic segregation of Horismenus parasitoids and how such segregation may lead to race formation and even speciation. The proposed research is divided into two projects. The first project investigates the role of the bean species and variation within bean species, wild and cultivated, in affecting the population structure of the three Horismenus parasitoids. The second project is aimed to determine the phylogeographies of all trophic levels involved and will look for correlates that reveal the driving forces in determining population structure of the parasitoids. The two projects are expected to provide the key evidence for a primary role of the bean species in mediating host race formation in H. butcheri and determine the relative importance of the bruchid hosts in the population structure of all three parasitoid species.