Population structure within coevolving groups determines which individuals interact with one another and thereby influences patterns of selection. In disease systems involving a diversity of hosts, population structure due to differential host use may cause coevolutionary trajectories to differ among subpopulations of one or more of the species. I documented the association between infection with barley yellow dwarf viruses (BYDVs) and fitness in three weedy grass species, and explored two possible contributors to heterogeneity in this association: host use by the aphids that vector these viruses, and host use by the viruses themselves.