Host‐race formation: promoted by phenology, constrained by heritability

Host‐race formation is promoted by genetic trade‐offs in the ability of herbivores to use alternate hosts, including trade‐offs due to differential timing of host‐plant availability. We examined the role of phenology in limiting host‐plant use in the goldenrod gall fly (Eurosta solidaginis) by determining: (1) whether phenology limits alternate host use, leading to a trade‐off that could cause divergent selection on Eurosta emergence time and (2) whether Eurosta has the genetic capacity to respond to such selection in the face of existing environmental variation. Experiments demonstrated that oviposition and gall induction on the alternate host, Solidago canadensis, were the highest on young plants, whereas the highest levels of gall induction on the normal host, Solidago gigantea, occurred on intermediate‐age plants. These findings indicate a phenological trade‐off for host‐plant use that sets up the possibility of divergent selection on emergence time. Heritability, estimated by parent–offspring regression, indicated that host‐race formation is impeded by the amount of genetic variation, relative to environmental, for emergence time.