Host‐associated differentiation (HAD) is considered a step towards ecological speciation and an important mechanism promoting diversification in phytophagous insects. Although the number of documented cases of HAD is increasing, these still represent only a small fraction of species and feeding guilds among phytophagous insects, and most reports are based on a single type of evidence. Here we employ a comprehensive approach to present behavioural, morphological, ecological and genetic evidence for the occurrence of HAD in the gall midge Dasineura folliculi (Diptera: Cecidomyiidae) on two sympatric species of goldenrods (Solidago rugosa and S. gigantea). Controlled experiments revealed assortative mating and strong oviposition fidelity for the natal‐host species. Analysis of mitochondrial DNA showed an amount of genetic divergence between the two host‐associated populations compatible with cryptic species rather than host races. Lower levels of within‐host genetic divergence, gall development and natural‐enemy attack in the S. gigantea population suggest this is the derived host.

No abstract