Although all major groups of vascular plants are used extensively as a food source by phytophagous insects, these groups are not utilized equally. A few insect species use ferns, belong mainly to Coleoptera, Hymenoptera, and Lepidoptera. Hymenopteran fern feeders provide a good model system for investigating evolutionary questions because the proportion of fern feeders in phytophagous Hymenoptera is higher than that in other insect orders.

Strongylogaster is notable for its use of ferns as host plants. We reconstructed a molecular phylogeny of Japanese Strongylogaster and then used the reconstructed phylogeny to infer the host-plant relationships in the genus. We found that the groups can be divided into two clades depending on whether they use Pteridium aquilinum and each Strongylogaster clades generally uses several species of ferns belonging to different families. In addition, Selandriinae (Hymenoptera; Tenthredinidae) involving Strongylogaster is composed of various genera with foraging different styles (cf. fern feeder, no fern feeder or generalist). To reveal fern specialist is phylogenetic monophyly group or not, we reconstructed a molecular phylogeny of nine genera of Japanese Selandriinae.