We investigated the guild structure of phytophagous insects on Brassicaceae in Poland and the influence of host-plant parameters (e.g. sampling effort, geographical distribution, taxonomic isolation, nitrogen indicator values) on the species richness of these guilds. The data were extracted from a study published by Lipa et al. The overall number of recorded ectophagous species is about twice the number of endophagous species. Irrespective of the feeding niche, species are predominantly oligophagous, feeding on more than two Brassicaceae genera. The relative importance of endophagous species within the fauna decreases with increasing host range. The sampling effort predicts a high proportion of the variance of alpha-diversity in oligo- and polyphagous insects, but only a low proportion in specialized species. After correction for sampling effort, most of the plant parameters do not explain an additional proportion of the variance in species richness. We hypothesize that the uniform chemical defence system across genera in Brassicaceae is an important factor triggering host range and diversity patterns in phytophagous insects on crucifers.