Impacts of insect herbivory have been hypothesized as a potential mechanism for suppressing the invasiveness of non-native plant species. Occurrence of the exotic bull thistle, Cirsium vulgare (Savi) Tenore, is sparse, and unexpectedly, bull thistle has not become a noxious weed in eastern NE. Evidence suggests that the thistle-feeding local insect community has been shifting over from the native tall thistle, Cirsium altissimum (L.) Spreng, to the non-native bull thistle, likely providing biological resistance to the invasiveness of bull thistle in eastern NE. However, it remains unknown which guilds or species of the insect groups on tall thistle have been transferring onto bull thistle. We aimed to: 1) determine species richness, abundance, and the proportions of thistle generalists and specialists on both thistle species, and 2) test if local spatial scale affects transfer of insects to bull thistle. By quantifying arthropod abundance and richness through intensive observations, we found that species accumulation on bull thistle occurred rapidly and that the rate of species build-up varied by season. In addition, the floral guild formed a large proportion of the entire insect community on both thistle species. These results quantify the structure and dynamics of the insect community reducing reproductive performance of the exotic bull thistle.