Insect mycophagy is considered common but generally lacking host-specificity. Larvae of some Mycodiplosis species (Insecta, Diptera) feed primarily on spores of rust fungi (Basidiomycota, Pucciniales). The number of rust-feeding species and their relative frequency, distribution, and degree of host-specificity are not known. A survey of 200 recent rust collections from around the world, and a systematic survey of 333 herbarium specimens from Maryland show that Mycodiplosis infestation is very common. Desiccated larvae were found on specimens dating back as far as 1886, the oldest collection in the survey. Greater than 20 % of all rust collections examined were infested with Mycodiplosis larvae. In Maryland infestation frequencies were similar at different spatial scales, but different rust species varied in their frequency of infestation. Primers were designed to target Mycodiplosis 28S rDNA, and sequence data revealed genetic variation between Mycodiplosis isolates from different rust species.