Abstract Neorthacheta dissimilis Malloch (Diptera: Scathophagidae) is a poorly known scathophagid fly that feeds and develops on iris (Iridaceae). A survey of its occurrence was performed at the Montréal Botanical Garden (Montréal, Québec, Canada) in 2018, 2019, and 2020. Iris species and cultivars from two subgenera, Iris and Limniris, were evaluated for larval infestation. When pooled for subgenera and years, data from 18 Iris classes revealed high levels of infestation per flower stalk, ranging from 34% to 100%. When analysed per bud or flower, levels of infestation remained high, ranging from 19% to 100%, but generally was lower than for flower stalks as the unit of replication. The mean number of N. dissimilis larvae per infested flower or bud was higher for the subgenus Limniris (1.13) than for the subgenus Iris (1.03), with a maximum of four N. dissimilis larvae per flower being observed. These figures are worrying for horticulturalists because the insect is prevalent and causes either abortion or aesthetic damages to iris flowers.