Delia sanctijacobi is critically assessed and given a revised description using data from scanning electronic microscopy (SEM) and DNA barcode analysis. This species is recorded for the first time in Brazil and Peru. We provide a morphological identification key (with figures) for Delia species from Brazil, a molecular identification based on COI (cytochrome C oxidase subunit I) barcode sequences and an updated distributional map. We also report the first occurrence of D. sanctijacobi feeding on Brassica species in Brazil. This potential pest was observed in broccoli roots (Brassica oleracea var. italica; Brassicaceae) in União da Vitória, Paraná, southern Brazil, in August and September of 2017. The infested plants displayed reduced growth due to damage to the stem base or death if severely attacked.