di·a·mond·back moth 
(dīə-mənd-băk′, dīmənd-)
(dīə-mənd-băk′, dīmənd-)n.
Any of several small moths of the family Plutellidae, having front wings that reveal diamond-shaped spots when folded, especially Plutella xylostella, whose larvae are pests of cabbage and other cruciferous plants.