The larvae gradually move downward through the sheaves to the crowns and rhizomes as they grow. Irises later develop loose, rotted bases and holes in rhizomes.
Bacterial rhizome rot readily attacks borer-infested plants.
The pink larvae are cutworm-like caterpillars with rows of black spots along their sides. Iris borer adults are medium-large brown moths with black markings. Borer larvae hatch in early spring from overwintering eggs. For a short time, they wander over the surface of the leaf but soon seek concealment. These caterpillars pierce leaves and tunnel into the sheaves.