Eggs usually are laid in crevices or rolled areas of the leaves. The egg overwinters and begins to develop in early spring. Eggs hatch during April and May and the young larvae begin to feed on young, rapidly growing iris leaves. After feeding on leaf surfaces, causing scars from which sap runs down the leaf, the larvae assume a boring habit, entering the leaves and gradually mining downward. Leaf mining injury is characterized by dark, water-soaked areas or slits in the leaves.
Some chewing also occurs on leaf margins.
As the borers feed, bacteria infect the rhizome, initiating the soft rot. One key to iris borer prevention is removal and destruction of the previous year's foliage before April 1. This sanitation practice will reduce the problem by destroying overwintering eggs in the residues. When resetting your iris beds, it is important to trim away damaged or rotted areas. To elimate soft rots caused by bacteria, cut off the damaged parts of the rhizomes and dip the undamaged portions in a solution of 1 part household chlorine bleach and 10 parts water.