Iris borer tunnels into the rhizomes of iris, causing dieback of the plants. Moths emerge and lay their eggs in the spring on the developing iris fans. The hatching larvae tunnel into the leaves and live as leafminers, causing brown, water-soaked-appearing streaks down the leaves. Once the caterpillars mine to the base of the leaves, they enter the rhizome. Tunneling in the rhizome increases the likelihood of rot in the rhizome. Both the borer and the rotting contribute to the potential decline and death of the iris.
Tall bearded iris is very susceptible to iris borer, but its large rhizomes allow it to survive attack; it may still grow and flower with moderate infestation. Many varieties with small rhizomes, such as Siberian, Louisiana, and Japanese irises, are much less likely to be attacked; however, even small infestations severely harm or kill these small-rhizome varieties.
Many growers successfully dig up the rhizomes in mid- to late summer, remove the borers, and replant the rhizomes. Other growers do not have much success with this method.
Iris borers are most susceptible to insecticides when traveling down the leaves as leafminers. When the fans are about 6 inches high, applying a systemic insecticide such as acephate (sold as Orthene) or spinosad (Conserve) is effective.