The iris borer (Macronoctua onusta) is the most destructive insect pest of iris. It directly damages leaves and rhizomes and introduces the bacterium that causes a foul-smelling soft rot.
Damage first appears in May or June when the iris borer caterpillar feeds inside the foliage and causes dark-streaked, watery areas or ragged edges on the developing leaves. The caterpillars move downward in the plant, and by July or early August, may have caused extensive destruction inside the rhizomes. Rhizomes may be completely hollow from borer feeding or decayed by the soft rot bacteria.
Iris borer caterpillars are usually found inside the rhizome at the time irises are dug and divided. Some may have tunneled a short distance away to pupate in the soil. The caterpillars are smooth and plump and resemble cutworms. They vary from light to deep pink with a brown head and are 1.5 to 2 inches in length.
Iris borer moths emerge from the pupae in the soil by late summer and lay eggs on the dying iris leaves and surface debris. The eggs overwinter and hatch the following April or early May. There is only one generation per year. Iris borers and borer-damaged rhizomes uncovered during dividing should be discarded. If you find empty, damaged rhizomes, search the nearby soil for shiny, dark chestnut-brown pupae and discard them, also. There is no benefit to treating the soil when replanting the irises.
Control: Sanitation is essential in managing iris borer. Discard all damaged and infested rhizomes. During the fall or early spring, remove and discard all old plant material and debris from the iris bed (to eliminate eggs before they hatch). If iris borer has been a persistent problem, you might consider spraying the foliage with insecticide in the spring when leaves are 5 to 6 inches in length. Insecticide sprays are only effective if used against small larvae before they tunnel into the plants. Use a general garden insecticide such as azadirachtin, pyrethrins, permethrin, or spinosad according to label directions.
Beneficial, insect-attacking nematodes may be a naturally-occurring biological control. They can also be purchased and added to the soil, though the effectiveness of commercially available nematodes has not been documented in Iowa. Nematodes are perishable and need a consistently moist environment to survive. Apply nematodes when iris leaves are almost fully expanded but before flowering and repeat one week later. For best results, apply during a rain shower or irrigate the soil before and after application.