How can I control the iris borer?
The iris borer is a serious pest of bearded irises. The mature stage of the iris borer is a grayish moth. Female moths lay eggs on iris foliage and other nearby plants in late summer/early fall. The eggs hatch the following spring. The small larvae (caterpillars) bore their way into the iris foliage and feed on leaf tissue. Over time, the larvae tunnel down through the leaves and into the rhizomes. The caterpillars continue to feed inside the rhizomes and eventually destroy much of it. When fully grown, the larvae move into the soil and pupate. Adults (moths) emerge in late summer.
Bacterial soft rot often invades rhizomes damaged by iris borers. Rhizomes infected with bacterial soft rot become soft and foul-smelling.
Iris borers can be controlled by sanitation and the timely application of insecticides. Remove and destroy dead iris foliage in late fall or early spring. This will eliminate many of the iris borer eggs. An insecticide should be applied in spring when the new shoots are 4 to 6 inches tall. An application of an insecticide at this time should destroy small iris borer larvae before they have the opportunity to bore into the iris foliage. Products that contain acephate, permethrin, or spinosad should be effective. As always, carefully read and follow label directions when using pesticides.