About iris borer
Iris borer is the most damaging pest of iris (bearded, Japanese, blue flag, and Belamcanda). Eggs hatch in April or early May. The tiny, white larvae (caterpillars) crawl up the leaves, bore into the leaf tissue where they mine the foliage. Eventually, they work their way down to the plant by boring down the stem until they reach the rhizomes. The damaged leaves appear water-soaked and ragged. The larvae reach the rhizome around early July where they feed. By the time they are mature they are flesh-pink colored, with brown heads and 1 and ½ to 2 inches long. After eating out the interior of the rhizome, the larva pupates in the soil. The adults (moths) emerge and are present from August through early October, flying only at night. The female moths lay their eggs at the base of old iris leaves and debris, especially at the base of old iris plants. There is one generation of this insect each year.
Iris borer damage symptoms. Photo: University of Maryland Extension
Iris borer damage to leaves and bulb. Photo: University of Maryland Extension
Iris borer larva in stem. Photo: John Davidson, University of Maryland
Iris borer damage to stem and rhizome. Photo: University of Maryland
Management
Remove old iris leaves and stalks in the late fall to eliminate the overwintering eggs. Monitor the iris plants in the spring for mining damage caused by the young larvae. These may be killed in the leaves by squishing between your fingers. Clean up infested iris beds after flowering, by digging up the plants. Remove any rotted or damaged portions and kill any larvae that are found.