The iris borer caterpillar is a pest that primarily targets the iris plant. According to the Missouri Botanical Garden, the iris borer causes the most damage to iris plants when compared to any other insect. After borer larvae feed on iris rhizomes, the stems become vulnerable to attack by soft rot, and an entire iris bed can be quickly destroyed.
Symptoms of damage by the iris borer include dark streaks, water-soaked areas and ragged edges on young leaves in the months of May and June. As the larvae continue to feed, they move lower and lower into the rhizome. Cutting open the stem of an infested plant reveals the borer.
The life span of an iris borer is about one year. Adult moths emerge in August and September, mate and deposit eggs on dead iris leaves. The eggs hatch in April or May as the new iris leaves begin forming. As the larvae grow, they bore holes in the iris leaves and eventually make their way into the rhizomes. By mid-July, the larvae reach the rhizomes, where they grow to be around 2 inches in length. After this, the larvae move into the soil to pupate. Adult moths begin to emerge in August, and the cycle begins again.