You see lovely blooms when you look at your iris bed in May and June. If you also see ugly brown streaks on the leaves, pay attention: you may have an iris borer infestation. These pale pinkish caterpillars are Enemy No. 1 for irises, because after they tunnel through the leaves they start feeding on the rhizome.
The evidence on the leaves is actually secondary to the real damage, which happens when the iris borer (Macronoctua onusta) finally reaches the rhizome, in midsummer, and feasts on it. What’s not eaten is often destroyed by soft rot. Infected iris leaves turn brown at the tips; later, if the damage gets worse, the leaves may turn yellow.
If you see brown streaks and chewing damage in May or June, pinch the leaves to kill the larvae inside. After the irises have bloomed, dig up rhizomes and cut out any that are rotten or infested with borers. Dip healthy parts in a 1:9 solution of bleach and water. Before next spring, remove and destroy the dead foliage, which is where the borer eggs overwinter.
—Photo courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden.