Iris borer (Macronoctua onusta) moths lay their eggs on the foliage of irises in late summer and early fall. The eggs overwinter on old foliage and emerge in the spring as tiny caterpillars that tunnel into the newly emerging foliage. Throughout the summer, they tunnel their way down to the rhizome, growing to about two inches long. By the time they are fully grown, they start tunneling into the rhizome. Once they are ready to pupate, they leave the rhizome to dig into the surrounding soil. In late summer, they emerge as moths, and the cycle begins anew.
Signs of Iris Borer
The first signs of iris borer damage are difficult to recognize unless you know what to look for. Tunneling through the foliage that looks waterlogged is a sure sign of iris borers. If the tips of the foliage turn yellow and then start to look dead, that's another sign. And, if the base of the stalk has become yellowish-brown and mushy, and has a sickly odor, they have gotten into the rhizome and rot has set in. If you are digging irises to divide or move them, you may notice holes in rhizomes as well.
Effect on Irises
In some cases, the plant will, surprisingly, live on if rot hasn't set in. Most likely, it will bloom less, or not at all. In many cases, however, we aren't as lucky and the caterpillars cause so much damage that the rhizome rots completely.
Organic Control for Iris Borers
Monitor your plants closely throughout April and May. If you begin to see signs of tunneling, prune off the affected foliage below where the damage occurs. The caterpillar, still quite small, will be trapped inside the section you've removed, and you can destroy. If you are noticing rot at the base of the stalk, dig up the rhizomes to inspect them. If you see tunneling into the rhizome but are unsure whether or not the border is still inside, soak the rhizome in water. The borer will drown. Inspect the rhizome. If the damage isn't too bad, cut away the damaged areas, let the cut dry for several hours, and replant.
The key here is tidiness. If there is no leftover foliage in which the eggs can overwinter, there will likely be no caterpillars to contend with. Always remove old stalks and foliage after the plants are hit with a frost. If iris borer is a persistent problem, consider planting resistant varieties. While iris borer can attack any type of irises, it is less likely to bother with Siberian irises, because the rhizomes are planted deeper in the soil.