Iris borer Macronoctua onusta
IDENTIFICATION — This pest may be the most dreaded problem for gardeners with iris. It is difficult to know if you’ve got a problem until you see the signs. If you find brown streaks down the leaves, you need to investigate further. You may also notice the fans falling over. Dig up the rhizomes and look for mushy parts and borer exit holes. Iris borers not only do significant damage to the foliage and rhizomes of iris, but they open the plants up to bacterial rot of the rhizome.
LIFE CYCLE — The iris borer is actually the larval stage of a small moth. The larvae hatch in spring and begin eating holes in the leaves and burrowing toward the base of the iris plant. They eventually reach 1½ to 2 in. long and are white or pale pink. They eat through the rhizome and pupate in the soil.
CONTROL — Unfortunately, most gardeners don’t notice iris borers until their iris have suffered some damage. If you see brown streaks in the foliage or the fans fall over, dig up the rhizome and examine the tissue. Cut away any mushy or holey areas of the rhizome. Dip the remainder in a solution of one part bleach to four parts water. Allow the rhizome to dry for a day or two before replanting it. You may want to consider moving your iris planting if you have a serious infestation.
There are some biological controls for iris borer. Two nematodes (Steinernema carpocapsae and Heterorhabditis bacteriospora) can be added to water and sprayed on the iris plants. These nematodes lay their eggs in the borers and prevent them from doing much damage.