How to Treat Iris Borer

For every climate extreme from the frigid winters of U.S. Department of Agriculture plant hardiness zone 2 to the mild, tropical climate of USDA zone 11, there's at least one iris (Iris spp.) variety willing to meet the challenge. A much greater challenge for any irises growing within the range of the iris borer moth (Macronoctua onusta) is to survive an attack by the insect's larvae. The tiny caterpillars bore into the leaves and tunnel down to the rhizomes. Unless treated quickly, they’ll turn your irises’ firm, healthy roots to mush.

If your irises have a history of borer infestations, treating your bed preemptively in early spring with ready-to-use pyrethrum insecticide dust may eliminate the caterpillars before they attack the leaves. On a calm, dry day with no rain in the overnight forecast, spread the insecticide around the bases of the iris plants with a dust gun or other hand-operated duster. Use between 1/4 and 3/4 cups of dust, or the label's specified amount, for each 250 square feet. Keep people and pets out of the area until the dust settles, and wait 12 hours before watering. Dress in a long-sleeved shirt, long pants, socks, shoes and protective eyewear when working with the dust, and follow the label's application instructions.

Beginning in late spring, check your irises' foliage for ragged, water-soaked margins, the early sign of borer infestation. Hold the leaves with the sun backlighting them to outline the caterpillars inside, and crush the pests between your thumb and forefinger.

If you're late in discovering borer damage to your irises' leaves, wait until the plants finish blooming before lifting the rhizomes to check for damage. The caterpillars usually don't reach them until early to midsummer. Bacterial rot often follows the feeding and tunneling; it softens and darkens the damaged areas and collapses the irises' leaves. If you're not squeamish and the infestation is light, kill the borers by inserting a flexible wire into their tunnels and stabbing them. Otherwise, cut away the soft, diseased portions of the rhizomes and dip the healthy remnants in a solution of 1 part household bleach to 9 parts water. Before replanting, set the rhizomes in a sunny spot until their cut ends heal.

The most effective way to keep iris borers out of your irises is to keep the borer moths out of your iris bed. The brown or grayish moths leave their soil-sheltered cocoons in late summer to deposit their eggs on dry, yellow iris leaves. A single female may lay up to 1,000 eggs. To stop the egg-laying, remove and dispose of your irises' dead leaves in spring before their new ones emerge. Peel them back all the way to the rhizomes and dispose of them in sealed bags.