Iris borer caterpillars (Macronoctua onusta) are the most destructive insect pests of iris. The pinkish caterpillars are two inches long when full grown. Adult moths have chocolate-brown front wings and lighter yellow-brown hind wings with a wing span up to 2 inches. Moths are seldom seen because they fly at night.
Life cycle and damage
Iris borers spend the winter as eggs on old iris leaves and plant debris at the base of iris stalks. In early spring, they hatch into tiny caterpillars and climb up new foliage. They chew pinprick-sized holes and tunnel inside leaves as they continue down toward the rhizome. Their feeding causes streaks that appear tan or water-soaked. Later, the tips of infested iris turn brown and appear to senesce (age prematurely), although the entire plant seldom dies.
The caterpillar tunnels through the leaves reaching the rhizome by mid summer (July in Minnesota). At that point, caterpillars have grown to about 1½ to 2 inches in length. Iris borers do severe damage to iris by consuming the rhizome. Iris borer feeding can allow the entry of a bacterial soft rot. Rhizomes infected by soft rot are slimy, soft and foul-smelling.
There are several iris diseases with similar symptoms that can occur when iris borer is absent, including bacterial soft rot, iris scorch (probably caused by a phytoplasma), and fungal leaf spot (Didymellina macrospora). All of these disease organisms will cause iris leaf tips to brown and iris plants to senesce, mimicking symptoms caused by iris borer.
In late July or early August, iris borer caterpillars move into the soil where they pupate. They emerge as moths in late summer or fall. Female moths complete the life cycle by laying eggs on old iris plants in August and September where eggs remain until next spring.
Management of iris borers is difficult. However, there are several steps that can be taken to reduce their damage in gardens.
Although all types of iris may be infested by iris borer, Siberian iris are more tolerant to iris borer attack.
Proper sanitation is important. Check iris during spring for evidence of chewing damage and water-soaked streaks. This injury can be easily overlooked so look carefully. If you discover iris borer damage early in the season, you can crush the insect while it is inside the leaf or remove the infested leaf.
Sanitation in July is critical if you are having problems with iris borer. If an iris plant has above-ground symptoms (brown leaf tips, early senescence), dig it up and examine the rhizomes. Discard rhizomes containing iris borer caterpillars and those with tunnels.
During fall, remove and destroy (e.g. burning or burying) old iris leaves, stems, and any nearby plant debris. This removes and kills overwintering eggs, minimizing the risk of iris borers next year. Clean up iris beds anytime after we get a hard frost when female moths are no longer laying eggs.
A well-timed insecticide application can help reduce an iris borer infestation. Two options are acephate (Orthene) or spinosad (Bulls-Eye). It is important to time the application when eggs are just hatching. This is approximately when new growth is about four to six inches high. A repeat treatment 10-14 days after the first application may be necessary. Dimethoate (often sold as Cygon 2E) was a commonly used product for iris borer management, however it was canceled in 2002 and is no longer available.
If your irises have been unaffected in the past, you do not need to use an insecticide to protect them. Maintain good sanitation practices to prevent problems and to help detect an iris borer problem when it first begins.
Beneficial nematodes are a natural option to help control some boring insects. These nematodes are microscopic worms that seek out and kill the caterpillars by introducing a bacteria. Research results using nematodes have been inconsistent, although they can potentially be effective in reducing iris borer populations.
Two species of nematodes are commercially available from natural garden supply catalogs or your local garden center. Both species (Heterorhabditis and Steinernema) have been shown to attack iris borer. Nematodes need a moist environment to survive and move. The easiest time to apply is during a rain shower. Lacking rain, water iris before and after applying nematodes. Apply nematodes when iris leaves are almost fully expanded but before flowering. Repeat the treatment approximately one week later. Nematodes are perishable; check the label for the expiration date.
Published in Yard & Garden Brief February 2003