The iris borer causes more damage to iris plants than any other insect. Feeding of the borer larva opens the rhizome to attack by soft rot that can quickly turn rhizomes into foul-smelling mush. A healthy bed of iris can be destroyed quickly.

Symptoms and Diagnosis

Damage is typified by dark streaks, water-soaked areas, and ragged edges on young leaves in May and June, caused by the feeding of the young larvae. As the larvae feed, they move down into the rhizome where they continue feeding. Soft rot can set in, causing leaves to yellow and fall over and rhizomes may become soft and foul-smelling. Cutting open the rhizome will reveal the borer, a 1- to 2-inch, fat-bodied, pink larva with a brown head.

Life Cycle

Adult moths emerge from the soil in August and September, mate, and deposit eggs on dead iris leaves. The crevices of dried and crinkled leaves or rolled leaf areas make a good place to lay their eggs. The base of leaves and other plant debris is the wintering place of the eggs. Eggs are laid in groups of 3–5 or more; a single female moth may deposit 1000 eggs before dying. Moths are typical millers— dark gray to brown, with a wingspan of about 2 inches. There is only one generation per year.

The eggs overwinter in the plant material and hatch in April or early May as the new iris leaves are expanding. The small, young larvae crawl up onto the new iris leaves and make tiny pinpoint holes as they enter. Once the larvae enter the foliage, they act as leafminers, tunneling to the base of the leaves throughout the spring. By early to mid-July the larvae reach the soil area and tunnel into the rhizomes.

In the rhizome, the larvae grow to be 1 to 2 inches in length. In late July to early August, the iris borer larvae move from the rhizomes into the soil to pupate. The pupa is a non-feeding transition stage between the larva and the adult moth. Pupae are dark brown to black and are usually found in the top 2 inches of soil. After two to three weeks in the pupa stage, the iris borer moth emerges. The adult brown moths emerge in late August and September to mate, lay eggs, and repeat the cycle.

Integrated Pest Management Strategies

1. Sanitation. The main key to iris borer prevention is the removal and destruction of the previous year’s dead foliage before April 1. This sanitation practice will reduce the problem by destroying overwintering eggs in the residues. This is best done in spring. Peel off the dead foliage down to the rhizome.

2. Inspect bed. If signs of borers are apparent, dig the rhizomes after blooming is completed. Inspect them carefully, cut out damaged ones, and replant them. During the spring, inspect the leaves for signs of borers larvae chewing and pinch down the leaves to kill any larvae that may be inside.

3. Precaution. An insecticide containing pyrethrum spread or sprayed around the base of plants in the spring may help in destroying the newly hatched larvae before they have a chance to enter leaves.

4. Insecticides. If the problem is severe, insecticides are available to help combat borers. When iris leaves are 3–4 inches long, begin spraying at two-week intervals, using azadirachtin (Neem oil), endosulfan (Thiodan), or methyl nonyl ketone. These are available under various trade names at nurseries and garden centers. Always follow label directions and read the entire label before using any pesticide.

5. Replanting. When resetting your iris beds, it is important to trim away damaged or rotted areas on the rhizomes. To eliminate soft rots caused by bacteria, cut off the damaged parts of the rhizomes and dip the undamaged portions in a solution of 1–part household chlorine bleach and 9 parts water.

Organic Strategies

Strategies 1, 2, and 3 are strictly organic approaches. For an organic approach to Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate spinosad products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against iris borer. Not following the pesticide label before usage is a violation of federal law.