University of Illinois Extension

University of Illinois Extension

Hort Answers

Insect Damage

Iris Borer
Macronoctua onusta

3 (1 = rare 5 = annual)
4 (1 = very little damage 5 = plants killed)
Bearded iris is most commonly affected; however, it can usually survive attack. Siberian and Louisiana iris are less commonly attacked, but attack normally kills the plants.

Plants Affected
The main indication that the iris has a borer is the yellow streak caused by a bacterial rot that grows down through the borer's tunnel into the rhizome. Between the borer feeding and the bacterial rot, the iris declines in vigor and appearance (yellowing, browning of foliage). Severe feeding and rotting may kill the iris.

Life Cycle
The borer egg hatches and the larva enters the leaf when the leaf is about 4-6 inches high. The insect works its way down the leaf into the rhizome. In the rhizome, the insect continues feeding till it is a full size larvae. The adult moths emerge, mate, and lay eggs on the overwintering plant.

Clean up plant debris before plants begin growing in spring. Use suggested insecticides if necessary when fans are 4 to 6 inches high. Dig up and remove borers, scrape out decay, dip in 10 percent chlorine bleach solution, dry for several hours in sunlight. Replant so that top of rhizome is visible after watering the rhizomes in. Cut leaves to 4-6 inches in height.

Related Resources
Home, Yard & Garden Pest Guide
Illinois Commercial Landscape and Turfgrass Pest Management Handbook
U of IL - Distance Diagnosis through Digital Imaging
U of IL - Plant Clinic