Iris borers spend the winter as eggs deposited on plant debris near or around the base of infested plants. Remove and destroy old foliage and plant residue now to eliminate an important source of problems with this destructive insect next year.
Pink caterpillars with brown heads do the damage. Tiny caterpillars that hatch from the eggs in early spring chew into iris foliage and eat their way downward to the rhizomes, arriving there around mid-summer. Continued feeding by the caterpillars takes its toll on the plant as well as providing entryways for soft rots.
An insecticide application, made next spring when there is 4 to 6 inches of new growth, can kill newly hatched borers before they enter the plant. A second spray about 2 weeks later will extend the protection. This is advisable in cases of severe damage from the previous year and / or lack of a good sanitation program.
by Lee Townsend, Extension Entomologist