Iris Borer

Iris borers enter through the leaves and exit through the rhizomes

By: Sandy Feather 2009
Penn State Extension

Q. Iris borers have attacked my iris plants. I cut them out of the root, but I am afraid I did not get all of them. Is there a soil drench I could use to keep the borers from surviving over the winter? 

A. Iris borer caterpillars spend the winter as eggs on old iris leaves and plant debris around the base of the plants. Consequently, late season insecticide applications will not reduce their population for next year. Most insecticides do not work on insect eggs. After spending the winter as eggs on the host plant, tiny iris borer caterpillars hatch in the spring as the irises are putting out new leaves. They climb up onto the tender new foliage and chew pinprick-sized holes to gain entrance into the leaves.

Method of Feeding

They tunnel their way through the leaves, feeding and growing, as they work their way down to the rhizome. This feeding damage causes tan streaks in the leaves. They usually work their way down to the rhizomes by mid-summer. At this point, the pinkish caterpillars are one-and-a-half to two inches long, and do tremendous damage by feeding on the rhizomes. Sometimes, rhizomes injured by their feeding damage become infected with bacterial soft rot. This disease is characterized by a foul odor.

Rhizomes infected with bacterial soft rot should be discarded, as there is no chemical control for this disease. By late July or early August, iris borers move into the surrounding soil to pupate. Adult moths emerge in late summer or early fall, and the females lay eggs on iris plants for the cycle to start anew the following year. Adult moths are active at night, so we usually do not see them.

Iris borer damage
Iris rhizomes showing borer damage

The most important thing you can do now to reduce their population for next spring is to cut back and destroy (burn or send out with the trash) old iris leaves, stems and plant debris after we have had a hard frost. The female moths will not be laying any more eggs, and removing the old leaves and plant debris removes most of next year's iris borers.


What to do Next Spring

When the irises start to grow next spring, monitor the plants for pinprick-sized holes and tan streaks in the leaves. You may be able to locate the borer and squish it while it is inside the leaf, or remove the infested leaf. You can also make insecticide applications to control the newly-hatched caterpillars in spring. Insecticides must be applied before the caterpillars tunnel into the leaves. The first application should be made when the iris leaves are about six inches tall, and a second application should follow in 10 - 14 days. Sevin (carbaryl) and Orthene (acephate) are labeled to control iris borers. Organic gardeners can use beneficial nematodes. Steinernema carpocapsae has been shown to attack iris borer.

Have you tried using Nematodes?

Cornell University recommends a rate of 500 nematodes per square inch, applied to a one-square foot area around the base of each plant. Application should be made when soil temperatures are between 50 and 90F, and when there is adequate soil moisture. If necessary, irrigate before and after applying beneficial nematodes. Beneficial nematodes are available from a number of mail order sources.


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