Iris Borers

June 24, 2013 | Common Garden Pests



Iris borers, Macronoctua onusta, are probably the most destructive pests of  the iris.  The iris borer overwinters as eggs on the stalks of iris plants or in the debris around the base of plants.   After hatching in the spring the caterpillars climb up the plant and begin boring down through the new foliage towards the rhizome.  The larvae will be fat, pink with a brown head, and a light strip down the back.  The iris borer will reach the rhizome by late June or July depending on your zone.   Towards the end of summer the borer will exit the rhizome and move into the surrounding soil and pupate.  At this point the borer can be up to 2 inches long.  The borer is most susceptible to Ecomask while in the rhizome and while in the soil.   Soon after the adult moth will emerge, mate, and lay its eggs on or around the old plants.

Iris Borer

Iris borer (Macronoctua onusta)

Treating for Iris Borers with Nematodes

Ecomask, contains Steinernema carpocapsae nematodes.  BioLogic recommends treating with Ecomask Spray for existing beds. You may want to try Ecomask Topdressing when introducing new rhizomes or when transplanting.  For heavy infestations or for your first treatment of nematodes apply at 70,000/square foot.  After the initial treatment it is important to monitor the population every year.  If the population begins to grow again, treat the beds at a rate of 35,000/square foot.  Like all beneficial nematode applications, it is important to apply at dusk to moist soil.  Apply with a sprayer or watering can directly around the base of the iris and the immediate area surrounding the rhizome.  Water again after application to ensure the nematodes are moist and in contact with the soil.  Once in the soil Ecomask will find the iris borers and enter their body via a pore hole, mouth, or anus. Inside the borer the nematodes will kill the host, reproduce, and the next generation of nematodes will emerge after about 3 weeks.

Other Ways to Help Control Iris Borers Without Chemicals

Reducing the number of eggs can be accomplished by cleaning up the litter around the iris beds after the first frost of the fall.  The litter should be either burned or composted. Reducing the number of eggs in the fall will limit the number of borers you will have the following spring.