by Don Janssen, Nebraska Extension Educator
Borers can be a major pest of irises. But they don't have to be.
A thorough garden cleanup in the fall can eliminate most overwintering eggs. This greatly reduces the chances of borer infestations next year.
The borer adult is a moth with purplish brown front wings and lighter, yellow-brown hind wings. The wingspread is about 2 inches. The moth emerges in late summer or early fall, flying only at night, and lays its eggs on the foliage and old stems of iris plants.
The eggs hatch in early spring, when new iris leaves are 4 to 6 inches high. The tiny larvae crawl up onto the new leaves, making pinpoint holes where they enter. At first they feed inside the foliage as leafminers, leaving water-soaked trails behind them. The caterpillars eventually work their way downward into the rhizome, where they feed and grow. While still in the leaves, the larvae are relatively slender. The mature larvae are plump, pink caterpillars about 1 1/2 to 2 inches long.
When fully grown, the caterpillars leave the rhizomes and pupate in the soil nearby. When they emerge as adults, they start the cycle all over again.
Gardeners can interrupt the cycle by removing and destroying iris foliage and old stems and other nearby plant debris in the fall. Another option is to spray or dust in the spring -- weekly from the beginning of new growth until early June -- with dimethoate or diazinon to control newly hatched larvae. Or monitor the foliage closely and crush young larvae in their water-soaked tunnels in the leaves.
Another chance to eliminate larvae occurs when you lift and divide iris rhizomes after flowering. Borer-infested rhizomes will often be soft, slimy and foul-smelling from a secondary bacterial rot. Discard and destroy infested rhizomes, and search out and destroy pupae in the soil around the plants. Pupae are dark brown, spindle-shaped and legless, and between 1/2 and 3/4 inch long. Healthy rhizomes can then be divided and replanted.
The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office