Answer: Iris borers can really ruin an iris bed! These moth larvae emerge in the spring as tiny caterpillars. They bore into new iris leaves and eat the soft interior tissue, gradually working their way down into the roots. In early fall the fat, pinkish, dark-brown-headed borers emerge from the rhizome and pupate in the soil.The gray-brown moths begin to appear in late August and are active at night. In late fall the moths lay their egg clusters on old iris foliage and other nearby garden debris.
To eliminate overwintering eggs, clean up and destroy old leaves, stems, and debris in fall or winter. Inspect the plants in spring and early summer and squish any larvae you see in the foliage. Look for signs of borer infestation -- yellow leaves that detach easily from the rhizome. Dig up affected plants and examine the rhizome for borers; remove and destroy any infested plants.
The borers not only damage the plants with their chewing but carry the bacteria that produce foul-smelling soft rot, one the serious diseases that affect irises. Make sure the iris are not receiving too much water, as this will also encourage rot.
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