Answer: Iris borers are moth larvae, hatched from eggs laid in old iris stalks and garden debris in late autumn, 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. 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. Clear iris beds of old leaves and stems, where borers lay their eggs in the fall. In early spring, inspect plants--if you see any borer "mines", or tunnels, in the leaves, pinch them to kill the hidden larva. After flowering, you can also dig up affected plants and remove any infested leaves and rhizomes, then replant the healthy sections. Make sure the iris are not receiving too much water..this will also encourage rot.
It is difficult to control borers with insecticides, because the caterpillars are protected inside the stems or roots so the spray can't reach them. Timing is critical, because the only window of opportunity is when the young larvae are emerging in the spring, before they bore in. One method you can try is spreading pyrethrum dust around the base of the plants in early spring, to kill the larvae as they hatch and begin crawling in search of a leaf. You may need to reapply the dust after a rain; follow label directions.
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