Iris borer moths lay eggs in the late summer. The following is more information on this pest.
Most of the time, the evidence of iris borer damage to rhizomes is discovered when people dig the rhizomes to transplant them during the summer. The iris borer is a caterpillar in the same family as the corn earworm and cabbage looper. The moths emerge in late summer to mate and lay eggs on the oldest, roughest, dead and bleached out iris leaves or on plants nearby. A single female may lay more than 1,000 eggs, usually in crevices or in folds of the leaves. The eggs hatch the following spring. The tiny caterpillars first feed on the new foliage and sometimes cause the margins of the leaves to be ragged. The holes caused by the young caterpillars bleed causing deposits of sap on the leaves. The caterpillars then mine in the leaves for a while before working downward toward the rhizomes. Narrow, watersoaked slits appear where the external feeding and mining have injured the leaves. As the caterpillars grow, they excrete slimy frass in which grow bacteria and fungi causing an unpleasant odor. The caterpillars are about half grown by the time they reach the rhizome. There they feed on the edge or on the underside of the rhizome sometimes boring right in. Often a single caterpillar may completely devour the insides of a rhizome.
To control the iris borer, it is important to remove all old iris leaves and other plant rubbish from the beds in fall or early spring before new growth emerges. If the borers are discovered later in the spring, it may be possible to crush the caterpillars with the thumb and finger inside the leaf. Imidacloprid (Merit) is a systemic and effective. Orthene is another product. Either could be applied in the spring and would be taken up into the plant reasonably quickly. Studies at the University of Maryland show that entomomopathogenic nematodes, Steinernema carpocapsae and Heterorhabditis bacteriophora, were as effective as chemicals against iris borer when applied correctly. For more information, see the Purdue University link at http://www.entm.purdue.edu/Entomology/research/cs/mg/mg.html
Pink iris borer caterpillar is exposed when heavily damaged leaf is split. Photo from Purdue University.
Information from Steve Bambara, Extension Entomologist, in the September 4, 2009 edition of the North Carolina Pest News http://ipm.ncsu.edu/current_ipm/pest_news.html