Wednesday, June 27, 2007

Another Boring Insect?- Iris Borers

Iris borer damage begins in the foliage and may be evident now. Most of the time, the damage is to rhizomes discovered when people dig them to transplant during the summer. The moths emerge in late summer to mate and lay eggs on the oldest, roughest, dead and bleached iris leaves or on plants nearby. 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 time before working downward toward the rhizomes. 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 inside. Often a single caterpillar may completely the insides of a rhizome before moving to another. The mature caterpillars are pale yellow/pink to pink in color with brown heads.

To control the iris borer, it is important to remove all old iris leaves and other plant rubbish from the beds in 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. By holding the injured leaf so that the sun shines on the far side, the silhouette of the small caterpillar should be easily visible through the leaf. John Weidhaas at Virginia Tech University has determined that Sevin and malathion give good control of the iris borer when these pesticides are applied early while the caterpillars are feeding on the outside of the leaves. (It may be late for this, now.) For more information and treatment suggestions, see Ornamental and Turf Insect Note No. 46 at Iris Borer

For more information contact your local Cooperative Extension Center and ask for the Commercial Horticulture Agent.