Iris Borer

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Iris Borer


Adult - Iris borer moths are mottled brown with yellow brown hind wings. Most of the time, the iris borer is represented only by its injury that is discovered when people dig the rhizomes to transplant them in late summer. The borer is a caterpillar in the same family as the corn earworm and cabbage looper.

Eggs - The eggs are almost spherical and are creamy with a greenish tinge to pale purple. The eggs are about 0.5 mm in diameter.

Larva - The mature caterpillars are fat and pale yellowish-pink to pink worms with brown heads. They grow to almost 3.5 cm long.

Pupa - The pupae are about 3 cm long and 6 mm wide. Pupae are reddish brown to dark brown.


Fig. 85 Iris borer. Macronoctua onusta Grote, Noctuidae, LEPIDOPTERA A, Female moth. B, Male moth. C, Larva.


Distribution - The iris borer is found throughout the eastern United States and parts of Canada.

Host Plants - iris borer apparently feeds on Japanese, German, and blueflag irises.

Damage - The tiny caterpillars first feed on the new foliage and sometimes cause the margins of the leaves to bleed sap and be ragged. Narrow, water soaked slits appear where the external feeding and mining have injured the leaves. As the caterpillars grow, they excrete slimy frass in which grow soft-rot bacteria and fungi. The bacteria cause an unpleasant odor and further damage the rhizome. Often a single iris borer may completely devour the insides of a rhizome before migrating to others. Several rhizomes may be injured by a single iris borer.

Life History - 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 are placed singly or in rows of three to five or even more. 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 caterpillars then mine in the leaves for a while 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 and sometimes bore right in. They pupate in late summer, and a new generation of moths emerges in the fall to lay eggs for the following year's generation of iris borers.


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 them with the thumb and finger inside the leaf. If the injured leaf is held so that the sun shines on the far side, the silhouette of the small caterpillar should be easily visible through the leaf. In summer during the digging of iris to thin the beds, the infested rhizomes may be probed with a soft wire to destroy the borers and the heavily infested rhizomes should be cut off and destroyed. The divided rhizomes should be laid in the sun for a few days to allow the cut surfaces to heal before replanting.

For specific chemical control recommendations, see the current Cooperative Extension publications on ornamental plant pest management or contact your Cooperative Extension Agent.

Reference to University of Florida/IFAS Pest Control Guides