The iris borer (Macronoctua onusta) is actually a caterpillar of a nocturnal brownish moth. In the autumn, it lays its eggs on old iris foliage where they spend the winter. In the warmth of the spring, they hatch and the tiny white caterpillars seek out the new iris leaves emerging from the rhizomes. After feeding all summer, they will transform into the moth stage which starts the process again by laying eggs.

Early in the season, damage is difficult to detect. The first signs may be feeding damage in the form of notches on the leaves and some frass. The real damage comes when they move down into the rhizome and may eventually hollow it out. There will be small, pencil lead (or is that graphite) size holes in the rhizomes.

Ultimately, the plant will begin to wilt and may die from a severe infestation. However, perhaps an even more serious side effect is the introduction of a bacterial rot into the damage caused by the borer. This will cause the rhizome to turn to a very smelly mush and kill the plant.

Iris borer tends to be more of a problem on iris that are already under stress from other sources such as crowding, poor drainage or compacted soils. Preventing these conditions or more frequent division may help to prevent infestations of iris borer.

Of course, preventing the problem by keeping the iris in otherwise healthy condition is the best approach.

Insecticide treatment of iris borer can be difficult. Since the critter is living inside the rhizome for much of its life cycle, many standard sprays are not effective. Certain systemic insecticides may work better.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

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