How to Cure Iris Borer and Save Iris Plants


The iris borer, Macronoctua onusta, which looks like a white or very light pink worm, is actually moth larvae. Not only does it have a voracious appetite for the iris, it even cannibalizes its own species. Signs of an iris borer infestation include the appearance of droppings, which resemble sawdust, at the base of the plant and typical bite-type notches in the leaves. As the infestation worsens, the entire plant may topple, exposing slimy holes at the base. The best time to treat the iris is in the spring, before the moth's eggs hatch.

Things You'll Need

  • Pruning shears
  • Granular imidacloprid insecticide
  • Parasitic nematodes
  • Cut back iris foliage in the fall. Do not compost the foliage. Dispose it.

  • Apply a granular insecticide containing imidacloprid to the soil around the iris. Apply the insecticide in the spring before temperatures reach 70 degrees Fahrenheit. This is when the eggs hatch. The iris needs time to absorb the insecticide into its tissues to be effective against feeding borers. Use the rate recommended on the package and wear protective clothing during the application.

  • Treat the iris organically by applying parasitic nematodes, such as Steinernema carpocapsae and Heterorhabditis bacteriophora. Apply the nematodes in the spring after the soil temperature reaches 50 degrees Fahrenheit. According to iris specialists at Iris Gardens, the soil needs to be drenched with 2 quarts of water per square foot before application. This treatment remains effective for five weeks.

Tips & Warnings

  • Purchase parasitic nematodes from organic gardening supply catalogs or nursery centers.

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