Iris Borer
Macronoctua onusta
The most destructive pest of iris, a pinkish caterpillar that tunnels down the leaves and into the rhizome, where its feeding opens the door to bacterial soft rot. A badly bored rhizome turns to foul-smelling mush.
🔎 How to spot it
The caterpillars are pinkish and up to two inches long when mature. The night-flying adult moth has chocolate-brown front wings and lighter yellow-brown hind wings, with a wingspan up to two inches, and is rarely seen. Early damage shows as pinprick holes and tan, water-soaked streaks on the leaves.
🥀 Damage it causes
Newly hatched caterpillars chew tiny holes and tunnel inside the leaves, leaving water-soaked streaks, then move down into the rhizome by midsummer and hollow it out. The tunneling wounds let in bacterial soft rot, which turns the rhizome slimy, soft, and foul smelling. Leaf tips brown and the plant weakens.
🛡️ Prevent it
The key control is fall sanitation: remove and destroy old iris leaves, stems, and nearby debris where the eggs overwinter. Cleaning up in autumn breaks the cycle before spring hatch. Siberian iris are more tolerant of borer attack.
🧯 If it is already here
Catch damage early and crush the caterpillar inside the leaf or remove the damaged leaf. Beneficial nematodes applied when leaves are nearly full grown but before bloom can reduce numbers. A well-timed spray of spinosad or acephate as eggs hatch, when leaves are four to six inches tall, is effective.
💡 Good to know
There is one generation a year, and the borer overwinters as eggs on old iris foliage, which is exactly why fall cleanup works so well. The link to bacterial soft rot means borer damage and rot often go together. Dig and divide congested clumps to find and discard infested rhizomes.
🌱 Plants it attacks
2 plants in the library can be attacked by this pest
For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.