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Iris borer

Filed under: — admin @ 2:58 pm


Iris borer (Macronoctua onusta) is a pinkish caterpillar with rows of black spots on their sides. When full grown, it is about 2 inches long. The larvae emerge from the ground in April or early May when iris leaves are about 4-6 inches tall. They enter the leaves through tiny pinholes and burrow their way down to the rhizome. By midsummer they eat their way through the rhizome leaving only the outside shell and then return to the soil to pupate. In August-October, iris borer emerges as a night-flying moth with brown front wings and yellow-brown hind wings and a wing span of 2 inches. The moths will then lay creamy white eggs which later turn lavender on foliage or debris which will winterover and start a new lifecycle the following spring.

Early signs of iris borer are disfigured foliage with irregular tunnels in the lower section of the leaf and chewed leaf edges that appear in early spring. Leaves may also have water soaked streaks.

bacterial soft rot iris borer infestation

Bacterial soft rot
One of the major problems that iris borer introduces to the leaves and rhizomes is bacterial soft rot (Pecotobacterium carotovorum). Rhizomes become soft, mushy and have an offensive odor. The leaf tips are withered and leaves have long brown streaks or appear water soaked. Bacterial rot can also be introduced through careless cultivation. Crowded, shaded growing conditions can make the plant susceptible to infection. More info on iris..

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3 Comments on Iris borer»

  1. My bearded iris seem to be dying all of a sudden, the leaves are going brown, what can I do, I have been growing them for years and moved them in to the rose beds some time ago they grew wonderfully until last week.

    Check the tubers for holes which would indicate iris borer causing bacterial rot and long brown streaks in the leaves.

    Comment by Gayle — 8/10/2007 @ 3:20 pm

  2. Treating iris borer
    Concerning my iris tubers with pinholes, how do I treat them? Someone told me about using a mild bleach solution but I don’t know how much or how long.

    Sounds like you have iris borer. The borer enters the leaf and works its way down to the rhizome. Dig up and remove borers, scrape away the mushy decay (soft rot), dip in 10% chlorine bleach solution and dry in sunlight for several hours. If it is too diseased discard the rhizome. Check the soil for borers that are pupating in the ground or replant at a different site. Replant the rhizome (top above the ground) water and cut the leave 3-4 inches. Make sure you remove all diseased leaves. Sanitation is very important all year round. Check next spring for any missed “straggler” borers that may have hatched; look for ragged edges and browning leaf tips.

    Comment by Lisa Tracey — 8/10/2007 @ 3:23 pm

  3. Thanks for all the info regarding Iris. Relative to the borer - please advise what do you do to the dirt in your beds to get rid of the borer? Need to know ASAP. Thanks

    Hi Glenda
    The borer is active Aug-Sept. in that it lays eggs on old iris plants. The eggs will winter over and hatch next spring. Therefore, wait till after the first frost and remove any old plant material and debris to minimize any borer. Burn, bury or compost well all debris. If you have a history of borer apply an insecticide next spring when new growth is 4′’high. University Ext. suggests Cygon 2E(dimethoate) which is labeled for iris control. Another way is to transplant your whole bed to a different site, but that can become a futile effort if you still have borer eggs on any debris transplanted. So the best solution is good sanitation.

    Comment by Glenda — 8/10/2007 @ 3:31 pm

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