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Iris borer

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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7 comments to Iris borer

  • Gayle

    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.

  • Lisa Tracey

    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.

  • Glenda

    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.

  • LORRIE CREECH

    I WAS TOLD TO DIG UP AND DIP RHIZOMES OF THE IRISES IN 1 PART OF BLEACH TO 3 PARTS WATER AFTER CUTTING OFF ANY ROT WITH A KNIFE AND TO CUT BACK YOUR IRIS LEAVES IN THE FALL TO 3 TO FOUR INCHES TO RID OF EGGS LAID ON TIPS.BEST OF LUCK…Lorrie

  • Joanne Hamm

    The best way to get rid of bacterial soft rot, which is caused by a bacteria not the iris borer, is to scrape the rot from the rhizome without removing it from the ground and then pour Dial Aniti-bacterial Hand soap directly on the rhizome and the surrounding soil. I grow 300 varieties of irises and used to use the chlorox, but the soap works much better

    Hi Joanne
    Thanks for the tip!! I stand corrected. Bacterial soft rot (Erwinia carotovora) is a soilborne bacteria prevalent in heavy,clay wet soils. The bacteria enters the tuber when the iris borer or any other pest injures and wounds on the tubers.

  • Diane in Iowa

    Freeze damage
    I have a few iris plants with leaves that have accordian like folds in them and some of the buds are deformed. Any answers Thx

    Hi Diane
    Distorted leaves on growing iris is a sign a frost damage. Freezing temperatures can kill growing cells and prevent expansion in emerging leaf foliage-causing an accordian like leaf. This will also affect the flower buds. The new foliage should be normal.

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