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Exterminating Iris Borers

Updated on May 2, 2016
Iris "Harvest Moon"
Iris "Harvest Moon" | Source

What Are Iris Borer?

Although iris are very hardy, they are troubled by one particular pest: the iris borer. The iris borer is actually the caterpillar stage of a nocturnal moth, Macronoctua onusta. The adult moths are rarely seen, but the caterpillars and the damage they cause are easy to identify. Iris borer caterpillars may be found near the rhizome or root part of the iris plant. They're pink-brown and about two inches long.

But it's the damage they do the adult iris plants that most gardeners notice first. Signs of an iris borer infestation are apparent. The tips of the iris leaves turn brown, and they may seem to shrivel or age quickly. Leaves have tan or brown streaks down the centers.

Several other iris diseases mimic iris borer, so before you take steps to eradicate iris borer, make sure you have it! Take a sample of the leaf to your local County Cooperative Extension Office and ask for help from the agents or Master Gardeners to make sure you're dealing with iris borer and not a fungal disease. Only after you're absolutely certain your garden has iris borer should you take steps to eradicate it.

Iris Borer Life Cycle

It's helpful to understand an insect's life cycle before trying to eradicate it; most treatments are intended to work during one or more of the insect's life cycle phases. "Know your enemy" is a good phrase to remember in war and in gardening.

Macronoctua onusta moths seek host plants such as iris. They emerge in late summer or fall, and after mating, the female lays her eggs on the foliage of iris plants. The eggs remain dormant until the following spring, when warmth triggers the caterpillar to emerge. He then chews his way into the leaves of the iris plant, boring a hole down to the rhizome. He may feed on the rhizome as well. After munching his way down the plant through the spring and into the summer, he settles nicely into the soil before entering the pupae stage. After pupating, the newly emerged moths rise and fly away to start the cycle all over again.

Iris "Cherub's Smile"
Iris "Cherub's Smile" | Source

Four Ways to Get Rid of Iris Borer

Now that you understand your enemy and have taken a sample to your local Cooperative Extension Office to confirm it's iris borer, it's time to take action. There are four steps you can take to eradicate iris borer.

  1. Buy borer-resistant varieties. Boring advice, I know; you want the super-duper double flowering iris with the great scent or whatever you want. I do, too. But sometimes you have to work with nature. Japanese and Siberian iris tend to be resistant to borers.
  2. Clean up the garden in the fall. Given that the iris borer eggs remain dormant on the iris leaves, clearing them out of the garden when the leaves die back in the fall is an important step to eradicating iris borer. By removing the eggs, you've disrupted their life cycle. Instead of composting spent iris leaves, bag them and throw them away in the trash.
  3. Insecticides. The Minnesota Cooperative Extension office recommends two insecticides against iris borer: Orthene (acephate) and Bull's Eye (spinosad.) Always read the package directions and follow them to the letter when applying insecticides.
  4. Organic controls: Biological controls for iris borer include beneficial nematodes. Nematodes are microscopic worms introduce a deadly bacteria to the caterpillars. You can purchase nematodes at your local garden center. They do need a moist environment in order to work most effectively against iris borer. As with commercial pesticides, read the label and follow all instructions when using organic controls. Just because something is labeled organic doesn't mean you should get sloppy with the care and application instructions.

I'm not sure you can completely eradicate iris borer, unless you get rid of your iris garden completely. And who wants to do that? Not me. I look forward to iris season every spring, and I never let a little caterpillar stop me.

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