How To Control Iris Borers

Encourage your irises looking beautiful from season to season by keeping this pest in check.

by Jessica Walliser
PHOTO: Bob Gutowski/Flickr

I’ve grown bearded iris in my garden for years; in fact, these perennials are one of my favorite spring bloomers. But in recent years, my bearded iris have been plagued by a troublesome pest: the iris borer.

In early spring, my iris flowers bloom beautifully, but by mid-July the foliage begins to turn yellow and slimy. Brown splotches appear all over the sword-like leaves, and when the plants are disturbed, sometimes there’s an unpleasant odor. These are all pretty clear signs of an iris borer infestation.

Spotting The Iris Borer

Iris borers (Macronoctua onusta) are the caterpillars of a species of moth. These caterpillars spend their entire larval stage inside of an iris plant, munching on the foliage and rhizomes. The brown, slimy leaves are caused by their feeding. Eventually, all the damaged leaves will turn completely brown and shrivel up, possibly impacting next year’s flower production.

Adult iris borer moths fly around the garden at night. They’re fairly non-descript, and the females lay eggs on iris leaves in August or September. These tiny eggs sit on the leaves all winter long, and the larvae hatch the following spring. The young caterpillars burrow into the newly emerged leaves and spend weeks feeding. They move down toward the base of the plant as the season progresses, and by the time July arrives, the borers are feeding on the rhizomes.

When they reach the rhizomes, the borers tunnel through these fleshy roots and often cause the rhizomes to rot. That’s what makes infested iris plants smell so badly. I think they smell like rotten potatoes or onions.

In early August, the mature caterpillars begin to pupate in the soil around the iris plants. Pupation takes only a few weeks before a new generation of adult moths emerge and begin to lay more eggs.

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Protecting Your Irises

Jessica Walliser

Trim Back Foliage

The reason these pests have begun to plague me the past few years is because I’ve neglected to cut my iris plants back in the late fall. Because the eggs overwinter on the foliage, it’s very important to spend some time every autumn removing all the leaves from the plants, all the way back to the rhizomes, and tossing them into the garbage or burying them. This is best done after we get a few hard frosts and the adult moths have stopped laying eggs. I haven’t gotten to this chore the past few seasons, and as a result, my iris plants are paying the price.

Discard Infested Rhizomes

If you have iris plants that are already infested with borers, you can dig up the plants and crack open the rhizomes to reveal any borers housed inside. Throw away any soft, rotted rhizomes and replant the healthy ones. If you happen to come across any chubby, pink, hairless caterpillars in the process, be sure to squish them or feed them to your ducks or chickens.

Apply Beneficial Nematodes

Another option for those gardeners who, like me, have neglected to trim back the foliage in the fall, is to apply beneficial nematodes to your plants in the spring. The correct species of this beneficial, microscopic roundworm are Heterorhabditis or Steinernema. These little critters are mixed with water and spread over the iris plants where they seek out and kill any iris borer larvae. They’re best applied when the iris leaves are fully grown but the plants have yet to flower.

The Silver Lining

Although I’d like to rid my bearded iris plants of borers to keep the plants looking and performing their best, it’s important to note that an iris borer infestation seldom kills an established iris plant. Even if a handful of the rhizomes are damaged, the rest will go on to produce healthy flowers the following year. Except in extreme cases, iris borers don’t mean the end of the road for your bearded iris plants. Gardeners like myself just need to put in a little effort to keep them in check.

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