Once one of the easiest perennials to grow, bearded iris is now fighting off infestation by an insect called an iris borer. It is one of the few insects to bother this rugged plant, but dealing with it can be a challenge.

What are the signs of iris borers?
First you will see streaks and wandering tunnels on your iris leaves that later turn yellow and brown. Iris that are severely infested may also fail to bloom.

What do the borers look like?
That depends on their stage of development. Most gardeners only see iris borer when they are digging up damaged plantsand looking over the rhizomes. At this stage, they are 1-2 inches long (pink-gray in color), with dots on their sides and a brown head.

How did they get into my iris?
Iris borers go through several stages of life. A small, nondescript moth lays eggs on iris rhizomes in late fall. The eggs over-winter in any iris foliage or other debris that remains on top of the rhizomes during winter. In spring, eggs hatch and tiny caterpillars crawl up the emerging foliage and bore into the leaves. Once in the tender young leaves, borers travel between the layers of the leaf, eating their way down to the rhizome. After feeding on the rhizomes, borers drop off into the soil and pupate until mid to late fall when they emerge as a moth to start the cycle over again.

What should I do if I suspect I have borers?
In early to mid-August, dig up iris plants that are showing any symptoms. Trim the foliage back to a 3-4 inch fan, and wash the soil off the rhizomes. Look them over carefully. Discard any pieces that are decayed. Examine the firm, fleshy rhizomes for holes where a borer may have dug in. If you cannot see the borer, but suspect it is still in the rhizome, you can either cut away that section with a sharp knife or soak the rhizome in a water and 10% bleach solution. This may take a few minutes to an hour or two. If the borer is still there, it will back out and drown. After soaking the rhizomes or cutting a section away, let them dry thoroughly before replanting. Once you have disposed of any decayed or infested rhizomes, go ahead and replant.

Should I treat the rhizomes with anything before I put them back in the soil?
To avoid any decay on the new breaks or cuts, dust the rhizomes with a powdered fungicide such as sulfur or Bulb Dust.

Can I treat the soil when I am digging and resetting my plants?
At this stage in their life, the iris borers are not susceptible to any kind of insecticides. As you work the soil, watch for borers pupating in the soil. Pupa are shiny, reddish-brown and segmented, about an inch or so long and narrow.

When I dug my iris there were several black, mushy rhizomes that were filled with little white worms. Are those borers?
As disgusting as this might seem, those are maggots and they are the good guys and do not hurt your iris. Maggots are like a clean up crew, going in where nothing else will and making use of the decaying tissue.

Are there any cultural methods of controlling iris borers?
There are several things you can do to minimize your borer problem. In the fall, do not allow any iris leaves or stalks to remain in the area or on the rhizomes. They're the perfect place for the eggs to overwinter.

Can I spray with an insecticide?
The only stage when borers are susceptible to insecticides is as young caterpillars in early spring. When new foliage fans are around 5-6 inches tall, apply a systemic insecticide that will be absorbed by the foliage and kill the young borers.

Are there any organic alternatives to using an insecticide?
Unfortunately, there are not any organic chemicals that are as effective. Organic gardeners have some success by dusting the base of the iris foliage with pyrethrums or rotenone several times in early spring. An interesting alternative that might work (depending on your gardening habits) is to introduce beneficial nematodes to prey on the iris borers. For details on this subject, refer to our information sheet Beneficial Nematodes. Other gardeners rely strictly on digging and replanting the bed each August and making sure there is no plant residue on top of the iris over winter.

Does that mean I shouldn't mulch my iris?
There are lots of opinions on this question. Iris that are not well established before winter need protection, but providing protection may encourage borers. If you planted your iris late or you feel they are in an exposed location where they may be damaged by winter, wait as late as you can to mulch.  Ideally you'll wait for the ground to freeze or for the first snow of the season. Then in spring, as soon as the snow cover is gone, remove the mulch.

Do borers bother all types of iris?
While all types can be damaged, tall bearded iris are most at risk. Iris with bulbs or rhizomes that are deeper in the ground are seldom bothered by borers.

Is there anything else that does that kind of damage to iris, but isn't the borer?
Slugs will cause ragged holes in leaves that may turn yellow and brown. There are a few types of leaf spot that cause water-soaked yellow/brown patches. Iris can develop soft rot from too much moisture in poorly drained soil.

Can iris borers affect any other plants?
Occasionally we will find borers in another perennial with fleshy roots that is planted near an infected iris patch, but it is not a common problem.

Life Cycle of the Iris Borer
Fall: Moth emerges from pupa and lays eggs on the rhizomes
Summer: Borers enter the rhizomes
Late summer: Borers leave rhizomes and pupate in the soil
Spring: Eggs hatch into small crawlers that enter new leaves

For the winter, clean all debris, especially iris leaves, from the rhizomes.
Spring, when new foliage is 4-6" tall, apply a systemic insecticide.
Optional: Apply beneficial nematodes to the soil as soon as the soil warms.
In early to mid August, dig, clean and reset all the rhizomes.

© Bachman's 2007