Growing Concerns 530: Iris borers

Late Summer!

Old ways still work on old fashioned bearded iris and their borers

Dear Janet,

My neighbor's giant black iris' that I gave her are all infested with those iris borers. She suggested that I check mine. Yes, some of mine are infested although not as bad as hers. Have you had any experience with this? I checked the web and found different ways to treat it. Some say to pull them all out and soak in bleach or Epsom salts then to treat the soil with certain insecticides. They are the biggest ugliest looking grub/worm thing I've ever seen....and so many eggs!


Dear C.,

Iris borers are big and ugly when they are old enough to have eaten all the way down into the root but they don't require weapons of mass destruction. Also, they're "normal" for iris. If you grow bearded iris eventually you will deal with borers because the moths whose young are iris eating caterpillars will  find those plants.

Each fall, these moths emerge from the soil, having rested there since August as pupae -- that state in between caterpillar and winged adult. The moths lay eggs on iris foliage. Where conditions favor the borers or weaken the iris, many of the borer eggs survive to hatch in spring and the borers eat without disturbance all summer. Then, the damage to the plants is bad. In other places, even just across the street, the irises may be tougher, natural predators of the borers more active or the gardener more savvy so there is less damage.

Borer damaged iris are small and weak, the leaves have dead streaks in them and the plants fall over, destabilized by rotting roots or chewed leaf bases. Late July and early August is a time to deal with them. Lift the iris, then cut off and discard parts of the root that have succumbed to a bacteria called soft rot that enters through borer holes. Cut off parts with grubs in them, squish the grubs or leave them where the birds can get them. You'll know the parts to cut because soft rot stinks and the grubs at this stage are 1-1/2 to 2 inches long so they and their holes are easy to spot.

You can dip the roots before replanting in a 10% bleach solution to clean inside remaining holes and cut edges. Alternatively, you can leave the salvaged roots in a warm place in the sun for a day or two -- gardeners where I grew up used to spread them on the concrete sidewalk.

Don't put pesticide in the soil. Even if you can get an insecticide to thoroughly penetrate the top few inches of the soil so it reaches all the pupae, it's nigh impossible to kill them because they're old, tough and in a resting state. In addition, you'll kill other insects in the soil, many of which are beneficial in this or other plant-insect systems working in your garden.

Instead, aim for the borer's weak link. In late fall or early spring (before April when borer eggs hatch), cut off and discard all iris foliage. That eliminates a lot of the eggs. If the previous year's damage was very bad in spite of the fact that you had done a good foliage clean up, you can use an insecticide to spray the iris foliage when it's 4" tall, about two weeks later when it's 10" tall and again just as you start seeing the flowering stalks rising up. Almost any insecticide will do since you're killing tiny larvae as they hatch, before they can start eating into the leaf. Using a systemic insecticide such as Cygon can reduce the number of sprays -- systemics stay inside the leaf to kill the borers as they first start to eat.

Wildlife lovers, avoid systemics. These toxins become part of the flower as well as the leaf and can sicken nectar drinkers, including hummingbirds that feed at iris.

Some people cut off and hot compost the top half of each iris leaf after bloom, in June. At that time of year borers are still chewing their way down inside the leaf toward the root so this practice may catch some insects that escaped earlier preventives.

About the eggs you mention. There shouldn't be any eggs, which only come from adult moths and are so tiny you can't see them without a magnifier.  You're probably seeing frass -- insect excrement or bug poop.

More about iris borer...


Green thumbs up

to that spunky gardener who stood up to her tree trimmers and refused to let them use climbing hooks to ascend and prune her trees. Those so-called experts tried to tell her that's acceptable practice but any certified arborist current even to 1990 standards will confirm that hooks, which pierce the bark and open the tree to infection and insect attack, should not be used unless the tree is being removed.


Green thumbs down

to spraying insecticide when you don't know that insects are the problem. Even when you're certain an insect species is seriously threatening your plants, don't spray at any but the time of year when that insect can be effectively controlled. Find that time in pest control books. In the hot sun, many pesticide sprays can do more damage to dry leaves and stressed plants than to dormant or absent pests!


Originally published 8/23/03