Irises are by far one of the easiest and most reliable plants to grow in Kansas, but sometimes a hidden insect pest affects their ability to bloom. Even my garden has been subject to an infestation of the semi-elusive iris borer, drawn to my attention only when the spring blossoms failed to appear a few years ago.

Fortunately iris borer is controllable with just a little attention to the plants in late summer and fall.

Irises are one of the easiest and most reliable flowers to grow in Kansas but are susceptible to the semi-elusive iris borer.

Timing is everything to control iris borer, and an understanding of the insect’s life cycle helps. Eggs hatch in late spring, and the short, fat worms that emerge begin feeding on and into the leaf tissue, working their way inside leaves and “boring” down into the centers of the roots (rhizomes) within a short time. This is where they do the most damage, hollowing the rhizomes as they feed on what the plant has stored away for energy for the following spring.

Iris borer larvae have whitish or pinkish bodies with dark brown heads and grow to about 2 inches at maturity. At the end of the summer, the larvae pupate and then emerge in the fall as a short-lived, nocturnal, brown moth whose only concern is to lay eggs for the following year.

Small piles of what looks like sawdust near the base of irises and pinholes in the leaves can also be telltale signs of the presence of iris borer.

For control, start by removing and destroying heavily infested plants. This can be done any time, but late July through August is the best time to divide and transplant irises, so you may wish to combine tasks.

For light infestations, some gardeners still prefer to destroy plants with any signs of the insect. Others prefer to try to destroy remaining larvae by stabbing a piece of wire into infested rhizomes.

Iris borer can also be controlled with parasitic nematodes. There are two species of nematodes that parasitize iris borer larva: Steinernema carpocapsae and Heterorhabditis bacteriophora. The former provided 100 percent control in research trials. Parasitic nematodes must be applied under specific conditions to be effective, so read and follow all instructions carefully if opting for this method.

There are also several insecticides labeled for control of iris borer that should also be applied according to label directions.

Replant healthy iris rhizomes as clumps placed 10 to 12 inches apart, with just enough soil and mulch to cover bottom half of the rhizome. Ideally, each clump would contain two fans of iris foliage connected by a stronger root, in what may be described as a double-ended Y shape. Water after replanting.

Removing iris foliage each fall is also suggested as a means to control iris borer. Because the adults lay eggs on the foliage in early fall, cutting the plants back presumably removes eggs before they hatch the following spring.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation. She is the host of “The Garden Show” and has been a gardener since childhood. Send your gardening questions and feedback to Lawrence Living@ljworld.com.