Iris gardeners in the eastern U.S. and Canada know the damaging effects of the iris borer, commonly known as 'Macronoctua onusta' in entomological terms. Before you read any further, please know that I do not have a “cure” for these pests, and I certainly am not an expert on these insects. I fight these vermin just like everybody else, and most of the time I come out ahead with few if any damaged plants from year to year.
I have followed the iris forums for some time here and at Cubits and I realize that some folks don’t know they have a problem until their fans and stems turn yellow, fall over, and decay. This condition is accompanied by a foul-smelling rot. These ideas are intended to offer help and information to gardeners searching for answers. Yes, I know from reading the forums that many, many experienced and seasoned irisarians already know what is happening, and have their own methods of control. The ideas presented here are intended to add to the knowledge base that already exists for the pros and veteran growers at ATP (& Cubits too), and to possibly assist the younger, less experienced gardeners who want answers.
I noticed the discoloration and holes in the iris fans in these two pictures at the left and right while I was in the garden. I walk the rows and beds weekly in May and June, carefully inspecting and flagging each clump where these unusual markings exist. After each weekly tour, I remove damaged rhizomes with a sharp serrated knife. Each is then marked with the cultivar name and brought to my work bench for "surgery" and rehabilitation. In the picture on the right, the damage is seen in the outside leaf, while the one on the left shows damage in the central fan. Both clearly show the signs of borer damage. If left in the garden, the borers would grow to maturity and eat from rhizome to rhizome, and even from clump to clump. By August the damage would be far greater if left unchecked and uncontrolled. Most of the rhizomes in these pictures are smaller non-blooming-sized ones taken from near the outside of the clump.
In the picture on the left, borer damage is clearly seen in this small rhizome; and on the right I've peeled away the leaves to show the extensive damage already done. I don't like to dwell on pictures like this, with excrement showing, but we need to understand how rapidly a borer can destroy a plant. And, if we are to save these rhizomes (especially the expensive ones), the borers must be removed. In this case the rhizome was far beyond saving. I will show in the pictures below the removal process in a series of steps. It is not a job for the squeamish or the faint hearted. I keep a tub of bleach/water solution handy for washing my hands and the rhizomes periodically. This is just good hygiene in the garden, and elsewhere, because hands often come into contact with many undesirables.
With close, regular inspection of leaves from April through June, iris borer larvae can be easily detected at the telltale spots of chewing damage, leaf discoloration, and other signs such as dark watery appearing marks. Fall sanitation is very important for iris borer control. Following the first hard frost, remove and destroy old iris leaves, stems, rotted rhizomes and nearby plant debris. This will help rid irises of the overwintering egg stage. Insecticides labeled for iris borer control may also be used in the spring of the year on new foliage. I have tried several "systemic" solutions for borer control over the years. The one I currently use is very effective when applied EARLY. As a rule of thumb, I spray AFTER spring cleanup, when all dead leaves, debris, and winter accumulation have been removed from the garden. This was done this year as soon as the first tulips bloomed (about mid-April). This is the time when any borer eggs will begin to hatch. I fully saturate the leaves and rhizome clumps at the base of the plants. A word of warning: Insecticides of ALL types are toxic. ALWAYS direct the spray to align with wind flow AWAY from you! Also, make sure pets are kept away from the area during spraying, and for at least one hour afterward or until the area is completely dry. Spraying is a personal choice and will vary depending on the individual gardener. (All images used are my own.)