Nightmare in the Garden

by Barbara Nicodemus

The insect goes through four stages in its life cycle: egg, larva (borer or caterpillar), pupa, and finally the adult, violet brown night-flying moth (Macronocture anusta). The main body is almost black with a crested head. Its brownish fore wings are marked with fine black lines, with the hind wings being lighter and whiter. The wingspan is approximately one and one-half inches across and a little over three-quarters inch long. Due to its secretive nature of becoming active at dusk or after dark and avoiding even night lights, it is seldom ever seen, keeping well hidden during the day with the exception of sometimes appearing on dark, cloudy days. The moths usually appear during September or October and begin mating within a few days. The female soon lays her 150-200 eggs in clusters of about 25 or more on old stalks, lower base of the plant (sometimes higher up), cracks on the rhizome, shriveling old leaves, and other debris around the iris plant. The one-fiftieth inch eggs are cylindrical shaped, having flattened tops and bottoms. When first laid, eggs are shiny white, but as they mature, the eggs go through different color changes until they are a lavender hue just prior to hatching.

In early April through June, upon hatching, the one-eighth inch, hairless, brown-headed larvae are light grayish to white with pinkish tones. This is the stage when the larvae start their journey of destruction! They crawl up nearby leaves and proceed to feed on the new growth. Some, however, may spin fine threads to become wind-borne, thereby, spreading to other iris plants in the garden. In a few days, the larvae will start "mining" in the leaf, working their way down inside the leaf sheath, eating their way to the base in about a week. Iris borers are cannibalistic, so only one is left per plant by the time it reaches the rhizome. There it will spend several weeks feeding on the rhizome, eventually hollowing it out, and may even feed on the roots. There still can easily be more larvae that have survived on nearby fans or by becoming wind-borne to other clumps, as mentioned previously.

Normally a larger tall bearded rhizome will feed the larvae to maturity, but the smaller rhizomes and some beardless, like Siberians, won’t be enough. Then, they will travel in the soil from one rhizome to the other until they destroy most, if not all, of the clump. In mid-August or September, the mature larvae are up to one and one-half inches long, soft, fat (approximately one-half inch wide), and pinkish tinged, with a brown head. It is at this time that they will leave the hollowed out rhizomes to pupate in the nearby soil.

The chestnut brown pupae are easily identified, shiny and oval in shape, up to one inch in length. The pupae can be found within a one foot radius of the clump, but are normally closer. They are buried about six inches deep and have been known to be even deeper. The pupation will take a month, and then the moths will emerge, starting their life cycle again.

The presence of the iris borer may first be recognized by the wet stains along the leaf edges, notched out leaves, small pinholes, fine silk threads left from their spinning, and "sawdust" looking remains at the base of the plant from their waste. Later, the newly developing central leaves will have larger, more ragged, "saw-toothed" edges, due to the growing borer feeding inside the lower base of the leaf sheaths. The outside base of the plant will become slimy and look water-soaked due to the "bleeding" of the leaves. In the advanced stage, the central leaf may yellow and will be easily pulled out, sometimes even the whole fan. Bloom stalks will topple over and, upon inspection, the base will show the slimy, riddled work of the borer. The preceding two signs might be all that is noticed on smaller rhizomes, like MDBs, etc. This is especially true on some beardless irises, like Siberians.

Preventive measures are the first defense against iris borers. Clean up the garden in early spring and late fall, thus getting rid of most eggs. Remove all of the dead leaves and other debris in and around the clump and burn it. In the spring, be alert to any signs on the leaves that the borers are present, especially if you had trouble with them the year before. Hand pick and kill any larvae found. If they are already ‘mining’ in the leaf sheath, many can be killed by squeezing all leaves which show wet stains, pinholes, or any damage. Starting at the leaf base, firmly squeeze the leaf between the thumb and forefinger, pulling upward. Even this might not get rid of all of them, especially if they are already tunneling from the base into the rhizome. In a heavily infested garden the best way to get rid of the borers is to dig up the plants after flowering. Examine all the rhizomes, cut out any rot or borers found, and treat the rhizome for soft rot. If this procedure is done when the iris should naturally be divided, the borers should be kept fairly well under control.

Pesticides can be very beneficial if used with some common sense. They can be hazardous to the environment and the user, if used excessively or recklessly, so only use them when everything else has failed. You also need to make sure that any pesticide you use is labeled for use on irises, including any mentioned in this article. Otherwise, it is against the law to use them.

The most effective chemical control of iris borers is Cygon 2E (dimethoate) [see note, below] or Lindane, which is a contact insecticide. Lindane is only useful early in the season since it is a contact spray and will only kill the larvae that are active on the surface of the plant when used. It will be ineffective against the larvae once they are mining inside the plant leaves and rhizomes. The first spraying should be timed when the larvae are starting to hatch in early spring, followed by weekly spraying for the next two or three weeks. Cygon 2E (dimethoate), or any sprays containing dimethoate, are systemic insecticides and, therefore, will be absorbed throughout the plant when used. For this reason it isn’t necessary to spray the whole plant, just the lower part of the leaves and base. Heavy spraying of Cygon 2E on the upper leaf tips during hot weather can burn them, so it should be avoided. Cygon 2E can be repeated with a second spraying if necessary, especially if you were heavily infested with borers the year before. It can also be beneficial if the borers are detected late in the season and have invaded the rhizomes. But when mixing insecticides, etc., for irises, a few drops of detergent or a commercial sticker must be used per gallon of spray, to prevent it from shedding off their waxy leaves. It must also be stressed that chemical control needs to coincide with the egg hatching in early spring because as the larvae grow they become much harder to kill. This is normally when new leaf growth is four to six inches high. The temperature will also affect the time of hatching due to being warmer (earlier) or cooler (later) in the spring than normal. This can make it less reliable to depend on just plant growth alone, so all considerations need to be taken into account to start watching or spraying for them in the spring. Once there are no further signs of borers, spraying can be reduced to once a year in early spring for one or two more years and then discontinued. But a person must always be vigilant every spring for their reappearance.

The time of month the borer first appears in spring will vary for different parts of the country, so it is very important to find out this information for your area. This can be done by contacting your state or local Agriculture Department Extension Office or local iris growers. They will also be a good source of more information about how to get rid of and effective treatment for the iris borers mentioned here; the goal is to keep this nightmare, the iris borer, out of your garden!

[Note: Dimethoate will not be re-registered by the EPA for use in home gardens by agreement of the EPA and the manufacturer after reviewing new data.]