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Iris Borer Information

Illustration of iris borer
Nematodes control
Cornell article - Iris borer of Past, Present and Future?
Control advice

Illustration of iris borer

Illustration of iris borer


Nematodes - An alternative control for iris borer

A discussion collected by Chris Hollinshead, Mississauga Ontario:

First, what are nematodes? Nematodes are considered one of the most abundant groups of living animals, and although morphologically they are very simple, they have exploited a wide range of diverse habitats including invertebrates. Nematodes can parasitize spiders, leeches, annelids, crustaceans, molluscs, and insects. If the entomopathogenic (insect-parasitic) nematode attacks an insect pest; kills or hampers the development of the insect host; and is capable of mass production it can then be used as an effective biological control agent.

A recent study was undertaken at the University of Maryland with regard to using nematodes as a form of control for iris borer. The report information will be of interest to most iris growers in Canada regarding a viable and effective alternative control to iris borer. This control is with nematodes rather than the standard recommendation of a chemical control in the form of Cygon.

The research report on iris and nematodes was published in the Journal of Environmental Horticulture, 15(2):108-110. June 1997. The report was written by Stanton A. Gill and Michael J. Raupp, University of Maryland.

Ross Bishop, Santa Fe, New Mexico, provides the following synopsis of the report:

I have had an opportunity to digest the U. of Maryland research and talk to some folks in the field and have some preliminary conclusions.

First, if you want to know more about nematodes per. se., please visit this following website of the University of Nebraska-Lincoln: http://ianrwww.unl.edu/ianr/plntpath/nematode/wormepns.htm

It will give you good general background about nematodes. (There are other sites and there are links to them from this one.)

Last April, researchers at the University of Maryland applied two kinds of nematodes and two commonly used chemical treatments to compare results on iris borer. Nematodes used were: Steinernema carpocapsae (known as Sc) and Heterorhabditis bacteriophora (known as Hb) . Chemicals tested were: dimetheoate (Cygon) and imidachloprid (Marathon). These were used on groups of iris that had been intentionally infected with borer the previous fall. As I said, the plants were treated in April, and dug up in mid July. The findings were that one nematode Hb (Heterorhabditis bacteriophora), dimetheoate (Cygon) and imidachloprid (Marathon) all eliminated 87% of the borer problem, and Sc (Steinernema carpocapsae) the other nematode, did better, eliminating the borer population completely.

I know many of you have been using Cygon, and given these results, I want to urge you to consider changing. Cygon is a nasty chemical, rating very high on the government's LD toxicity list. Its high toxicity is the reason it works, but on the other hand, using chemicals like this is like conducting nuclear war on your garden. It kills everything. Unfortunately, most of what it kills are beneficial organisms. It is also a little risky to use. I have often wondered if Cygon wasn't a major contributor to the reason virgin soil seems to do so much better for iris. Marathon is fairly new (at least to me), and I don't know how dangerous it is. It is a systemic poison.

By contrast with both, nematodes are so safe that the government does not even bother to regulate their use. Nematode suppliers will also tell you that nematodes (depending on the variety) will control up to 300 other garden pests, while disturbing few beneficial garden organisms. (Nematodes are relatives of earthworms.) I find that a fascinating prospect.

The one (sort of) advantage we have with iris borer over other pests is that we know where to find them. Unfortunately where we find them is in our iris plants! In the research study, nematodes were applied to a one square foot area around each infected plant at a rate of 500 nematodes per square inch (remember that nematodes are microscopic). (The researchers also tested a 1,000 per square inch rate and found that the 500 rate was just as effective.) Nematodes are routinely purchased per million, which would mean that at the 500 per square inch rate, a million nematodes would treat 2,000 square inches. The researchers treated a square foot around each plant (144 square inches), but I think one might be able to do less, perhaps an 8-inch square around each plant. The reason I suggest this is that normal nematode rates for broadcast application run more like 4-5 per square foot, so 500 is considerably higher than normal.

Anyway, using a 12" square, a million nematodes could treat 14 plants. Using an 8" square you could treat closer to 30 iris plants. All these numbers are approximations because when you receive the nematodes they usually come in a damp sponge and you then disperse them in a bucket of water. The mix is then poured or sprayed over the area you want to treat.

Although possible, foliar treatment with nematodes does not seem to be a good option Opinion is not based upon research, however. The general consensus is that chances of success are greater using nematodes applied to the soil. A factor to remember is that nematodes don't like cold soil. So when you treat, for optimal results, make certain that your soil temperature is at least 50 degrees to a depth of 5". Nematodes will last at least two months in the soil if there is nothing for them to feed on and reproduce. I know of one fellow in Vermont that has had nematodes remain in his garden for six years.

One of the suppliers used in the study, HydroGardens of Boulder Colorado (800-634-6362) sells nematodes at $8 per million, 6 million for $30. Because nematodes are living organisms they must be shipped by two day air, and this adds about another $7.50 to the cost. Several people or a club could get together however, and reduce the cost. Biosys of Columbia, MD, the other nematode supplier used in the study, apparently had formulated a water dispersible granule for the nematodes which may have made normal shipping possible. However, they are apparently out of business. There are many other nematode suppliers around, so shop around. I can recommend M&R in Durango Colorado (800-526-4075) from personal experience, I have not checked their prices recently. The fellow who supplies nematodes to Garden's Alive has an excellent reputation, but at the moment they are offering a different nematode species than SC.

I hope I've added enough additional info to make the research results usable to iris growers. In addition to the university folks I talked with a number of other researchers and nematode suppliers. Use the piece in good health. If you have additional questions on the research, Dr. Michael J. Raupp, Chairperson of Department, Entomology, Phone: (301) 405-3912
Email: mr7@umailsrv0.umd.edu has been great at answering my questions.

A reminder that for best results, wait until soil temp is at least 50 F to apply nematodes.

Simone, Nematode Project, CSIRO Entomology, Canberra, Australia adds:

It was great to see those results. I would suggest one thing though that is that everybody uses the application rate and areas used in the study.

Changing things like that may mean you don’t get the control you are looking for and therefore your money may be wasted. Do make sure your supplier is selling you Steinernema carpocapsae, as from the advertising I've seen from some sellers they make a lot of unsubstantiated claims for lots of different nematodes. I would put that 4-5 per square foot application rate in that category. I don’t know what that was recommended to kill but it seems very low. Some sellers seem to be recommending rates like that so their product doesn't seem so expensive. Anything that doesn't work is very expensive.

I hope everybody over there with borer problems tries some S.carpocapsae, I am very interested in hearing what you all think of them, whether they worked (who you bought them from) and how they compared to using chemicals (ease of use etc).

Iris Borer of Past, Present and Future?

Here is an excerpt from a Cornell University Bulletin [July 1935] that makes for some interesting reading. The control measures use chemicals no longer in use or, for that matter, even legal! Keep in mind when this was written!




Until recent years iris has been thought to be comparatively free from the attacks of insects. With the spread of the iris borer and the increasing damage caused by this pest, iris growers are becoming conscious of the injuries which insects may inflict. In the following paragraphs some of the more common insects that attack iris are discussed and measures for controlling them are suggested.

The iris borer

The iris borer (Macronoctua onusta Grt.) is probably the most injurious insect that attacks iris, causing more damage to this popular plant than all of its other insect foes combined. Not only is this borer an enemy on its own account, but the tunnels it makes in a rhizome are ideal places for the development of a serious iris disease (page 28). The iris borer attacks practically all kinds of iris, including the Japanese and the Siberian, although it appears to be more common in home gardens than in large field plantings. The injury is most evident during July and August.

It is not always easy, however, to tell where the borers are at work. If the infestation is a heavy one, entire plants will be killed; but if there is only one borer in a rhizome, the damage may not be evident aboveground. There are several ways in which one can tell that borers are present. A plant may have leaves that are loose and rotted at the base, so that they can be easily pulled off. A rhizome may be exposed, showing holes in the top that the borers have made. The leaves of the new growth may be small and poorly developed. Some of the leaves may have badly eaten places along the edges.

A careful study of the iris borer is being carried on at Ithaca and it is planned to publish a detailed report on this work in the near future. The adult borer is a medium-sized, night-flying moth, quite inconspicuous, for its coloring is mostly dark brown marked with black. The moths usually appear about the second or third week of September and are on the wing through October and even into November. They mate, and the females lay eggs that do not hatch until the following spring. The egg is a tiny object only about 1/50 of an inch across and less than half as high. When first laid the eggs are creamy-white, but eventually they become distinctly lavender. The eggs are laid in groups or clusters and are carefully glued down. In our studies, eggs have been found pasted to practically everything in the cages that had a roughened surface-twigs, dead leaves, rusty nails, cloth, bits of wood, and wire screen. Only in one instance were any eggs laid on fresh green iris leaves. A single female may lay as many as 600 or 700 eggs in 24 hours and a total of more than 1400.

As previously stated, the eggs do not hatch until the following spring, usually about the last of April or the first week in May in the vicinity of Ithaca. The newly hatched larvae are tiny creatures, only about 1/16 of an inch in length. Almost at once they crawl up the iris plants and make small holes, like pinpricks, in the leaves. Then each larva gnaws out the soft tissue between the upper and the lower surface of the leaf, crawls inside, and becomes a leafminer. From the small holes made in the foliage, droplets of sap exude. Later the larva bores down towards the base of the plant, chewing the edges of newly developing leaves. Finally, it gets to the rhizome where it makes great tunnels. At this time the larva is about 1 1/2 inches long, has a reddish-brown head and a body that is distinctly pinkish. It eventually leaves the rhizome and goes into the soil to pupate. Pupation lasts for about five weeks and, as previously stated, the adult moths begin to appear about the middle of September.


Iris plants should be carefully watched during the summer months. If there is any evidence that borers are at work, the plants should be dug up, the borers removed and destroyed, and the rhizomes carefully cleaned before being reset. Any rhizomes that are at all rotted should be given special attention, as explained on page 30.

In fighting the iris borer a thorough clean up of the garden late in the fall and again early in the spring is most essential. (See page 26.) Since egg clusters are glued to dead leaves and other debris, all possible trash should be removed and burned. Some growers have found it effective to burn over their iris plantings late in the fall or early in the spring. This should be done with a quick, hot fire. An old broom dipped in kerosene will make a good torch which can be swung back and forth over the ground about the plants; or one can cover the plants with a light coating of dead leaves or straw and then apply a match here and there.

Unfortunately, burning-over the plants seems to injure certain types of iris.

Another method of control is to spray the iris foliage early in the spring with a stomach poison such as arsenate of lead. Many of the tiny, newly hatched larvae will be killed by the poison when they crawl up the plants and chew holes in the sprayed leaves. If the spray solution also contains a contact insecticide, then any small larvae that are hit by the mixture will be killed. The following combination makes a good spray: 1 ounce (about 4 level tablespoonfuls) of lead arsenate, 2 teaspoonfuls of nicotine sulfate, 1 tablespoonful of mayonnaise dressing, and 1 gallon of water. The mayonnaise dressing helps greatly to make the mixture cover and adhere to the glossy leaves of the iris plants. Treatments should start when the eggs begin to hatch, about the first of May in the vicinity of Ithaca, NY and earlier in the southern part of the NY State. A second application should follow a week or so later.

Some iris borer control advice

Question: What to do to control or rid my garden of iris borer?

In order to get an effective control on the situation you will have to do a couple of successive sprays of cygon in the springtime in order to interrupt the borer life cycle and reduce its population. If you miss just one or two they will still lay enough eggs in the autumn to cause you grief the next season!

Check the following item from Kathy Guest of East Aurora, NY to advise you of the action to take:

Irisborer are probably the biggest problem we have to face in eastern North America (except for soft rot...but I don't have any solid advice for that!) The borer is the larva of a night-flying and very shy moth. The life cycle goes like this... despicable moth flies into your iris patch and lays her eggs on iris debris (a good cause for total clean up in the fall.... and very early clean up in the spring). The eggs overwinter and hatch in early spring and the worms released burrow into your precious iris... usually along a leaf somewhere (don't ask me why we never see the worms between the hatch and the entry... I never have, anywho). So, being a totally charming life form, the worms then eat each other... a kind of insectoid Donner Party... ending up with one superworm. The superworm eats its way down the leaf sheath to the rhizome... growing all the time... and enters the rhizome. While there, it hollows it out... exits into the soil and finds a place to pupate... which becomes a moth and so on.

SO .... the controls are 3-fold... and you can use any combination or the ones that appeal.

1. Your iris bed should be extremely clean. No dead foliage should be allowed to linger... and both a fall total clean and a very early spring cleaning are imperative! I know one hybridizer who actually BURNS his iris field each year in the late fall.

2. You should get right down and personal with your plants... especially if you're of the organic persuasion. Watch for any telltale signs... notched leaves.... shiny appearance at the base of the fan... frass or sawdust. If you see anything like this... you can pinch the bejesus out of the leaves and hope you mash the dastardly grub... or you can go in with a knife and drag it out of your iris and send it to it's maker in a bucket with clorox water.

3. You need to set up a spraying program with Cygon 2E. This systemic pesticide is applied 3 times in the spring (and proactively)... once at the first sign of iris growth (or when the tulips bloom), 10 days later and 10 days later. This should rid you of almost 100% of the critters...

...be vigilant!

Question: (prompted by the unusual warm weather experienced through late autumn, November and December 1998) If the borer start their cycle this side of winter, will the very cold winter weather (if it ever comes) kill the larvae off?

It's unlikely that borers will emerge during winter. Insect eggs (the borer passes winter in the egg stage) undergo a process called diapause, in which they become completely dormant for a fixed period of time. Only when this period is up will they become sensitive to rising temperatures and begin to develop. This is an adaptation to prevent just the thing that the question refers to--emerging during a freak warm spell and then getting zapped. Diapause can occur in various stages of insect life cycles in different species, but it usually happens in the stage that passes the winter, or the dry season.

Answer courtesy of: Bill Shear
Department of Biology, Hampden-Sydney College
Hampden-Sydney VA 23943