Leaf Strip

Controlling Iris Borers with Pesticides

Imidacloprid (Merit): a New Systemic Pesticide

Imidacloprid is a systemic, persistent pesticide manufactured by Bayer Corp. with low toxicity to mammals. Merit is the brand marketed for home use while Marathon is used in the nursery trade. Imidacloprid is also the active ingredient in Advantage, Bayer's spot-on flea killer for dogs and cats. It an analog of nicotine sulfate, the pesticide naturally found in tobacco and petunias. It was first synthesized in Japan in 1987 and registered for use in the United States in 1994.

To control borers, Merit only has to be applied once in the spring. As with all pesticides, read the directions carefully and follow them. Do not apply near vegetables or plants that will be eaten. The recommended dosage is 60 pounds per acre but Don and Ginny Spoon have successfully controlled borers with 30 pounds per acre, half that dose. Wear rubber gloves and long pants during the application. Use a granular applicator to sprinkle the Merit granules in the soil in spring, ideally before the borer hatch which occurs after two consecutive days of air temperatures 70 degrees or greater. The Spoons often apply Merit after the borer hatch has begun because they like to observe which cultivars are borer resistant and they still get good borer control so there is some leeway on when to apply Merit. Water deeply after application or apply it before a drenching rain so the granules can dissolve quickly. Once the imidacloprid enters the soil, it will be taken up into the iris through the roots.

If you can not locate Merit in your area, Bayer Advanced Lawn Season Long Grub Control Ready to Spread Granuals is an alternative. This product contains Merit and might be easier to find. Make sure that you are buying Season Long Grub Control or a product with Merit because Bayer Advanced Lawn has several grub formulations that do not contain Merit and have other more toxic pesticides. The recommended dose of Season Long Grub Control for lawn grubs is 2 tablespoons per ten square feet of garden bed or 1.25 cups per 100 square feet.

Merit comes in a granular form, which is an improvement over the other pesticides used to control borer that are sprayed. There is less risk of the pesticide being carried off the unintended locations, less risk of inhaling the pesticide and the pesticide can be applied on breezey days. Merit is much less toxic to humans than many of the pesticides commonly used to control boreres.

Be judicious in your use of imidacloprid. Some whitefly species and the Colorado Potato have already developed resistance to it. Imidacloprid is persistent so if you apply Merit carefully, you should only need to treat once a year to control borers.

Imidacloprid works by interfering with the nervous system of insects. Its primary effect is paralysis of the insect mouth parts resulting in starvation. Unfortunately French and Canadian sources contend that imidacloprid maybe implicated in "Mad Bee Disease". In France, the beekeepers contend that the trouble began when a form of imidacloprid, Gaucho, was used to treat sunflowers seeds. In response, Bayer has research that shows no link to imidacloprid and "Mad Bee Disease". Bayer contends that "Mad Bee Disease" is caused by a virus or a spiraoplasm that produces similar symptoms. However, we all should be aware that the chemical is known to be highly toxic to honey bees based on laboratory studies at very low exposure dosages. Since it is a systemic pesticide, imidacloprid can enter the pollen and nectar of treated plants. Studies in Canada of canola (rapeseed) have shown minute trace residues of imidacloprid residues in 13 of 70 pollen samples and all were less 20 parts per billion. Further research is underway in Canada.
More information on this is available on the internet.

Orthene and Isotox Formula 4

Orthene and Isotox Formula 4 have replaced Cygone 2-E as the commonly recommended systemic pesticide for iris borers. Older formulations of Isotox contained lindane, a pesticide banned 15 years ago , so be careful with old pesticides labeled Isotox! These pesticides are applied when the Forsythia bloom or when the tulips bloom, followed by one or two more additional applications at two week intervals. Watch your iris for signs of borer to determine how long you need to spray Spraying must be done on a windless day. Follow the directions carefully.

Orthene and Isotox use the same chemical, acephate, to kill iris borers. Acephate may be a less hazardous pesticide than dimethoate (Cygon) but it is not totally benign. It is highly toxic to bees and other beneficial insects. It is hazardous to birds. Many agricultural workers have been poisoned by acephate while on the job. A healthy, young man who spent the day mixing and spraying acephate without safety gear, collapsed and died. Follow the label directions and apply it carefully!

The recent EPA review of acephate is almost complete. The prelimary results indicate that the EPA will continue to allow garden use of acephate except on lawns. The lawn and indoor use ban is to protect children from exposure to the pesticide. The use of acephate on food crops like lettuce, celery and mint can continue but based on studies of agricultural workers exposed to acephate, the EPA will probably tighten the agriculture useage rules. The EPA will not present its final decision on the use of acephate until the organophosphate pesiticide review is completed.

Isotox Formula 4 contains acephate and an additional pesticide, Vendix (hexakis), that kills mites. It does not do anything to iris borers. Unless you need a mitacide for other uses, it probably would make more sense to use Orthene.

Obsolete Pesticides: Cygon 2-E

In Massachusetts, many gardeners start spraying Cygon 2-E, a systemic pesticide, when the Forsythias bloom followed by a second spray 2 weeks later if necessary. Sometimes a third spraying is required in a severe borer year with a long hatch. Watch your iris for signs of borer to determine how long you need to spray Spraying must be done on a windless day. Follow the directions carefully.

For the last 25 years, Cygon 2-E has been the most widely recommended pesticide for iris borer control. Dimethoate, the active ingredient in Cygon 2-E was introduced as a pesticide in 1956. Cygon 2-E was used on a wide variety of plants for many different pests with effective results. Cygon 2-E was popular with many daylily enthusiasts. However a recent review by the EPA has shown that Cygon 2-E is not as safe as previously thought and there are significant risks associated with its use. Dimethoate is highly toxic to bees and moderately to very highly toxic to birds. The EPA found there are some uses of dimethoate pose acute risks to humans and the environment.

Dimethoate (Cygon 2-E) will not be re-registered by the EPA for use in home gardens by agreement of the EPA and the manufacturer after review. The EPA discovered in 1999, that in residential use, dimethoate had one of the highest rates of poisoning per million containers when compared to other organophophate pesticides. Most of the cases of dimethoate poisonings were caused by mishaps handling or diluting concentrated Cygon 2-E. Dimethoate will no longer be sold for garden use or used in areas where the general public could be exposed to it.

Go to previous page Previous page Iris borers and a borer damaged rhizome
Back to Controlling Iris Borers
Go to next page Next page
Mail to kaneonapua@yahoo.com
Home Icon

G.E. Dodge iris print