Final Results
Experiments Fighting The Iris Borer
Jack Worel -- Minnesota

One more year of results have been added to the article that appeared in TALL TALK in the spring of ’04 This is the final report.

The object of the experiment was to see how effective various approaches to killing and controlling iris borers really were. The method was to simply apply the treatment and by the end June, when most borers have hatched, count any live borer remaining.

Treatments and effects: In order to figure out how many borers there would be without treatment, I discovered on the average, about 51 borers per iris bed; more in established clumps, less in new plantings.

Cygon:
The usual method I had been using for many years was a spring spraying of Cygon. I discovered over three years that this approach killed almost three-fourth of the borers. I am convinced that two sprayings before bloom and one right after would wipe out almost all borers. The problem is that Cygon is no longer being produced.

Borer Deterrent:
Iris Borer Deterrent by Garden Shield is effective if applied generously every week or so (after significant rains). With less application from the spray bottle, it was not effective. The 2002 borer count shows that. In 2003, Borer Deterrent was as effective as Cygon. In 2004, I was not able to purchase the deterrent.

Grub Control: by Bayer Corporation (Active ingredient, imidacloprid, .02 Granular)
I began using this in 2003 with poor results. The method was one application in the spring. Next, I laid down grub control in the fall of 2003 and again in the spring of 2004. The results then were almost as effective as Cygon. I cannot be sure if the improvement was from the fall and spring applications or the more timely rains received in 2004, or a little of both.

Liquid Grub Control: by Bayer (same active ingredient)
In 2004, I sprayed two beds with Liquid Grub Control (which is listed to kill birch borer and other tree borer). I applied 3 ounces per gallon at the same time of the year that I applied Cygon. It turned out to be as effective as Cygon.

Summary: Cygon, Borer Deterrent by Garden Shield and granular and liquid forms of grub control under the Merit label by Bayer Corporation are all effective in the control of borer. It is important to learn how to use each, their application strength, timing, the use of water with granular applications, and the effects of rain.
I also learned it was quite effective to pluck the borers out be hand when I did my counts. I found the large majority of them and, if I did this before the borer reached rhizome, I could save most plants.

© 2005 - 9 American Iris Society Region 8