Answering some of your questions on insecticide rotation

As a researcher and educator, I receive questions that touch on the minutia of how to build a successful insecticide rotation program as I preach and write about the importance of developing a rotation program. In this article, I’d like to take the opportunity to explore some of these direct questions, writes Juang Horng “JC” Chong, Clemson University Department of Plant and Environmental Sciences 

I’m going to proceed with the assumption that everyone who reads this article has some basic knowledge of how to rotate pesticides. If you’ve never heard of the terms “pesticide rotation” or “pesticide stewardship,” or if you are a novice to the practice of rotating pesticides, I invite you to read one of my older AFE’s Growing Further newsletter articles on building an insecticide rotation program for whiteflies.

In the simplest terms, rotating pesticides is like playing a game of “follow the numbers.” The three main groups of pesticides have their own classification systems. For insecticides, that’s IRAC (Insecticide Resistance Action Committee) classification. For fungicides, that’s FRAC (Fungicide Resistance Action Committee) classification. And, for herbicides, the dominant classification system used in the United States is developed by WSSA (Weed Science Society of America). Each mode of action (MOA), i.e., the way a pesticide kills a target pest, is given a number. An IRAC Group 4 insecticide has a MOA that’s different from a Group 9 insecticide. If you sprayed a Group 4 insecticide last week, spray a Group 9 insecticide today, and plan to spray a Group 28 insecticide against the same pest population next week, congratulations, you have developed an insecticide rotation program!

Okay, let’s get to some interesting questions from growers across the U.S. For their privacy, we are using only first names.

Question (Sam): I have thrips. I have botrytis. I have whiteflies, aphids, powdery mildew, bittercress, phytophthora, spider mites, oxalis, and green slimy crud that I haven’t figured out what it is. Do I need to build a rotation plan for each of these pests?

Answer (JC): Wow, Sam, your greenhouse sounds like a paradise for entomologists, pathologists, and weed scientists. Can I visit? Joking aside, you need to have a rotation program for each of these issues to do pesticide stewardship right. That’s because pesticides or, more specifically, the MOA available for and effective against each issue are different. (Remember: Rotate among different MOA, not among trade names.) Some MOA are specific to certain pest species or life stages. Some pests may need treatment more frequently than others because it has a shorter life cycle; in this case, you’ll have to rotate to the next MOA sooner. The MOA selected to start the program may be different depending on the predominant life stage at the time of application. Some products only allow a certain number of applications or a certain amount allowed in each crop. These moving parts make building a single rotation program that covers all pests impossible; there is no one-size-fits-all rotation program. The simplest way is to create a rotation program for each issue. That sounds like a lot of work, but the guiding principle is the same—whether you are dealing with a population of spider mites, a patch of oxalis, or that green slimy crud, you need to rotate among MOA.

Question (Sam): I forgot to mention that the problems don’t always show up at the same time.

Answer (JC): Well, why don’t you say so earlier? That simplifies things. You know the crops you grow, so you should be well-educated on the pest issues these crops can suffer. Prepare for these issues. Knowing that pests don’t show up all at the same time allows you to use the same MOA on multiple problems. For example, MOA used in the rotation program for aphids on calibrachoa in the spring can be reused for chrysanthemum aphids on mums, or some of the MOA can be repurposed for whiteflies on poinsettia in the summer or fall. That way, you don’t have to keep all 1,500 insecticides in your shed. Three to four MOA in a single rotation program for each pest are sufficient, and some of these MOA can be the same if they are effective against multiple pests. But, when you develop a rotation program for each pest, pay attention to what you’ve included to avoid the issue of doubling up when dealing with multiple pest species at the same time (see my answer to John’s question below). Be flexible and willing to change your rotation programs to adapt to the situation at hand.

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