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Answering Some of Your Questions on Insecticide Rotation

JC Chong

Article Author: Juang Horng “JC” Chong, Clemson University Department of Plant and Environmental Sciences

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.

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.

Question (Danielle): How do I know which mode of action or product to include in my rotation program?

Answer (JC): This is a biggie, Danielle. I recommend select products or application rates that are proven highly effective; no so-so, meh, or suppression. You can find efficacy information by digging through the IR-4 Environmental Horticulture Database or reading IR-4’s Research Summaries. Another great resource is the Comparative Efficacy and Ecotox table from Rutgers University’s Protecting Bees website. Or, call or email your local or favorite entomologists, plant pathologists, or weed scientists for recommendations.

Question (Danielle): But what if I also use biological control agents? How do I build a rotation program that won’t hurt my bio?

Answer (JC): That’s one of the minutiae to be considered when building a rotation program. You have to ask yourself, “How do these effective products fit into my existing pest management programs?” whenever you build a rotation program. For example, if you use cucumeris mite to control your thrips population, you want to exclude insecticides or miticides that can reduce the predatory mites’ survival and foraging efficacy. And, it’s not just thinking about insecticides and predatory mites, but also other biological control agents you’re using. Let’s say you make frequent applications of the entomopathogenic fungus Beauveria bassiana for whitefly management. In this case, you’ll have to be careful with the rotation of different fungicide MOA, which can impact the efficacy of the entomopathogenic fungi. Talk to your biological control agent suppliers for the most current information on what to use and what not to use. Don’t be surprised if you get to a point in the discussion when you have to decide which is more important to you—botrytis or whiteflies. If managing botrytis is a more critical issue, you may have to build a rotation program for whiteflies that doesn’t include entomopathogenic fungi and won’t be affected by fungicide applications.

Question (John): I just found lots of whiteflies and thrips! They are eating my verbena crop alive! What do I do?

Answer (JC): John, what has your scout been doing? Okay, no sense pointing the finger at this time. Let’s solve the problem on hand. First, let’s find the big guns (i.e., the most effective products) for both pests by using the resources I’d just provided to Danielle. You may find a product or MOA that’s effective against both whiteflies and thrips; excellent if that’s the case. If not, I think this is a great time to think about tank-mix. From a pesticide rotation standpoint, I usually shy away from tank-mixing multiple pesticides, but tank-mix has a place in dealing with two different pest species. I suggest tank-mixing products of different MOA if you choose to go this route. You could tank-mix insecticides with just about any other fungicides, herbicides, and fertilizers as long as the combination isn’t prohibited in the label and if the combination doesn’t cause pH, compatibility, or phytotoxicity issues after preliminary tests.

insecticide rotation

Spider mites

Question (John): Thank you for not pointing the finger because it’ll eventually point to me. Like you, JC, I just don’t like tank mixing too many things because something might go wrong somewhere in that combination. So, I’m going to spray something against whiteflies tomorrow and then something else against thrips two days after the whitefly treatment. Anything else in this plan I need to consider?

Answer (JC): Glad you ask, John! When applying two products close to each other, you need to sift through the two rotation programs to avoid doubling on the same MOA. For example, if you use an IRAC Group 4A product for whiteflies this week, your application for thrips on the same crop two days later should avoid using an IRAC Group 4A product. Otherwise, from the whitefly management perspective, you have doubled up on applications of Group 4A. This same principle of avoiding doubling up should apply to all subsequent applications. This principle should also be considered when building all rotation programs.

Question (David): When do I need to switch to the next MOA?

Answer (JC): My guiding principle is to use one MOA for each pest generation. Pesticide resistance has its root in genetic mutations, which can only occur between generations. So, each generation should be the basis for rotation. That means that the time between two MOA varies depending on how fast your pest develops. For example, the generation time of spider mites is about seven days. If you plan to spray once a week, you’ll have to change to a different MOA in every application. The generation time of mealybug is about a month. If you plan to spray every other week, you can apply the same MOA twice before changing to another MOA. What about fungi or bacteria, which have generation times in hours or days? You may have to switch MOA at just about every one or two applications in this case. Remember that development or generation time is shorter in higher temperatures but longer in lower temperatures. The time between changing from one MOA to the next MOA will change depending on how fast the pest develops or the season. Again, be flexible and adjust the time between switching.

Question (Kate): I always heard that you shouldn’t build a rotation program with only single-site MOA. Is that true?

Answer (JC): I am a subscriber to the principle of building a rotation program interspersing with single-site and multiple-site MOA. Single-site pesticides, which act on only one biochemical or physiological pathway within a pest, are more prone to resistance development. A multi-site pesticide is one that acts on multiple parts of the physiological pathways. Examples of multi-site pesticides include horticultural oil, insecticidal soap, and biopesticides. If you include a multi-site pesticide, the pest will have a lower chance of developing resistance, meaning that your rotation program will last longer.

I’m sure you have many more questions about developing a pesticide rotation program. As much as I enjoy congratulating myself for having a place in the ivory tower, being snarky with my colleagues, and torturing students with mid-terms or papers, I feel elated when I know I can be helpful to a grower. I and your local Extension agents or specialists are ready to help. Ask us!

Additionally, check out AFE’s Grow Pro Webinar Series, which presents new research findings specifically for growers every month. Each session includes ample time for Q&A directly with AFE’s expert researchers. Questions and suggestions for future topics can be submitted to communications@afeendowment.org.

By Juang Horng “JC” Chong, Clemson University
Department of Plant and Environmental Sciences