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Resistance Management - Pesticide Rotation

Author: Graeme Murphy - Greenhouse Floriculture IPM Specialist/OMAFRA
Creation Date: 01 December 2005
Last Reviewed: 01 December 2005

Table of Contents

Last month's article discussed how pesticide resistance develops and touched briefly on some of the strategies that can be used to manage it. Pesticides control pests in many different ways, however, pesticides with similar chemical structures (classed as being in the same chemical family), usually work in the same way. If a pest becomes resistant to one particular pesticide, it is very likely it will be resistant to other products that work in the same way (i.e. with the same mode of action).

One of the most commonly used approaches to manage the development of resistance is to limit the use of pesticides with the same mode of action. This is done by rotating through the chemical families so that the insect is constantly being exposed to pesticides with different modes of action. In theory, any pest individuals that may be resistant to a product and survive its application, will be killed by the next application of a product with a different mode of action. Pesticides should not be used just once and then rotated; rather, the same pesticide or pesticide family should be used for a length of time equivalent to one generation of the pest. This may change slightly depending on the time of year, but for thrips, one generation is about 2 weeks, while for whiteflies it may be 3-4 weeks. (Obviously the label also needs to be consulted for directions on how to use individual products).

So, in what chemical families do all the greenhouse registered pesticides (insecticides and fungicides) belong and how can we use this information to set up a pesticide rotation. I should warn you in advance. This is likely to be a frustrating exercise in some ways, not just for you as the reader, but for me in trying to put it together. For some pest problems, it will not be that difficult as there is a reasonable number of effective products from different families, that allow a rotational program to make sense. For others however….

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Insecticide families

Let's begin with insecticides. Going back to the start of the pesticide era (in the 1940s and 50s), the first synthetic organic pesticides were the organochlorines followed in the succeeding 20-30 years by the organophosphates (OPs), the carbamates and the synthetic pyrethroids. For a long time these four families of pesticides were pretty much all that there was, and each family consisted of dozens of different pesticides. A pesticide rotation program did not have to be very creative especially since the OPs and carbamates have a very similar mode of action often resulting in cross resistance between the two. We still have a number of products registered for use in the greenhouse, that fall into these families. For example:

  • Organochlorines - Thiodan and Kelthane
  • Organophosphates - Orthene, Pyrate, DDVP, Malathion, Dibrom
  • Carbamates - Trumpet and Pirliss
  • Synthetic pyrethroids - Decis, Pounce

There are some problems with some of these products. Because these pesticides are in general very old, insects have been exposed to them for many years, and in many cases they are not nearly as effective as they used to be. Also, the organophosphates and carbamates are currently under review by the PMRA and chances are that many of those still registered, will not be for too much longer. Many of these products are very persistent, with residues that are much more effective against biological control agents than they are against pests. With the interest that many growers are showing in biocontrol these days, many of the pesticides listed above are not compatible with the direction that growers want to take.

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In the last 10-15 years, many new chemical families with novel modes of action have been developed. This means that they are unlikely to be affected by any cross-resistance to earlier chemical families. However, it does not mean that resistance will not develop to them. It can and does. Some of the newer chemical families include:

  • avermectins (containing Avid)
  • triazines (Citation)
  • substituted benzoylurea (Dimilin)
  • chloronicotinyls (Intercept and Tristar)
  • insect growth regulators (Enstar II and Confirm)
  • pyridine azomethines (Endeavor)
  • pyridazinones (Dyno-Mite).

When we look at individual pest problems to try to develop a rotational program, we start to see how difficult this can be with the limited number of pesticides available.

Pesticides for which pests?

For example, with thrips, the only registered products (not necessarily registered on all greenhouse crops - check the label) are:

  • Trumpet (carbamate)
  • Malathion, DDVP, Pyrate, Orthene (organophosphates)
  • Nicotine (botanical)
  • Decis (synthetic pyrethroid)

Don't forget that carbamates and OPs often have the same mode of action, which may be important to know if any of these were very effective products. However, none of these are used with consistent success by growers. Also of interest is that Decis is quite ineffective against thrips.

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For mites, developing a pesticide rotation is equally difficult. The following pesticides are registered for mite control in the greenhouse (check the label for crops).

  • Floramite (unclassified)
  • Avid (avermectins)
  • DynoMite (pyridazinone)
  • Insecticidal soap (salt of fatty acid)
  • Malathion, DDVP, Dibrom (organophosphates)
  • Kelthane (organochlorine)

As with the thrips control products, the OPs and Kelthane are very old products and not as effective as they used to be. In recent years, growers have come to rely very heavily on Avid and DynoMite and most would admit that neither of those products is as effective as when they were first developed. Floramite is a newly registered active which is very effective against spider mites, but the concern is that in the absence of other effective products it will be overused and resistance will develop to it.

For leafminer, the situation is even worse, the lack of registered products compounded by the resistance of leafminer to the few we have.

  • Avid (avermectins)
  • Citation (triazine)
  • Pounce (synthetic pyrethroid)

The effectiveness of these products depends on the origins of the leafminer strain and its previous history of exposure to pesticides.

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For aphids and whiteflies, the situation is not quite as bad with several recent registrations that allow pesticide rotations to have some meaning. Registered products include (check label for crops):

  • Intercept and Tristar (chloronicotinyls)
  • Endeavor (pyridine azomethine)
  • Enstar II (insect growth regulator)
  • Pirliss (aphids only) and Trumpet (carbamates)
  • Insecticidal soap (salt of fatty acid)
  • Thiodan (organochlorine)
  • DDVP, Malathion, Dibrom, Orthene (organophosphates)
  • DynoMite (pyridazinone) - whiteflies only
  • Pounce (synthetic pyrethroid) - whiteflies only
  • Nicotine (botanical) - aphids only

So, how do I rotate these products?

You will have noticed that apart from providing a list of registered products and their families, I have conveniently skipped over giving any sort of rotation recipe for the various pests. There is a reason for that; I can't do it! It is almost impossible to generalize for all growers. A rotational scheme depends on several things:

  • the pesticides available
  • the resistance status of the pest to the various products
  • the nature of the pest problem (i.e. is it sporadic, appearing only occasionally; or are you dealing with a resident population that could be a problem 12 months a year?)

The pesticides available are the same for all growers in Canada - they don't change. However, the resistance of the pests can. Pests such as moths, leafhoppers or tarnished plant bugs, may fly in from outdoors every year and don't usually survive in the greenhouse from one year to the next. A pesticide rotational program may not be as critical in this type of situation. On the other hand however, a resident pest population such as thrips, whitefly or mites that is present in your greenhouse all year, every year, is a different situation completely. It has likely been heavily exposed to all the pesticides in the arsenal for a prolonged period and resistance may already be a serious issue. And for a pest that you bring in on plant material from a propagator in another country? Not only may it have been subjected to heavy pesticide pressure from the products we have registered here, it may also have been exposed to (and possibly be developing resistance to) products that are not yet registered in Canada.

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What then, is a grower to do?

  • Firstly, you have to know what is happening in your own greenhouse. Careful monitoring of pest populations will not only tell you what the trends are, but you will also be able to get a good idea of the effectiveness of different pest control products. This is critical to developing a rotation program that fits your situation. Once you know which products are effective against which pests, use the information from the lists of chemical families above to put together a program that allows as much time as possible before returning to the same family.
  • Control pest populations early. The smaller the population, the fewer resistant individuals will be present. Don't allow populations to build up.
  • Use a variety of control strategies. Pesticides should not be your only parachute. Consider screening your greenhouse. Good weed control and sanitation are essential. What about the use of sticky tape to control adults and slow down the population build up? Look at your crop/variety mix. Are highly susceptible varieties really critical to your business?
  • Finally, the ultimate resistance management strategy is biological control. Probably one of the reasons that Canadian growers are leaders in the use of biocontrol is because of the limited choice of pesticides. And a key reason given by growers for the adoption of biocontrol is that none of the registered products were effective.

And just to finish up. I mentioned much earlier in this article that I would begin with insecticides. That implied that I would also deal with resistance management in fungicides. And so I will, but that will have to wait for another article.

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