A resistance management plan is a critical component of any integrated pest management (IPM) program. In order to develop a resistance management program you need to understand the mode of action of the products available and the biology of your target pests.
For years entomologists have been telling growers to rotate insecticides by mode of action. The mode of action is the mechanism by which the pesticide kills the pest. Determining the mode of action of an insecticide has become easier over the last few years. Newer insecticide labels have the mode of action group listed on the top of the first page. You can also find a complete listing of the mode of action groups in a number of locations. These include OHP’s Chemical Class Chart (http://www.ohp.com/) and BASF’s Pest Management Guide (http://www.betterplants.com/).
There is still debate over how many times a product can be used before you need to rotate to another product and how different products should be in the rotation program. Your first step should be to read the label and see if there are any label restrictions regarding resistance management. Most products will limit the total number of applications per year. One extreme is the Pedestal® label which states “Do not make more than two (2) applications of Pedestal per crop per year.” In addition to a limit of the total number of applications that can be made, a products label may also have other use restrictions.
A general rule of thumb is that you want to rotate to a different mode of action for each insect generation. Products that kill by desiccation or smothering, such as soaps and oils, can be used anytime in a pesticide rotation scheme without negatively impacting resistance management programs.
There are a few instances where products in different mode of action groups have the same or similar modes of action, examples include 1) cross-resistance between organophosphates and carbamates and 2) pymetrozine (Endeavour®) has been shown to be cross-resistant to neonicotinoids in Bemisia tabaci. It is also important to take note if there is a letter after the mode of action group number, such as 4A with the neonicotinoids. Products with different letters within the groups have different modes of killing the insect, although the end result appears similar or the active ingredients have very similar chemical structures. One example is MOA Class 9. Although Endeavor (9B) and Aria (9C) are both selective feeding blockers, they can be rotated with each other since each product has a different target site on the insect.
Unfortunately, many people do not understand how resistance develops. Insecticide resistance is the genetically based, inherited ability of an individual to survive exposure to an insecticide that is lethal to other individuals in the population. That is a nice scientific definition. Here is a very simplistic way to visualize resistance. You have 1000 aphids on an oleander and because you have not been rotating insecticides 10 individuals or 1% of them are resistant to your favorites insecticide. After you spray there are 100 aphids left on the plant. Assuming the resistant individuals were not impacted by any other mortality factor they now comprise 10% of the population. You decide that a second insecticide application is need a month later. The there are now 2000 aphids on the plant and 200 are resistant to our favorite insecticide (2000 x 10% = 200). Your spay tech does a phenomenal job spraying and kills all the susceptible aphids, but the 200 aphids not impacted by the spray are still on the plant. A week later you decide you want to sell the plants without any aphids and have the plants sprayed again with your favorite insecticide and to your surprise you are unable to detect any decrease in the population. This is a very simplistic hypothetical situation, but hopefully give you a way of visualizing how resistance can develop.
As illustrated above, it takes a number of generations for an insect species to develop resistance to an insecticide. Generally, the shorter the generation time the faster the species can develop resistance. According to the Insecticide Resistance Action Committee (http://www.irac-online.org/) there are four mechanisms that may cause an insect population to become resistant to insecticides. Some insects may actually exhibit more than one of the mechanisms at a time.
Metabolic resistance. This is the most common mechanism of insecticide resistance. Resistant insects use internal enzymes to detoxifying or destroy the insecticide’s toxin more rapidly than a susceptible insect. The resistant insect may also be able to rid its bodies of the toxic molecules.
Altered target-site resistance. The second most common mechanism of resistance is a result of the target site where the insecticide toxin binds becoming altered. This modification reduces the ability of the insecticide to kill the insect.
Behavioral resistance. Resistant insects detect or recognize the insecticide and avoid the toxin. The insect may stop feeding or move to an area where there is no insecticide.
Penetration resistance. The resistant insect’s outer cuticle develops barriers that slow an insecticides ability to penetrate into the insect.
The first evidence of resistance is usually reduced efficacy against the target pest even when the pesticide was properly applied at the recommended rate. If you suspect a pest population is developing tolerance to a particular chemical, continued use or increasing the rate of the product will only accelerate the rate of resistance selection, eventually leading to complete control failure. If you are applying an insecticide that is not longer effective you are wasting money on labor and insecticides that will have no impact on the pest population.
If you suspect insecticide resistance or need further information, please do not hesitate to contact me, the insecticide company, or your local extension specialist.
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