Mixtures & Resistance
Question: Can pesticide mixtures be used to mitigate resistance?
Answer: First of all, pesticide mixtures or tank mixing involves combining two or more pesticides such as insecticides and/or miticides into a single spray solution. The mixture exposes individuals in an insect and/or mite pest population to each pesticide simultane- ously, possibly resulting in improved pest suppres- sion. Although there are benefits associated with pesticide mixtures (e.g., synergism), there are a number of issues that should be considered such as antagonism, incompatibility and phytotoxicity. It is important to read the label and understand why certain pesticides are being mixed together. Pesticide mixtures should be used that are appro- priate based on the mode of action of each pesticide and developmental stage(s) of the target pest(s) on which the pesticide mixture is most effective. For example, tank mixing two pesticides (in this case, miticides) that have activity on the adult stage of the twospotted spider mite (Tetranychus urticae) would not be appropriate because both pesticides are active on the adults. However, tank mixing a pesticide with adult activity with another pesticide that is active on the eggs, larvae and nymphs would be appropriate because this pesticide mixture tar- gets all the life stages of the twospotted spider mite.
The main reason for mixing pesticides together is convenience. It is less time consuming, costly and labor intensive to mix two or more pesticides and make one application as opposed to making two or more applications. Another reason is the potential for improved pest suppression. For instance, tank mixing two pesticides may result in greater mor- tality than applying either pesticide alone. This is referred to as synergism. Furthermore, pesticide mixtures may be more effective on certain develop- mental life stages of insect or mite pests. Pesticide mixtures, in general, are primarily used to broaden the spectrum of activity against targeted insect and mite pest populations.
The issue regarding pesticide mixtures and resistance is still not well-understood although applying two or more pesticides at different inter- vals may offer similar advantages as a pesticide mixture. However, this may not be entirely correct because each individual pest in the population may not receive a lethal dose or concentration of each pesticide. Therefore, resistance may evolve faster than what would occur with a pesticide mixture.
Another factor to consider, and one that cannot be controlled by the greenhouse producer, is the presence of resistance mechanisms in a given insect and/or mite pest population. Different mecha- nisms may confer resistance in various insect and/ or mite pest populations. The two primary mecha- nisms associated with resistance are metabolic and physiological. Metabolic resistance is degradation of the active ingredient by the pest. When a pes- ticide enters the body, enzymes attack and convert and/or detoxify the active ingredient into a non- toxic form. These detoxifying enzymes convert pesticides, which are hydrophobic (water-hating) to more hydrophilic (water-loving) and less bio- logically active compounds that are eliminated via excretion. Physiological resistance is also referred to as target site insensitivity. This is affiliated with the interaction between the pesticide and the des- ignated target, which is similar to a key (pesticide) fitting into a lock (target site). Decreased binding associated with physiological resistance is analo- gous to the lock having been changed so that the key does not fit, and consequently the pesticide is no longer effective. There are many examples of insect and/or mite pests that are resistant to insecticides or miticides based on either metabolic or physiological resistance mechanisms.
It is extremely important to tank mix pesti- cides with different modes of action. The mixing of pesticides with different modes of action may delay resistance within a given pest population as the mechanisms associated with resistance that are needed to resist the pesticide mixture may not be widespread or even exist in the pest population.
Furthermore, it may be difficult for individuals in the pest population to acquire resistance to several modes of action simultaneously. It is possible that individuals in the pest population resistant to one or more pesticides would succumb to the other pesticide in the mixture. The ability of insect and mite pest populations to evolve resistance depends on a number of factors including previous expo- sure to pesticides with similar modes of action. Moreover, using pesticide mixtures to mitigate resistance will only be effective if there is no cross resistance (based on a single mechanism confer- ring resistance to pesticides in the same chemical class and/or having similar modes of action) among individuals in the pest population to any of the pesticides in the mixture. Therefore, it is important to understand that pesticide mixtures should not be used to mitigate resistance as it is not known what resistance mechanisms may be present in the insect and/or mite pest population. The best way to avoid resistance developing in insect and mite pest populations is to rotate pesticides with different modes of action. For instance, use the same mode of action within a generation and then switch to a pesticide with a different mode of action.
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