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What to Consider when Mixing Pesticides
Aug 28, 2018

A pesticide mixture involves exposing individuals in an insect or mite pest population to each pesticide simultaneously. One of the potential benefits of pesticide mixtures is an increase in the suppression or mortality of insect and/or mite pest populations. This increase is due to either synergistic interactions or potentiation between or among the pesticides that are mixed. Synergism refers to the toxicity of a given pesticide being enhanced by the addition of a less- or non-toxic pesticide, or other compound such as a synergist (a non-toxic substance used to increase pesticide effectiveness by preventing insect pests from detoxifying the active ingredient). Potentiation is an increased toxic effect on an insect or mite population when mixing two active ingredients. However, a potential problem when mixing pesticides is antagonism in which the pesticide mixture provides less mortality compared to the pesticides being applied separately. Greenhouse producers mix pesticides primarily to suppress the multitude of insect and mite pests with one application instead of making multiple applications. However, there are a variety of factors that may compromise the continued effectiveness of pesticide mixtures. 

1. What are three important factors that greenhouse producers should be aware of when mixing pesticides?

The three most important factors to consider are 1) half-life 2) persistence and 3) mode of action of the pesticides being mixed.

Half-life is associated with the pH of the spray solution in regards to how long it takes for the active ingredient to degrade. Each pesticide has a different half-life, so it is important to only mix pesticides that have a similar half-life. Don't combine products with contradictory pH requirements. Check the label for information pertaining to the desirable pH range of the spray solution for each pesticide.

Persistence refers to the residual activity of the pesticide following an application. Pesticides have either short or long residual activity. Therefore, it is critical to mix pesticides with similar persistence (residual activity) to avoid exposing individuals in an insect or mite pest population to just one pesticide for an extended length of time. This would place undue selection pressure on the pest population leading to pesticide resistance to the product with the longest residual activity.

Mode of action is how a pesticide affects the metabolic and physiological processes in an insect or mite pest. It is important to only mix pesticides with different modes of action avoid the potential of resistance. The primary reason for this is that the mechanisms required to resist pesticide mixtures may not be wide-spread or exist in an insect or mite pest population, and it may be difficult for individuals in the population to develop resistance to several modes of action simultaneously. This is referred to as delayed resistance. Insect or mite pests in the population resistant to one or more pesticide would likely succumb to the other pesticide in a mixture. 

2. What pesticide mixtures would be appropriate for use?

Pesticide mixtures, in general, that contain pesticides with different modes of action are appropriate to use. An example of a proper pesticide mixture involves abamectin (Avid) with clofentezine (Ovation); both have very different modes of activity. Abamectin is a GABA (gamma-aminobutyric acid) chloride channel activator and clofentezine is a growth and embryogenesis inhibitor (or mite growth regulator). Another example of a legitimate pesticide mixture is spinosad (Conserve) and pymetrozine (Endeavor). Spinosad has a dual mode action; nicotinic acetylcholine receptor agonist and GABA chloride channel activator whereas pymetrozine is a selective feeding blocker.

However, continued use of the same pesticide mixture for an extended period of time may result in resistance to both modes of action by an insect or mite pest population. Especially those that have the capacity of developing multiple resistance, which involves resistance to different compounds resulting from the coexistence of more than one resistance mechanism in the same individuals. Pesticide mixtures provide greenhouse producers the opportunity to suppress multiple insect and/or mite pest populations with fewer applications; however, it is important to understand the characteristics of pesticides including half-life, persistence and mode of action to maximize effectiveness and avoid the possibility of pesticide resistance.