Insecticide Resistance

Pesticide resistance may be defined as a decreased response of a population of animals or plants to a pesticide or control agent as a result of previous exposure to the pesticide. Resistance is different from “tolerance,” which is the innate ability to survive a given toxicant dose without prior exposure and evolutionary change.

Insecticide resistance can be thought of as “accelerated evolution” or a population responding to an intense selective pressure and survival of those individuals that possess genes conferring resistance. Insecticide resistance occurs as a response to insect management practices over multiple years.

Resistance develops as a result of random mutations, producing a small number of individuals which possess traits that allow survival of normally lethal doses of insecticides. The insecticide itself does not produce a genetic change.

Insecticide Resistance Action Committee (IRAC)

IRAC is an international industry consortium providing a coordinated response to prevent or delay the development of resistance in insect and mite pests. Its website (www.irac-online.org) has a great deal of additional information. The following text from the IRAC website has been modified with permission.

Mode of Action, Target Site Resistance and Cross-Resistance

In most cases, not only does resistance render the selecting compound ineffective, it often also confers cross-resistance to other chemically related compounds. This is because compounds within a specific chemical group usually share a common target site within the pest, and thus share a common mode of action. For example, both carbamates such as Furadan® and Sevin® and organophosphates such as Lorsban® and Counter®, are acetylcholine inhibitors. Carbamates and organophosphates are subgroups with a similar mode of action.

It is common for resistance to develop based on a genetic modification of this target site. When this happens, the selecting compound’s interaction with its target site is impaired and the pesticide loses its pesticidal efficacy. Because all compounds within the chemical subgroup share a common mode of action, there is a high risk that the resistance that has developed will automatically confer cross-resistance to all the compounds in the same subgroup. It is this concept of cross-resistance within chemically related insecticides or acaricides that is the basis of the IRAC mode of action classification.

Effective IRM Strategies Use Alternations or Sequences of Different Modes of Action

The objective of successful Insecticide Resistance Management (IRM) is to prevent or delay the evolution of resistance to insecticides, or to help regain susceptibility in insect pest populations in which resistance has already arisen. Effective IRM is an important element in maintaining the efficacy of valuable insecticides. It is important to recognize that it is usually easier to proactively prevent resistance from occurring than it is to reactively regain susceptibility.

Experience has shown that all effective insecticide or acaricide resistance management strategies seek to minimize the selection for resistance from any one type of insecticide or acaricide. In practice, alternations, sequences, or rotations of compounds from different mode of action groups provide a sustainable and effective approach to IRM. This ensures that selection from compounds in any one mode of action group is minimized. The IRAC mode of action classification is provided as an aid to insecticide selection for these types of IRM strategies.

The Mode of Action Classification

This classification was developed and endorsed by IRAC and is based on the best available evidence of the mode of action of available insecticides. IRAC companies have agreed to the classification details and internationally recognized industrial and academic insect toxicologists and biochemists have approved the classification.

Table I. Mode of action of insecticides

Based on information obtained from www.irac-online.org, IRAC (Insecticide Resistance Action Committee) Mode of Action Classification Version 8.1, issued April 2016.

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