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Resistance Management

Resistance to pesticides originally drove the development of Integrated Pest Management, and today reducing the risk that insects, pathogens, and weeds will develop resistance to pesticides remains a primary goal of IPM. Modern pesticides generally have much more specific modes of action than older pesticides. A pesticide developed fifty or more years ago often has multiple ways that they can kill a pest, while today pesticides often have a single, specific aspect of pest physiology that they disrupt. Pests are more likely to develop resistance to single-site pesticides than to multi-site pesticides, so in terms of potential to develop resistance, single-site pesticides are considered “high risk” and multi-site pesticides termed “low risk”.  Some key pests, such as the apple scab fungus, have developed resistance to many types of high-risk fungicides. In order to maintain a full set of management options, it’s critical that growers manage pesticides resistance.

There are several ways that IPM addresses resistance, starting with only applying a pesticide when needed. In addition, IPM incorporates non-chemical tactics in addition to pesticides, rather than focusing exclusively or heavily on chemical control. When chemicals are required, a good IPM program recommends tactics for pesticide selection and application that reduce the chances of resistance development.

Tank Mixes. One way to manage resistance with pesticide selection is to apply mixtures of pesticides in a single application. Each pesticide must have a different mode of action, as indicated by different FRAC, IRAC or HRAC codes. Usually, these codes are on the pesticide label, and this guide lists them with pesticides as well. In recent years, companies have developed pesticides that come pre-mixed, with two, or even more, different modes of action. Usually, the mixes are made up of high-risk pesticides. While this is better than applying only one high-risk pesticide, putting a low risk, multi-site pesticide into a tank mix provides more powerful resistance management. Even with a pre-mix, it is worthwhile adding a low-risk partner.

Alternate Pesticide Modes of Action. Another way to manage resistance is to alternate the modes of action used in consecutive applications. This may be done by selecting a high-risk pesticide for one spray, a different high-risk pesticide in the next spray, and returning to the first mode of action for a third application. Even better would be alternating applications of a high-risk pesticide with a low-risk pesticide. Better still, apply a high-risk pesticide, then a low-risk pesticide, then a high-risk pesticide with a different mode of action from the first, then a low-risk pesticide. The more often the mode of action changes, the better it is for resistance management.

Limit the Use of Any Single Mode of Action in a Season. The more often a given mode of action is used, the more likely it is that a pest will develop resistance to it. By using a variety of modes of action in a year, the chances of resistance development go down. Pesticide manufacturers have increasingly put label restrictions on the number of applications per year, the total amount of a material that may be applied in a year, or both. This is to prolong the useful life of their products.

Use the Highest Recommended Rate. At the beginning of the modern drug era, the famous microbiologist Paul Ehrlich recommended that drugs be used to “hit hard and hit fast”. That is, use high rates of drugs as early as possible in the infection to prevent bacteria from becoming resistant to them. The same holds true for pesticides. Use the highest rates on the label to reduce the chances of resistance development in insects, pathogens or weeds.

Treat Early - Avoid Using Pesticides as a “Bail Out”. Similarly, address pest problems as soon as it is clear that a pesticide application is needed. For diseases, using post-infection or eradicant treatments increases the risk of resistance. Treating insects after they have had a chance to build up their populations, or treating more mature weeds, particularly if they go to seed, is also boosting the chances of resistance.


The Insecticide Resistance Action Committee (IRAC)

IRAC is dedicated to prolonging the effectiveness of insecticides and acaricides by countering resistance problems. IRAC provides a coordinated industry response to prevent or delay the development of resistance in insect and mite pests. The IRAC Mode of Action (MoA) Classification Scheme is recognized globally as the authoritative reference for defining the MoA of commercial insecticides. This information provides growers, advisors, extension staff, consultants and crop protection professionals with a guide to the selection of acaricides or insecticides for use in an effective and sustainable acaricide or insecticide resistance management (IRM) strategy.

IRAC has just published the latest version of the MoA Classification (, now incorporating bio-insecticides.

In addition to presenting the MoA classification, this document outlines the background to, and purposes of, the classification list, and provides guidance on how it is used for Insecticide Resistance Management (IRM) purposes. IRAC has taken the step to systematically integrate non-chemical and biological products with insecticidal activity into the IRAC MoA Classification Scheme. This is one of the most significant updates to the scheme since its creation. Now included are bacterial agents, plant-derived extracts and unrefined plant oils, fungal agents, non-specific mechanical disruptors, peptides, and viral agents.