2018 Crop Protection Guide for Tree Fruits in Washington

Pesticide Resistance Management

Sunday, February 17, 2019

Pesticide Resistance Management

Pesticide resistance can be defined as a heritable change in the response of a pest population to the application of a pesticide, such that the pesticide does not work as well as expected based on past experience (i.e., a pesticide does not work as well as it used to due to a genetic change in the pest population as a result of previous applications).  Repeated or prolonged use of pesticides, particularly insecticides, fungicides, and herbicides, can lead to rapid evolution of pest resistance to the chemicals.  Resistance management, as part of integrated pest management, develops strategies in attempt to mitigate the risk of losing the pesticides’ effectiveness.  In the past decade, several new insecticides, fungicides, and herbicides have been registered that affect pests in different ways than past chemicals.  Practicing resistance management for the new chemicals can prolong their useful lives, while at the same time preserving or enhancing the utility of older pesticides.

The most basic practice for preventing resistance is to not spray.  Using alternative IPM tactics, such as biological control, mating disruption, cultural control (e.g., mowing, sanitation), and host plant resistance can reduce the need for pesticide applications.  In practice, however, pesticide applications are often necessary.  An important resistance management (and IPM) strategy is to apply pesticides only when necessary.

Pesticide use is often necessary to control pests in Washington tree fruit, therefore, the most important resistant management strategy is the rotation of modes of action (MOA).  The MOA, the biochemical mechanism by which the pesticide kills the pest, has been determined for most pesticides and an alphanumeric code assigned to unique MOAs, and in several cases, MOA subgroups. (see MOA Table for fungicide and insecticide MOAs; they are also included in the General Recommendation tables).  Generally, the likelihood of resistance occurring when rotating among different (unique) MOAs is much lower than when repeatedly using chemicals within the same MOA.  Thus, repeated use of the same material or materials within the same MOA subgroup is most likely to lead to resistance, followed closely to repeated use within the same MOA but different subgroup, with the best option for slowing resistance being rotation among different MOAs. 

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