Chapter 24

Greenhouse Pesticides

Managing Pesticide Resistance

Resistance is a genetically-based characteristic that allows an organism—insects, for instance—to survive exposure to a pesticide dose that would normally have killed it. Resistance genes occur naturally in individual pests because of genetic mutation and inheritance. They spread throughout pest populations due to a process of selection brought about by repeated pesticide use. Resistant populations develop because the resistant individuals survive and subsequently reproduce, and the trait for resistance is “selected” in the next generation, while the susceptible individuals are eliminated by the pesticide treatment. If the treatment continues, the percentage of selected survivors will increase and the susceptibility of the population will decline to a point that the pesticide no longer provides an acceptable level of control.

General Guidelines in Managing Pesticide Resistance

The success of any pest management program depends of the correct application of pesticides when needed. Careful use makes pesticides more effective and reduces the likelihood of pesticide resistance, pest resurgence, secondary outbreaks, crop injury, and hazards to humans and the environment. Consistent with IPM principles, the following management strategies are recommended in managing pesticide resistance—whether fungicides, insecticides, or herbicides—and how to limit its development while continuing to protect crops from pests.

Adopt an Integrated Pest Management Plan

It is highly recommended for a resistance management plan to be developed within the framework of an overall integrated pest management approach for a given pest and cropping system. This should ensure that rational pest control strategies based on IPM principles—including the use of pesticides only when necessary and the use of alternative pest management techniques whenever possible—are designed to manage resistance.

Consider Alternative (non-chemical) Pest Management Measures

In keeping with IPM principles and strategies, a resistance management plan should comprise as many alternative, non-chemical pest control tools and methods as possible, as long as they contribute effectively to managing the pest.

Avoid Persistent Chemicals

Pests with resistant genes will be selected over susceptible ones whenever pesticide concentrations kill only the susceptible pests. An ideal pesticide quickly disappears from the environment so that persistence of a “selecting dose” does not occur.

Apply Only Recommended Pesticide Application Rates

The correct application rate should always be used. Reducing pesticide application rates to reduce costs may appear to provide the pest control desired, but this is only temporary.

Optimal Spray Coverage

Another means of retarding pesticide resistance is to increase the effectiveness of pesticide application. This involves maintaining and calibrating equipment and following recommendations for water volumes, spray pressures, and temperatures, thereby providing more uniform coverage.

Pesticide Mixtures

Pre-formulated mixture products and some tank mixes have proven to be relatively successful in controlling insect pests and in delaying resistance development. Pre-formulated mixtures have the advantage that resistance management is built in by the manufacturer.

Use Different Modes of Action

When resistance to a pesticide arises, not only does this resistance render the selecting compound ineffective, but it also confers cross-resistance to other chemically related compounds. This is because compounds within a specific chemical group usually share a common mode of action (MoA). Mode of action or mode of activity refers to how a pesticide affects the metabolic or physiological processes in a pest. Pesticides with the same modes of action have been assigned the same group number by their respective pesticide resistance action committees (IRAC [Insecticide Resistance Action Committee], FRAC [Fungicide Resistance Action Committee], and HRAC [Herbicide Resistance Action Committee]).

Use Long-Term Rotations

Resistance management strategies for insects, weeds, and fungal pathogens all include rotating classes of pesticides (e.g., pesticides with the same mode of action such as pyrethroids, organophosphates, carbamates, etc.). For example, with fungicides, it is suggested that classes be rotated every application. With insecticides, a single chemical class should be used for a single generation of the target pest followed by a rotation to a new class of insecticide that will affect the next generation and any survivors from the first generation.

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