Pest Resistance to Insecticides and Miticides

Pests are deemed resistant to a pesticide when they survive exposure at rates that previously controlled them. Individuals are either born resistant or susceptible to a given chemical and dose; resistance to pesticides doesn't develop within an individual over the course of its lifetime. Resistance also happens at a population level, not at a species level. This means that some populations of a pest may be resistant while others may not be, due to differences in their genetics and regional / local pest management practices. In agricultural systems, pre-existing mechanisms that allow for resistance development are selected by pesticide use. Generally, resistance to a pesticide develops after repeated exposure to a specific chemical. Sprays applied to control one pest can affect the status of resistance of other pests within the complex.

Resistance occurs because a few naturally occurring resistant individuals survive after each spray, while the susceptible portion of the population is killed. The ability is passed on from one generation to the next. These resistant survivors multiply and gradually replace the susceptible ones. Eventually the resistant population dominates, and the pesticide loses its effectiveness. In some cases when a pest becomes resistant to a chemical or chemical family it will not become susceptible again, even if that chemical is discontinued for many years (stable resistance). In other cases a resistant population becomes susceptible again over time (non-stable resistance).

Resistance to pesticides can develop very quickly. Do not use the same chemical repeatedly unless used in rotation with a different chemical or in combination with other chemicals having a different mode of action. Compounds within a chemical group usually share a common target site within the pest and mode of action. When a pest develops resistance to a chemical due to a mutation, there is a risk that the resistance will also result in cross-resistance to all the other compounds in the same sub-group. A pest population may develop cross resistance to closely related chemicals if a common detoxification mechanism exists, even in the absence of a selection pressure against individual compounds. Multiple resistance occurs through selective pressure on separate detoxification mechanisms for unrelated pesticides. In some species-specific cases, resistance develops due to an increased ability to metabolize toxins. Where these metabolic resistance mechanisms are not linked to a specific site of action, resistance to several different chemical families or mode of action can occur. Both metabolic and multiple resistant create serious challenges to the success of integrated resistance management strategies.

An effective insecticide or miticide resistance management (IRM) program will prevent or delay the development of resistant populations.

• Consider all available tools when planning pest management strategies; these include cultural, mechanical, biological, and chemical options. Avoid broad-spectrum products which may adversely affect beneficial organisms if an alternative narrow-spectrum or target-specific chemistry is available.
• Spray only when necessary: Monitoring for pests to determine their presence and the abundance of natural enemies. Insecticides are warranted only when pests are numerous enough to cause economic losses that exceed the cost of treatment. Use established thresholds where available.
• Time applications correctly: Use degree-day models, thresholds and trap catch data to target vulnerable life stages of arthropod pests. Timing may vary depending on the product being used. Use recommended spray intervals.
• Apply recommended registered materials as directed: Consider dose, timing, water volume, coverage, pH, and calibration of spray equipment.
• Alternate different chemical classes for a given pest generation: Rotate insecticides across available classes, as these will have different modes of action. Avoid the repeated use of the same insecticide or other insecticides in the same insecticide class to avoid the development of resistance or cross-resistance. Several sprays of the same product may be applied to manage a pest, but do not spray successive generations of the pest with products from the same chemical class. Keep in mind that sprays applied to manage a pest may have an impact on the status of resistance of other insects within the pest complex. Be aware that chemicals with the same active ingredients or same mode of action are marketed under different brand names.
• Avoid broad-spectrum products: Resistance is more likely to occur with persistent pesticides, particularly where one application affects several generations of insects. If using persistent products, resistance can be avoided by restricting their use to periods when growth of foliage dilutes residues to non toxic levels over shorter intervals.
• Avoid the use of insecticide cocktails to manage a single pest: The practice implies better results will be achieved where a mixture of insecticides from different chemical families exposes insects to more than one toxin, each with a different mode of action, at one time. This assumes that individuals resistant to both classes are very rare, that resistance involves a single, recessive gene, that cross-resistance does not exist, and that both products persist equally. Unfortunately, these assumptions are not typically all met in practice, and can promote the development of multiple resistance.
• Consider area-wide management programs and cross-commodity pests: Some insects are highly mobile.

Resistance can be costly to the grower. The development and registration of products with new chemistries is expensive and time-consuming. Judicious use of pesticides will help reduce the development of resistant populations and conserve the effectiveness of existing products. Alternations or rotations of compounds with different modes of action can provide a sustainable, effective approach to IRM. Keep in mind that there are many causes for failure of a product to manage a pest population. Prior to assuming a population is resistant to a product, consider the following factors in the pest management program: product choice, water volume, rate, calibration and coverage, timing, pH in spray tank water, time required for knockdown of pest, and weather conditions.

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