3.4 Pesticide Resistance Management & Mode of Action

The Threat of Pesticide Resistance

Pesticide resistance is a critical challenge in modern agriculture. It occurs when a pest population evolves the ability to survive a pesticide application that once successfully controlled it. Resistance is a localized genetic phenomenon; an herbicide that kills pigweed in one county might be completely ineffective against pigweed in a neighboring county.

Mechanism of Selection Pressure

Resistance does not develop because the pesticide mutates the pest. Instead, it develops through natural selection driven by selection pressure.

In any large pest population, a tiny fraction of individuals will possess naturally occurring genetic traits that make them slightly less susceptible to a given pesticide. When the applicator sprays that pesticide, all the susceptible individuals die. However, the rare, resistant individuals survive. These survivors then mate and pass their resistant genes to the next generation.

If the applicator repeatedly uses the exact same pesticide year after year, this intense selection pressure continues. With each generation, the percentage of resistant individuals in the population grows. Eventually, the entire population consists of resistant pests, and the pesticide completely fails.

Mode of Action (MoA) Classifications

To manage resistance, one must understand how pesticides work at the cellular or physiological level. This is known as the Mode of Action (MoA). A pesticide's MoA might involve disrupting the insect's nervous system, inhibiting a weed's ability to undergo photosynthesis, or preventing a fungus from synthesizing cell membranes.

Simply switching between different brand names is not enough; many different brands contain active ingredients that share the exact same Mode of Action. To make it easy for applicators to identify and rotate MoAs, international committees have established standardized classification codes, which are prominently displayed on pesticide labels.

• IRAC (Insecticide Resistance Action Committee): Assigns numbers to insecticide MoAs. For example, Group 1 insecticides are Acetylcholinesterase inhibitors, while Group 3 insecticides are Sodium channel modulators.
• HRAC (Herbicide Resistance Action Committee) / WSSA: Assigns group numbers to herbicides. Group 9 (Glyphosate) inhibits the EPSP synthase enzyme, while Group 4 contains synthetic auxins.
• FRAC (Fungicide Resistance Action Committee): Assigns numbers and letters to fungicides based on their biochemical target site.

Strategies for Resistance Management

The goal of resistance management is to reduce selection pressure and delay the evolution of resistance, thereby preserving the useful life of valuable pesticide chemistries.

1. MoA Rotation

The most critical tactic is rotating between different Mode of Action groups. If you apply a Group 3 insecticide for the first generation of a pest, you must use an insecticide from a completely different group (e.g., Group 28) for the next application. This ensures that any pests surviving the first spray because of a specific genetic trait will be killed by the second spray, which attacks a different biological pathway.

2. Tank-Mixing Non-Cross-Resistant Partners

Another strategy is tank-mixing two distinct pesticides that have different Modes of Action but target the same pest. The principle is mathematical: if the chance of a pest being naturally resistant to Chemical A is 1 in a million, and the chance of being resistant to Chemical B is 1 in a million, the chance of a pest possessing simultaneous genetic resistance to both distinct mechanisms is 1 in a trillion. It is imperative that the two chemicals do not exhibit cross-resistance (where resistance to one confers resistance to the other due to similar structural properties).

3. Avoiding Sub-Lethal Dosing

Always apply pesticides at the full, label-recommended rate. Cutting rates (applying a sub-lethal dose) to save money is a primary driver of resistance. A sub-lethal dose might kill the weakest individuals but allows moderately tolerant individuals to survive and reproduce. Over time, the population shifts toward higher tolerance. Full rates ensure that even moderately tolerant pests are eradicated.

4. Refugia Management

Refugia involves intentionally leaving a small portion of the crop untreated (or planting a non-genetically modified variety alongside a GMO crop like Bt corn). The purpose of the refuge is to provide a safe haven where susceptible pests can survive and reproduce.

When the few resistant pests that survive in the treated field mate with the abundant susceptible pests emerging from the refuge, the resulting offspring are usually susceptible (as resistance is often a recessive trait). This dilutes the resistant genes within the overall population, significantly delaying field-wide failure.