6.5 Resistance Management

6.5 Resistance Management

Quick Answer: Repeated use of one mode of action (MoA) selects for resistant pest individuals until they dominate the population. The three industry committees that classify MoA by group number are HRAC (herbicides), FRAC (fungicides), and IRAC (insecticides). Rotate MoA groups between successive pest generations, tank-mix different MoA groups at full rates, integrate non-chemical controls, and never reapply the same product after a control failure.

How Resistance Develops

Resistance is a heritable change in a pest population that reduces sensitivity to a pesticide used at the label rate. It is not the pest adapting during its lifetime; it is the selection of individuals that already carry a resistance trait.

The process has four steps:

1. Natural variation. Any large weed, insect, or pathogen population contains a few individuals with a genetic trait that makes them less sensitive to a given pesticide.
2. Selection pressure. Applying a pesticide with one MoA kills the susceptible individuals and leaves the resistant ones alive.
3. Survival and reproduction. The resistant survivors mate and pass the trait to the next generation.
4. Population shift. After several rounds of selection, the resistant biotype dominates the field. The pesticide that worked for years now fails.

Selection pressure is highest when the same MoA is used repeatedly, at high rates, over large areas, and with no other control tactic. Iowa waterhemp populations illustrate the endpoint: a single field can contain plants resistant to Group 9 (glyphosate), Group 2 (ALS), Group 14 (PPO), and other sites of action.

Mode of Action vs. Active Ingredient

The mode of action (MoA) is how the pesticide kills the pest at the molecular or physiological level. The active ingredient is the specific chemical. Two products with different active ingredients can share the same MoA, and rotating between them does not relieve selection pressure.

Example: glyphosate (HRAC Group 9, EPSPS inhibitor) and clethodim (HRAC Group 1, ACCase inhibitor) have different MoA; rotating them does help. But glyphosate and any other Group 9 product share the same MoA and rotating between them does not.

The Three Resistance Action Committees

CropLife International sponsors three committees that classify pesticides by MoA group number. Each scheme is independent; the same number means different things across committees.

Product labels show the MoA group number in a prominent box (for example, "GROUP 9 HERBICIDE"). Read it before you buy or rotate.

Selected HRAC, FRAC, and IRAC Groups

HRAC (herbicides, key Iowa groups):

FRAC (fungicides, key groups):

IRAC (insecticides, key groups):

Cross-Resistance and Multiple Resistance

• Cross-resistance: one resistance mechanism gives resistance to multiple active ingredients that share the same MoA group. A target-site mutation in ALS confers resistance to all Group 2 herbicides.
• Multiple resistance: a population is resistant to two or more different MoA groups. Iowa waterhemp with Group 9 + Group 2 + Group 14 resistance has multiple resistance.

Cross-resistance is the reason rotating active ingredients within the same MoA group does not help. Multiple resistance is the reason a single field can become very hard to control.

Resistance Management Practices

The same practices delay resistance across weeds, diseases, and insects:

1. Rotate MoA groups between successive pest generations. After a Group 9 herbicide, use a product from a different group the next pass.
2. Tank-mix different MoA groups at full labeled rates. A mix of Group 9 + Group 14 + Group 15 controls survivors of any one group.
3. Use full labeled rates. Sub-lethal rates let partially resistant individuals survive and breed.
4. Integrate non-chemical controls. Crop rotation, cultivation, cover crops, resistant varieties, and planting date shifts reduce selection pressure.
5. Do not reapply after a control failure. Switch to a different MoA with no known cross-resistance and investigate the cause.
6. Scout after application. Confirm control; if escapes appear, do not let them set seed or reproduce.
7. Follow refuge requirements for Bt corn and similar traited crops; the refuge produces susceptible insects to mate with any resistant survivors.
8. Use labeled weed-free seed and clean equipment to avoid moving resistant biotypes between fields.

The single most important message from all three committees: rotate MoA groups, not just trade names. Read the group number on every label.