Grower FAQ: Managing Resistance in Weeds, Diseases, and Insects

Resistance to herbicides, fungicides, and insecticides is a growing challenge for Canadian growers. These Frequently Asked Questions from Manage Resistance Now shares science-based answers and practical guidance about resistance management to help you protect yield, rotate control methods effectively, and sustain the long-term productivity of your land.

Pest Resistance

Resistance is the genetically inherited ability of a pest to survive a control method that would normally be effective under recommended use conditions. Resistance develops as an evolutionary response to repeated and prolonged use of the same pest management practices. Typically, a small proportion of the pest population is naturally resistant; when the same mode of action is applied repeatedly over multiple seasons, these resistant individuals survive and reproduce, leading to an increase in the resistant pest population.

Calibrate sprayers each season or after equipment changes to ensure accurate application rates. Proper nozzle and pressure settings are critical in dense canopies.

Weeds and Herbicide Resistance

Herbicide resistance is the genetic ability of a weed to survive an intervention, such as a herbicide application that would normally control it under typical conditions. A small number of weeds can be naturally resistant; repeated use of the same herbicide mode of action (also known as Chemical Group) allows those survivors to multiply over time.

Learn more about Herbicide Resistance.

Yes. Studies show that adopting integrated weed management strategies can may reduce profits in the first year of implementation, but increases profitability in the second year and beyond, delivering a 14–17% higher return over 20 years. Effective weed management programs that manage resistance also pay off by preserving the effectiveness of crop protection tools for future use.

Best Management Practices (BMPs) are proven, practical methods or techniques that are used to achieve a specific goal, like managing or reducing the risk of herbicide resistance.

Recommended BMPs are strategies and practices that help minimize the risk or impact of herbicide resistance issues on Canadian farms, including:

  • Rotate crops each season to enable herbicide rotation and reduce pest pressure.
  • Mix and rotate herbicide mode-of-action (MOA) groups and use true multi-MOA mixtures where each MOA controls the same target weed.
  • Always read and follow the label. Apply the full label rate and correct water volume.
  • Scout before and after spraying to identify escapes.
  • Clean equipment between fields and manage volunteer plants.

Visit the weeds resource page for more herbicide resistant BMPs.

Herbicide-tolerant (HT) crops such as canola, corn, soybeans, and wheat can simplify weed control, but resistance can still occur.

  • Rotate herbicide MOAs even in herbicide-tolerant systems.
  • Manage volunteers from herbicide-tolerant crops in subsequent seasons.
  • Avoid over-reliance on a single tolerance trait.
    See Herbicide-Tolerant Crops 101.

Disease and Fungicide Resistance

Fungicide resistance is the ability of a plant disease (fungus) to survive a fungicide application that would normally control it. A small number of fungi may naturally be resistant – when the same fungicide mode of action (also known as Chemical Group) is used repeatedly, these survivors can increase over time. As a result, the fungicide becomes less effective at controlling the disease.

Learn more about Fungicide Resistance.

Disease and fungicide resistance has the potential to affect all crops grown in Canada, and is already an issue in several crops, including pulses, potatoes and fruit, weather, disease pressure, and repeated use of the same fungicide group increase the development of resistance.

Visit Disease for crop-specific information.

Three main factors contribute to fungicide resistance:

  1. Agronomic – crop rotation, variety, and stress levels.
  2. Disease risk – pathogen presence and weather conditions.
  3. Fungicide risk – chemistry and repeated use of the same mode of action.  See How to Manage Fungicide Resistance in Your Crops.
  • Choose resistant varieties and high-quality seed.
  • Rotate crops and manage crop residue.
  • Optimize planting date, rate, and depth.
  • Sanitize equipment and remove diseased plants.
  • Match the fungicide to the disease and growth stage.
  • Rotate FRAC groups within and between seasons.
  • Mix two or more fungicides only if each is effective on the same pathogen.
  • Avoid repeated use of single-site fungicides.

Visit Fungicide Resistance Best Management Practices for more information.

Follow label directions, including application rate, proper timing, and recommended water volume. Combine scouting data and predictive models to time applications effectively.

More details are available on the Fungicide Resistance Management page.

Insects and Insecticide Resistance

Insecticide resistance occurs when an insect population changes over time so that a product no longer provides effective control. Some insects may naturally be less sensitive to an insecticide – when the same mode of action (also known as Chemical Group) is used repeatedly, those insects are more likely to survive and reproduce. Over time, this can lead to a reduction in the ability to control the pest.

Learn more about Insecticide Resistance.

More than one-third of Canadian growers expect insecticide resistance to rise within five years. Rotating chemical groups and following label directions are essential to maintain control.

See the Insects page for crop-specific insights.

Integrated Pest Management (IPM) can slow the development of resistance. It includes a combination of cultural, biological and mechanical pest controls.

Visit the Integrated Pest Management page for more information. 

  • Evaluate the need for chemical applications using economic thresholds.
  • Rotate effective mode of action groups among generations or application windows.
  • Limit use of the same MOA to fewer than three applications per generation.
  • Alternate MOAs between crop cycles or years for single-generation pests.
  • Read and follow the label. Use full label rates and proper water volumes.
    Full guidance is outlined in Follow Insecticide Best Practices.

Insecticide mixtures, or tank mixes, and seed treatments can target multiple pests and life stages but do not replace rotation. Rotating mode of action for insecticides and seed treatment is essential for preventing resistance development, keeping records helps to remember what happened the past year and the efficacy of the intervention.

Economics and Long-Term Benefits

Herbicide resistance costs Canadian growers an estimated $1.1 to $1.5 billion each year in extra herbicide use, yield loss, and reduced crop quality.

Read the Economics of Herbicide Resistance for more information.

Strategic crop management to reduce the development or spread of resistance may require more time or inputs at first but will pay off for growers.  Adopting resistance management methods will improve yield stability and return on investment, enhance the sustainability of Canadian farms and protect the effectiveness of existing crop protection tools for the future. Learn more in the Economics of Herbicide Resistance.