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Insect Resistance

IRM involves implementing multiple tactics, such as integrated pest management, the high-dose/refuge strategy, and monitoring to delay the evolution of resistance in pest populations.

Ultimately, resistance evolution is inevitable. Resistance management strategies are developed to delay the evolution of resistance and prolong the lifespan of pest management tools.

Resistance is a natural process that evolves due to genetic variability in insect populations in response to selection pressure. Resistance can develop against any management tool including transgenic corn (Bt, RNAi), insecticides, fungicides, and even cultural controls such as crop rotation.

Please watch the video linked here as an overview.

How Resistance Develops

Resistance prevention or delay is accomplished in the following ways:

  1. Within any pest population, a proportion of the population will possess genes that confer resistance to a pest control tool (rr). The majority of the population is typically susceptible to the pest control tool (ss) and some are heterozygous susceptible (sr). It is assumed that resistance genes are initially rare in the target population; however, the exact frequency of resistance genes is typically not known when a pest control tool is first released.
  2. Once the pest control tool is deployed, this creates selection pressure that favours survival of pests with resistance genes.
  3. With continued selection pressure, more resistant pests will survive with each generation, mate with heterozygous susceptible pests (sr) creating resistant offspring (rr), and eventually resistant pests will dominate the population.

The speed at which resistance develops depends on the pests’ biology (e.g. how many generations per year), the initial frequency of resistance alleles with the population, and the amount of selection pressure.

Another important factor in resistance evolution is the dose at which the pest control tool affects the target pest. High dose products kill >99.9% of the target population. Moderate dose products kill approximately 95% of the target population, and low dose products are less effective. The higher the dose, the lower the risk of resistance. Because high dose pest control tools cause mortality of almost all susceptible pests (ss) and heterozygous pests (sr), this reduces the availability of mates for resistant pests.

Mechanisms of Resistance

Resistance can develop in many different ways.

  • Reduced penetration of a pesticide
  • Increased sequestration or excretion of a pesticide
  • Modification of the target site molecule in the target pest resulting in lower toxicity of the pesticide
  • Metabolic resistance
    • Increased production of an enzyme that breaks down a pesticide to a less toxic form
    • Modification of an enzyme to break down a pesticide more easily
  • Behavioural resistance
    • Modification of target pests’ behaviour to avoid a pest control tactic

Insect Resistance Management (IRM)

  1. Integrated Pest Management (IPM)
    1. Use pest management tools judiciously, not prophylactically
    2. Scout to determine whether pests are present at damaging levels
    3. Use thresholds to decide whether treatment is economical
    4. Rotate, rotate, rotate
      1. Rotate host crops
      2. Rotate pest management tools
      3. Rotate insecticides with different modes of action
      4. Rotate different Bt-corn traits for the target pest
  2. High-Dose/Refuge Strategy
    1. The “high-dose/refuge” strategy was originally recommended to prevent resistance development in European corn borer (ECB).
    2. A high-dose protein will kill >99% of the target pest population. Transgenic Bt corn for ECB expresses a high dose of Bt protein (Cry1Ab, Cry1F, Cry1A.105, Cry2Ab2) against this pest.
    3. Planting a non-Bt refuge allows a proportion of the insect population to avoid selection pressure from the Bt protein, therefore, a small number of susceptible insects will survive and mate with any rare, resistant survivors from the Bt corn. The refuge ensures that resistance genes remain rare in the pest population, being diluted by the more abundant susceptible insects.
      1. For a corn hybrid expressing a single Bt protein, 20% refuge is recommended.
      2. For corn hybrids expressing a pyramid of Bt proteins, 5% refuge is recommended. It is assumed that the probability of resistance evolving simultaneously to more than one Bt protein is low.
  3. Monitoring
    1. Scout transgenic or pesticide-treated crops for unexpected insect injury. Be aware of re-entry intervals in pesticide-treated crops.
    2. If injury is observed, contact your seed provider, agronomist, extension personnel, or a CCPC representative for follow-up.
    3. Follow-up must include:
      1. Verification of planting (e.g. was the seed planted where it was intended?)
        1. Accurate planting records are key!
      2. Confirmation of Bt expression in the plants
      3. Assessment of the level and extent of injury in the field
      4. If unexpected injury is confirmed, insects should be collected from the field by the seed company or extension/research personnel to test their susceptibility to the pest control tool using laboratory tests.

Grower Responsibilities

Your Responsibilities as a Bt Corn Grower


Growers are required to plant a refuge when using transgenic corn. The amount and placement of refuge depends on the targets pest(s) of the transgenic hybrid purchased.

Go to the REFUGE SELECTOR to find out how much refuge is needed for your transgenic hybrid and what refuge hybrids are available in your area.


  • Scout for insects and insect damage in both the Bt and refuge planting.
  • Notify seed company representatives, the CCPC, or extension personnel immediately if you suspect a problem.


  • Keep accurate records of where transgenic and refuge corn hybrids were planted. This helps when comparing the performance, insect presence, or injury between transgenic and refuge hybrids.