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Competency Area 6: Management of Pesticide Resistance

PO 40. Define pesticide resistance, and be able to describe how it develops in a pest population. Know examples of resistant field crop pests in the Northeast.

See also Pest Management performance objectives 21 & 29

Pesticide Resistance

Pesticide Resistance is the ability of a life form to develop a tolerance to a pesticide. Pests (weeds, insects, mites, diseases, etc.) that become resistant to a pesticide will not be affected by the pesticide. When pests are resistant, it is more difficult to control the pest. Therefore, it is important to try to prevent pesticide resistance.

Sometimes scientists breed pesticide resistance into plants. These plants will not be affected by that pesticide, and when the pesticide is used on a resistant crop it does not harm the crop. For example, roundup ready canola can be treated with roundup to control weeds. The weeds will die, but the canola is not affected.
Plants may also be resistant to certain pests. These plants will not be affected by the pest. Using resistant plants is an effective control method that should be part of IPM programs.

How Resistance Develops

Resistance usually develops by genetic mutation and selection. Types of mutations can include: a change in processes in the pest that make the pesticide harmless, a change in the place where the pesticide enters the pest so it cannot enter, or a change in the behavior of the pest so that it avoids the pesticide. Resistant pests are selected when the pests reproduce. For example, in any pest population there may be some pests that will not be killed by the pesticide. When the pests that survive breed, some of their young will inherit the pesticide resistance. These pests will not be affected the next time the pesticide is used. With each generation, the pest population becomes more difficult to control with the same pesticide. If the same pesticide is applied often, there will be more resistant pests than susceptible pests.

Managing Resistance

Pesticide resistance develops when pesticides are used too often and when the same pesticide or similar pesticides are used over and over again. Reduce the development of pesticide resistance by:

  • Only using pesticides when necessary
  • Using pest tolerant or resistant plant varieties
  • Using cultural controls
  • Using biological controls
  • Monitoring to make sure pesticides are applied at the most effective time
  • Using selective pesticides that break down quickly
  • Only using tank mixtures of pesticides that have different sites of action
  • Using the recommended application rate
  • Getting complete coverage so all plant parts receive the proper pesticide dose
  • Avoiding use of low rates with marginal pest control
  • If there is more than one generation of pest, alternate different pesticide groups
  • If the pesticide doesn’t work, do not re-treat with a pesticide in the same group
  • Alternating pesticides or pesticides in different groups

All these practices are part of integrated pest management (IPM).

Resistant Pests

Triazine-resistant weeds in New York State

  • Common Lambsquarters (Chenopodium album) -1977
  • Smooth pigweed (Amaranthus hybridus) -1980
  • Common groundsel (Senceio vulgaris)-1989
  • Common ragweed (Ambrosia artemisiifolia) -1993

The information above has been provided by "About Pesticides: Resistance management" by the Government of British Columbia (

More information can also be obtained from: