Pesticide Resistance

John Unsworth
3rd April 2010

Pesticide Resistance


Through evolution plants and animals have developed defensive mechanisms, including chemical repellents and toxins, against attacking organisms. In turn the attacking organisms have developed mechanisms that enable them to detoxify or otherwise resist the defensive chemicals of their hosts. Thus, it appears that most pest species already contain genes that enable them to degrade enzymatically or otherwise circumvent the toxic effects of many types of chemicals that have been developed as modern pesticides1. Pesticide resistance, therefore, is a genetically based phenomenon and occurs when a pesticide is used on a population containing some individuals genetically predisposed to be resistant to that pesticide. Repeated applications and higher treatment rates will kill increasing numbers of the pest but resistant survivors will pass the resistance genes to the next generation. Unless a different treatment regime is used the population will contain increasing numbers of resistant pests and where reproductive rates are high, e.g. in insects, the entire population will quickly become resistant2.


Once a pest has developed resistance to a particular pesticide it is necessary to have other means of controlling it. One method is to use a different pesticide, especially one in a different chemical class that has a different mode of action against the pest2. Care must be taken, however, that multiple resistance, i.e. resistance to several classes of pesticides, is not introduced into the population. The best strategy is to avoid building up a resistant population and various procedures have been developed for this. Various management strategies have been recommended to avoid the build up of resistant populations due to the use of pesticides. The US EPA and the Canadian PMRA have drawn up a voluntary labelling scheme, together with recommendations for avoiding resistance, giving mode of action and target site information to users3.


Various “Action Committees” have been set up by industry and, with the input of academic and government scientists, have proposed management strategies to avoid the build up of resistance to pesticides.


Insecticides – A summary of the recommendations made by The Insecticide Resistance Action Committee (IRAC) is given below. Full details can be found on the IRAC website4.


  • Consult an advisor for insecticide resistance management and IPM strategies. Consider the pest management options available and map out a season-long plan to avoid unnecessary applications of insecticides.
  • Before planting, consider the options for minimizing insecticide use by selecting early maturing varieties or varieties that are resistant to insect attack.
  • Consider an integrated approach incorporating as many different control mechanisms as possible.
  • Select insecticides with care and consider the impact on future pest populations.
  • Watch the pest population during the growing season. Regularly monitor fields to identify pests and natural enemies, estimate insect populations and track stage of development. Insecticides and acaricides generally should be used only if insect counts exceed the local economic threshold or the point where economic losses exceed the costs of insecticide plus application.
  • At the end of the season remove crop residues, as appropriate, to eliminate food sources and over wintering habitats for pests.
  • Prevention is the best strategy, but if you suspect resistance, first eliminate other possible causes.


Fungicides – A summary of recommendations made by the Fungicide Resistance Action Committee (FRAC) is given below. Full details can be found on the FRAC website5.


  • Do not use the product in isolation, apply it as a mixture with one or more fungicides of a different type.
  • Restrict the number of treatments applied per season and apply only when strictly necessary.
  • Maintain manufacturers’ recommended dose.
  • Avoid eradicant use.
  • Use integrated disease management, a particular aspect of the concept more generally referred to as IPM (Integrated Pest Management)
  • Make use of the chemical diversity of different fungicide families.


Herbicides - A summary of recommendations made by the Herbicide Resistance Action Committee (HRAC) is given below. Full details can be found on the HRAC website6.


  • Rotate crops in order to allow the use of herbicides having a different mode of action and the use of different cultural techniques which can be employed to manage a particular weed problem.
  • Use non-chemical weed control methods which can assist greatly in reducing the soil seed bank.
  • Use herbicides with different modes of action against a given weed species.
  • Follow label use instructions for timing and treatment rate carefully.
  • Routinely monitor results of herbicide application and keep detailed field records of herbicide use so that cropping and treatment history are known.


Rodenticides – The Rodenticide Resistance Action Committee (RRAC) have issued a monograph on the strategy for anticoagulant resistance management, the recommendations of which are summarised below. Full details can be found on the RRAC website7.


  • Ensure that all baiting points are inspected weekly and old bait replaced where necessary.
  • Undertake the treatment according to the label until the infestation is cleared.
  • Remove all unused bait when treatment is complete.
  • Do not use anticoagulant rodenticides routinely as permanent baits.
  • Ensure complete elimination of the infestation is achieved
  • Apply Integrated Pest Management measures e.g. removal of food and water sources, remove harbourage and proof susceptible areas against rodent access.
  • Record details of treatment.




1.       The Magnitude of the Resistance Problem, G.P. Georghiou, Pesticide Resistance: Strategies and Tactics for Management, National Academy Press, Washington DC (1986)



2.       Pest Resistance to Pesticides, R.G. Bellinger, Dept. of Entymology, Clemson University, South Carolina (1996)



3.        US EPA, Pesticide Registration (PR) Notice 2001-5, Guidance for Pesticide Registrants on Pesticide Resistance Management Labelling (2001)



4.      General Principles of Insecticide Resistance from IRAC



5.       Fungicide Resistance in Crop Pathogens : How Can it be Managed ? K.J. Brent, FRAC Monograph No.1 (April 1995)



6.       Guideline to the Management of Herbicide Resistance, HRAC




7.      Anticoagulant Resistance Management for Pest Management Professionals, Central and Local Government and Other Competent Users of Rodenticides, RRAC Technical Monograph 2003






Last modified 3rd April 2010






Date added: 2010-05-08 00:39:56   
Last Updated 2010-05-10 03:08:56   
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