What is pesticide resistance?

Pesticide resistance is the inherited ability of a pest (weed, pathogen, insect, mite, etc.) to survive and reproduce following application of a pesticide (herbicide, fungicide, insecticide, etc.) that would normally be lethal. Pest populations contain natural diversity that results from random mutations. Spraying pesticides does not cause the resistance mutation to occur. An individual pest either has traits that allow it to survive exposure to a given pesticide or it doesn’t. Overuse of the same pesticide or others with the same site of action selects for individuals in a population who have resistant genes, and since they survive the application, they pass these traits on to the next generation. Resistance manifests itself at the population level. Over time, there is a shift in the frequency of individuals from being mostly susceptible to mostly resistant, and the pesticide loses efficacy.

Not all resistance is stable. In some cases where resistance is not stable, populations may revert to being susceptible if the selective pressure of the pesticide is removed. This occurs when there is a biological disadvantage associated with being resistant such as reduced size, fertility or fecundity, increased development times, decreased longevity, or other factors. If resistant individuals are disadvantaged when the pressure is removed, their frequency in the population will decrease. This is referred to as a fitness cost. Unfortunately, resistance is often not associated with a fitness cost and is stable, even decades after a product or group of related products are no longer in use.

Resistant pests can be costly to the grower. Direct crop loss can be a result of resistant pests not being controlled during a critical crop stage. Pesticide resistance reduces the options available for pest management and complicates the development of resistance management strategies using remaining chemistries. The discovery, development and registration of new pesticides is both lengthy and costly. Judicious use of pesticides as one component of a broader Integrated Pest Management (IPM) program will help reduce the development of resistance.

Pesticides need to be viewed as a valuable, non-renewable resource to crop production.

Certain pests are more prone to develop resistance to pesticides than others. Pests with a short life cycle and multiple generations per growing season are more likely to develop resistance. Production systems that do not allow the migration of susceptible individuals from untreated areas (e.g., greenhouses) offer few opportunities to reduce the proportion of resistance genes in the population and are at greater risk for the development of pesticide resistance.

Two important and related concepts help describe how pesticides are classified: mode of action and site of action (or target site). In general, mode of action refers to the cellular process or physiology that is affected by the pesticide. For example, some pesticides affect proteins involved in cellular respiration, while others affect growth and development, cell membranes, or the nervous system. Site of action refers to the specific location on the target protein where binding of the pesticide molecule occurs. For example, diamide insecticides activate specific receptors associated with muscles (ryanodine receptors), depleting calcium stores in the cell, and resulting in paralysis. Diamides are all classified as Group 28 insecticides, based on the site of action designated by the Insecticide Resistance Action Committee (IRAC).

Cross resistance results from a single mechanism of resistance and can result in the loss of efficacy of all products within a site of action group, and sometimes between different sites of action groups. For example, strobilurin fungicides (Fungicide Resistance Action Committee (FRAC) group 11) have a very specific target site and when the predominate fungal population is resistant to one of the fungicides (e.g., pyraclostrobin) the other fungicides within that group (e.g. azoxystrobin) are no longer effective, even if they’ve never been used.

Multiple resistance is resistance to multiple pesticide groups with different sites of action where more than one mechanism is involved. This can happen if one pesticide or a group of related pesticides is overused until the pest population displays resistance, followed by overuse of replacement products. For example, there are populations of waterhemp in Ontario that are resistant to herbicides with 4 different sites of action: imazethapyr (WSSA group 2), atrazine (WSSA group 5), glyphosate (WSSA group 9) and fomesafen (WSSA group 14).

Resistance is not the only cause of a pesticide failure. Before assuming a pest population is resistant to a product, consider the following factors, which may impact the effectiveness of pest control products:

  • Failure to keep accurate records of pest stage and development within a crop
  • Incorrect pest identification
  • Application to the wrong stage of pest on which the product is not effective (e.g. if applied during the egg stage and it has no effect on that stage)
  • Wrong pesticide choice
  • Incorrect use of pesticide (e.g., wrong rate, degradation of active ingredient due to long storage)
  • Inadequate preparation of pesticide (e.g., tank mix, adjuvants, pH, concentration)
  • Incorrect spraying/application time (e.g., time of day, environmental conditions)
  • Faulty equipment (e.g., incorrect calibration, poor maintenance, inconsistency, worn nozzles, low pressure)
  • Insufficient or improper pesticide coverage
  • Relying only on pesticides to manage pests

How do I know if I have resistant pests?

It can be challenging to know after a pesticide application why there may still be pests at an economically high level present in the field. Use this list to determine if the cause of pesticide failure is due to resistance:

  • Was the same pesticide or pesticides that have the same site of action used over a period of several growing seasons?
  • Has the uncontrolled pest been managed efficiently by the same pesticide in past years?
  • After the pesticide application, is the surviving pest (weed, insect, pathogen) growing and thriving?
  • Have all the other susceptible pests died?
  • Has a decline in the control of this particular pest been noticed in recent years?
  • Is there any known report of pest resistance around the farm area or within the province?
  • Is the pest being well managed on neighbouring farms?

Documenting and reporting resistance early is an important mitigation strategy. If you suspect pesticide resistance, contact your local agronomist, diagnostic lab or OMAFRA specialist.

General Resistance Management Strategies

Using management practices to avoid or slow the development of pesticide resistant pests is to be implemented with all pesticide use, not only when resistant pests have become an issue in your crop. Pesticide resistance management must be a community practice. Once resistance develops on one farm, it usually cannot be reversed and will spread to the rest of the industry over time.

At an individual field level, think about how to effectively use integrated pest management strategies including cultural, mechanical, biological, and chemical options. Any reduction in use of pesticides will result in less selection pressure on the pest population.

When pesticide use is warranted, here are some general management strategies:

  • Know your product
    • Many chemicals with the same active ingredients are marketed under different brand names. For example, propiconazole is sold under the trade names Tilt, Jade, Bumper, Topas. Consider all of these the same product for resistance management.
    • Different chemicals may also have the same mode of action. For example, both Success (spinosad) and Delegate (spinetoram), although different active ingredients, have the same IRAC classification (Group 5). Using Delegate after Success is equivalent to using Delegate after Delegate, since resistance to both chemicals develops at the same time, even though only one may have been used repeatedly. The same concept applies to herbicides. For example, both Authority and Chateau are in the same herbicide group (Group 14). Using Chateau after Authority to control pigweed species is equivalent to using Chateau after Chateau.
  • Rotate between products from different chemical groups. Avoid the repeated use of any one pesticide or group of pesticides.
  • Always follow the pesticide label. Many products make specific recommendations about the maximum number of sequential applications and the maximum number of total applications permitted in one season.
  • When applying a pesticide, use the appropriate rate, timing, water volume, nozzle selection and water pH. These application instructions are designed to maximize efficacy while minimizing the potential for resistance to develop.
  • Do not make decisions on tank-mixing products during loading; do so during the off-season. Before tank-mixing pest control products, ensure the following:
    • All potential tank-mix partners are registered for use on the crop in Canada and being used according to the label.
    • Tank-mixing of these products is permitted on product labels, and no product is specifically excluded on any of the other labels.
    • If more than one insecticide, fungicide or herbicide is included in a tank-mix, each should belong to a different group with a different mode of action.
    • All products are compatible as a tank-mix. Compatibility issues may result in reduced control or crop damage due to chemical interactions or because of problems in the tank such as inability to mix homogenously or development of a precipitate.
    • The tank-mix only includes an adjuvant when specifically required by one of the product labels, and is not incompatible with any of the others.
  • Resistance management needs to be considered for each pest. Often, products applied for one pest will not have efficacy on another pest. Know which products have efficacy on pests present in the field to avoid practices that may increase the resistance risk.
  • If a control failure occurs after using a registered product, do not reapply the same pesticide.
  • Keep accurate records of the type of product used during each application throughout the season. Document the effectiveness of the application, including if sufficient control was achieved.

Managing Resistance to Fungicides

  • Know the fungicide FRAC groups. Over a season, choose fungicides from different groups whenever possible. Limit the total number of applications, and the number of sequential applications, of a particular fungicide group per season. Look for specific resistance management strategies on the product label. For example, some product labels specify that the product should account for no more than a certain percentage of that season’s applications against a particular pest.
  • For high-risk pathogens with fungicide options from many groups, rotation to a different group is advisable after a single application of a resistance-prone fungicide (as designated by FRAC), although this is not necessarily required by the label.
  • Where possible and especially for high-risk pathogens, tank mix two effective fungicides. This can include FRAC group M (multi-site modes of action) and other single site fungicides that have efficacy on the target pathogen.
  • For pathogens controlled by only a few registered fungicide groups, use no more than 2 consecutive applications of a fungicide and then alternate to a different fungicide group.
  • Know which disease is targeted by which fungicide group. For combination products, know which fungicide component is controlling which disease. Rotating or tank mixing fungicides only helps in resistance management if both (or all) fungicides are effective against the target disease.
  • When a product contains active ingredients from more than one group, each application counts as a single use for each group. For example, one use of Miravis Duo counts as a single use of difenoconazole (Group 3) and a single use of pydiflumetofen (Group 7).
  • In some cases, a single fungicide group can control more than one pathogen. In this case, the maximum number of consecutive and total applications per season should be based on the pathogen with the highest risk of developing resistance.
  • Apply fungicides before disease occurs, applying at the recommended crop stages for effective management. Applications of fungicides after the disease is established are more likely to select for resistant individuals because there are more individuals in the pest population in which a mutation that confers resistance can occur.
  • Make use of Group M and BM fungicides if they are registered. These fungicides are known as multi-site inhibitors. They affect a wide range of metabolic processes in pathogens and are less prone to the development of resistance. While there is minimal risk of resistance development in these fungicides, resistance management should still be applied.

Managing Resistance to Insecticides and Miticides

  • Use thresholds and degree day models to predict the need for pesticides and ensure appropriate timing of application.
  • Consider the use of mating disruption where available and practical. Non-specific mechanical and physical disruptors including dormant and summer oils have very low risk in terms of pesticide resistance.
  • Encourage biological control by choosing pesticides less harmful to beneficial insects and by providing flowering plants in unsprayed habitat for these natural enemies. This may reduce the need for insecticides or miticides, particularly those targeting indirect pests such as aphids and mites.
  • Know the insecticide groups. Rotate products from different IRAC groups when available. Avoid sequential applications of the same group or repeated use of any insecticide or group of insecticides.
  • Insecticide group 4 has been divided into subgroups (4A= neonicotinoids, 4C=sulfoximines, 4D=butenolides). Compounds from these subgroups are structurally distinct but share the same binding site. The risk of cross-resistance between these subgroups is considered low. However, where alternatives are available, rotate with other groups. If only Group 4 insecticides are registered against the pest but more than one subgroup is included, rotate between subgroups only if it is clear that cross-resistance does not exist in the target populations.
  • For insects with multiple, discrete generations (e.g., oriental fruit moth, codling moth), manage each generation as separate units or “treatment windows”. Use products from a single insecticide group to manage a given generation of a pest. If the pest emergence or activity of that generation is prolonged, apply a second application of the same product. This exposes each generation to only one group. Rotate to another insecticide group (or groups) for subsequent generations. Avoid tank mixing two products from different insecticide groups if planning to spray more than one generation per season. This enables rotating to a different insecticide group the pest hasn't yet been exposed to for each generation.
  • For pests whose populations build quickly and with multiple, overlapping generations (e.g., aphids, mites), rotate between products in different insecticide groups for each spray. For short cycle crops, it may be necessary to consider the duration of the crop cycle as a window. In this case, it is recommended to alternate to different modes of action in the next crop cycle.
  • Avoid unnecessary or repeated applications of miticides and rotate among products in different groups. Many labels limit the number of applications of a product to one per season. Consider a multi-year rotation of miticides, so that mites are not exposed to products with a similar mode of action more frequently than once every few years.
  • Time sprays to contact the most susceptible life stage of the pest. Consider the time of day when the pest is most active and location in the crop to maximize exposure with the treatment. For example, some insecticides are only effective against scale insects when the susceptible crawler stage is present.
  • Monitor problematic pests to detect shifts in sensitivity to a group of pesticides.

Managing Resistance to Herbicides

from manageresistancenow.com (with permission)

Rotate Crops

  • Rotating crops within a field each growing season is essential to managing herbicide resistance.
  • Rotating crops allows for rotation of herbicide groups, making it more challenging for weeds to develop resistance to repeated use of the same mode of action.
  • Rotate crops with different seeding and harvesting dates. Risk of weed resistance is shown to be the lowest in fields with fall-seeded crops, forage crops, or where three or more crop types (e.g. cereal, oilseed, pulse) are grown over a six-year period.
  • Include crops that compete well with weeds. Plant a range of different crops including a mix of dicots and monocots, winter and spring planted, and annuals and perennials in your rotation.

Mix and Rotate Herbicides

  • Rotate herbicides within and between growing seasons. Use herbicide mixtures and rotate the mixtures for even more impact.
  • Rotate the use of one herbicide group with other herbicide group(s) that control the same weeds in a field. Rotate groups both during a growing season and across years in a field.
  • Herbicide mixtures – the combination of two or more herbicides having different modes of action applied as a single mixture – should be used to delay the onset of resistance to any herbicide. You can mix various combinations of herbicides according to label
    instructions. Use the recommended label rate of each herbicide for maximum weed kill.
  • Rotate from one herbicide mix to another during a growing season and from one season to the next. It’s easy for weeds to become resistant to simple, predictable weed control. Mixing and rotating makes it unpredictable for weeds and creates diversity for your crop plan.
  • For a mixture to truly be multi-mode of action, both modes of action need to be effective on the same weed species. If you are targeting one weed species, ensure the herbicides you are using target that weed species.
  • Consider herbicide layering if there are weed escapes after a soil applied herbicide in the fall or early spring. For example, follow up with a post-emergent application with different modes of action that target the same weed species during the growing season.


Use Recommended Rate and Timing

  • Using below-label rates of herbicides can contribute to the development of resistance. Use the full rate, timing and water volume indicated on the label.
  • Survey your weed populations before spraying so that your weed management is field- and site-specific. Scout fields after herbicide application so that you know how successful you have been in controlling the targeted weeds.
  • No herbicide is 100% effective on a susceptible weed species. Scout for weeds that have escaped an application and ensure that they do not disperse seed
  • Be mindful of spray techniques. Be aware that low travel and wind speeds will allow for more uniform application. Consider boom stability for more uniform droplet deposit. Also, keep in mind that sub-lethal doses can occur in a field on the periphery or outside of turns and lead to herbicide resistance.


Other Best Management Practices for Managing Herbicide Resistance

  • Keep accurate records to make informed crop management decisions for each field and even specific areas of a field.
  • Maximize crop competitiveness by using agronomic practices that promote competition with weeds such as planting at high seeding rates.
  • Use weed sanitation practices like planting weed-free crop seed and applying only composted manure to reduce weed seed additions in the soil seed bank.
  • Prevent and eliminate weed escapes, when possible, in field borders and fence rows. These are breeding grounds for weeds, including herbicide-resistant weeds.
  • Consider strategic tillage since the risk of weeds developing resistance is higher when no-till practices are in place.
  • Connect with a crop advisor who is familiar with weed biology to help trouble shoot when needed.


Additional resources