Pest Resistance to Fungicides, Insecticides and Miticides

Pesticide resistance is the ability of a pest to survive exposure to a pesticide at a rate that previously controlled it. It can occur in any pest population, including fungi, insects and weeds, and can occur very quickly. Resistance to a pesticide develops after repeated exposure to the same pesticide or pesticide family. In any population, there are a few individuals with naturally occurring resistance to a particular chemical. When the pesticide is applied, those resistant individuals survive, while the susceptible portion of the population is killed. These resistant survivors multiply and gradually replace the susceptible ones. Eventually the resistant population dominates, and the pesticide loses efficacy. Modern pesticides often have very specific modes of action on pests, which makes the development of resistance more likely.

Resistance management is based on knowledge of a pest control product’s mode of action. The mode of action refers to the way the product affects the pest — for example one mode of action may target a protein in an insect’s gut which affects its ability to eat, while a different mode of action may target an insect’s nervous system. A pesticide family is a group of products and active ingredients with the same mode of action. When a pest becomes resistant to one product in a pesticide family (or group), it is often resistant to all members of that family because they all work in the same way.

The development of resistance can be prevented or delayed by rotating pesticides with different modes of action. This is because, while there are always individuals in a pest population that are resistant to one mode of action, there are far fewer that are resistant to two modes of action. A pest individual that survives the first application is therefore much less likely to survive a second application if it has a different mode of action.

Certain pests are more prone to developing resistance to pesticides than others. Pests with a short life cycle and many generations per growing season are more likely to become resistant. Pests are also more likely to become resistant to pesticides that have a single mode of action than those with multiple modes of action.

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

  • Product selection – Does the product actually have activity against the target pest? Was the product applied as directed on the label (foliar, trunk, soil drench, etc.)? How long does the product take to knockdown pests? How long does the knockdown effect last
  • Rate – Was the rate used that which was listed on the product label for the target pest?
  • Weather conditions – Was the product applied under the right conditions specified on the product label (prior to rain, evening, etc.)? Could weather conditions during or after application have affected spray coverage or pesticide efficacy? Does the label specify the time required for the product to become rainfast?
  • Timing – Was the product applied at the appropriate pest life stage
  • Water volume / quality – Was sufficient water volume used to ensure adequate coverage where the target pest is found (under leaf, on trunk, etc.)? Was the pH of the water higher or lower than what is listed on the product label? Was the sprayer properly calibrated?

Resistance Management Strategies

Resistance management strategies include rotating products from different groups and limiting the total number of applications from a single group within a growing season. Specific knowledge is required for growers to manage resistance effectively.

General Resistance Management Strategies

Follow an IPM program that makes use of a variety of different pest control strategies, including resistant varieties when available, monitoring, crop rotation and cultural, biological and chemical control options.

  • Do not use pesticides at levels below label rates.
  • Use adequate water volumes to deliver the pesticide to all tissues.

  • Spray only when necessary. Use established thresholds where available.

  • Spray at the best timing for the pest and the product you are using.

  • Make each spray application count. Be sure the sprayer is calibrated, the correct rate is applied and spray coverage is complete.

  • Read the product label. New products include resistance management recommendations on the label.

  • Know the active ingredient of a pesticide. Many chemicals with the same active ingredients are marketed under different brand names. For example, the insecticide permethrin is marketed under the brand names Perm-Up and Pounce.

  • Know the product group. Choose products from different groups when possible in your spray rotation. For example, both Assail and Calypso are in the same insecticide group (Group 4A). To use Assail after Calypso is equivalent to using Assail after Assail, since resistance to both chemicals develops in the same way.

For a list of groups and their modes of action, see Table 2–1. Fungicide/Bactericide Groups, Table 2–2. Insecticide/Miticide Groups or Table 3–3. Products Used on Apples. In addition to these general resistance management strategies for all products, more specific strategies have been developed for fungicides, insecticides and miticides.

Managing Resistance to Fungicides

  • Know the fungicide 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.

  • Know which disease is targeted by which fungicide group. For combination products, know which fungicide component is controlling which disease.

  • Apply fungicides before disease occurs. Wherever possible, use disease prediction models. Applications of fungicides after the disease is established are more likely to select for resistant populations of the pathogen. Fungicide groups such as Group 9 and U12 should not be used after prebloom. Do not use Group 3, 7, 9 and 11 if apple scab or powdery mildew infection is present in the orchard.

  • Where permitted, make use of Group M fungicides. These fungicides are known as multi-site inhibitors (Table 2–1. Fungicide/Bactericide Groups). They affect a wide range of metabolic processes in fungi and are less prone to the development of resistance. While there is no significant risk of resistance development, integrated resistance management should still be applied. For example, bacteria causing fire blight or blister spot can develop resistance to copper products.

  • Tank-mix products from different groups. Where permitted, one of the tank-mix partners should be a fungicide from Group M, with a multi-site mode of action. This is an accepted resistance management strategy for fungicides, although not recommended for insecticides.

Two components of a resistance management strategy for a fungicide group are limiting the number of consecutive applications before rotating to a different group and observing a maximum number of applications per season. The following strategies reduce the risk for resistance development and may be more stringent than label guidelines:

  • 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, although this is not necessarily required by the label. For example, products in Groups 1, 3, 7, 9, 11 and U12 should be used once for apple scab before rotating to another group.
  • For pathogens controlled by only a few registered fungicide groups, use no more than 2 consecutive applications of a resistance-prone fungicide and then alternate to a different fungicide group.

  • When a product contains active ingredients from more than one group, each application counts as a single use for each group. For example, one application of Pristine counts as a single use of boscalid (Group 7) and a single use of pyraclostrobin (Group 11).

  • 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. For example, Groups 3, 9 and 11 should not be used more than 2 times per season solo or as a tank-mix partner due to the high risk of apple scab resistance to these products.

Managing Resistance to Insecticides and Miticides

  • Know the insecticide groups. Rotate products from different groups. Avoid sequential applications of the same group or repeated use of any insecticide or group of insecticides.
  • In some cases, insecticide groups have been divided into subgroups. For example, Group 1 has been divided into subgroups 1A and 1B, and Group 4 has been divided into subgroups 4A, 4C and 4D. Compounds from these subgroups are structurally distinct but share the same mode of action. The risk of cross-resistance between these subgroups is considered low. However, where alternatives are available, rotate with other groups. If only insecticides from these groups 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.
  • 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.
  • 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.
  • Consider annual delayed dormant oil or summer oils to suppress mite, aphid, midge or scale populations and reduce the need for miticides when numbers exceed the treatment threshold(s).
  • 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, insecticides are only effective against San Jose scale when the susceptible crawler stage is present.
  • Use mixtures with caution. Tank-mixes and pre-formulated mixtures are pest management tools, not insecticide resistance management tools. Mixtures can provide a broader range of target pest control; however, their repeated use increases the probability that the target pest population(s) will develop multiple resistances. Alternating or rotating among products with one active ingredient, rather than mixing them, is the preferred strategy for insecticides and miticides in most situations.
  • Consider the use of mating disruption where available and practical.
  • Use regional or area-wide tactics rather than crop-by-pest management for cross-commodity pests, such as oriental fruit moth in stone and pome fruits.
  • Encourage biological control by choosing pesticides less harmful to beneficial insects and by landscaping to provide flowering plants and 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.
  • Monitor problematic pests to detect shifts in sensitivity to a group of pesticides.