Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Managing Pesticide Resistance

Pesticide resistance can develop over time when pesticides with the same mode of action (same way of affecting pests) are repeatedly applied in the same area. Resistance occurs when a pesticide exhibits reduced effectiveness or no longer controls the pest population at the formerly effective rate. If the pesticide or others with the same mode of action continue to be applied, eventually not even higher rates or more frequent applications provide control. At this point the pest population becomes dominated by individuals that are not susceptible to pesticides of that particular chemical class or mode of action.

Pesticide mode-of-action codes are listed on the label of many trade-name products. The codes are assigned for

  • bactericides, fungicides, and oomycides by the Fungicide Resistance Action Committee (FRAC)
  • herbicides and plant growth regulators by the Herbicide Resistance Action Committee (HRAC) and Weed Science Society of America
  • insecticides, miticides (acaricides), nematicides, and molluscicides by the Insecticide Resistance Action Committee (IRAC)

Manage pesticide resistance using strategies that include avoid, delay, learn, and reverse:

  1.   Avoid the development of pesticide resistance by using integrated pest management (IPM), which employs a combination of control methods and commonly reduces reliance on pesticides.
  2. Delay resistance by using pesticides only when needed, as indicated by monitoring and using action thresholds when available. Make an application only when pests are present at the life stage(s) susceptible to that particular product. Do not apply pesticides on a regular schedule regardless of monitoring results or thresholds.
  3. Learn the mode of action, the physiological way target organisms are killed, for the products you use. Do not make more than two consecutive applications of pesticides with the same mode of action even when allowed by the label. Alternate (rotate) applications between pesticides with different modes of action, which are generally those from different chemical classes (e.g., Bacillus thuringiensis, Beauveria bassiana, carbamates, chitin synthesis inhibitors, neonicotinoids, organophosphates, pyrethroids, etc.).
  4. Reverse resistance in some instances by not applying for an extended time any pesticides with a mode of action that is known or suspected of becoming less effective because of resistance development. Suspending use of certain products for several generations of a pest's development may cause pesticide-resistant individuals to become less prevalent and pesticide-susceptible individuals to become relatively more abundant in the local pest populations. For more information see What Is Integrated Pest Management (IPM)?


Ask your University of California Cooperative Extension county advisor how to establish an IPM program and consult University of California publications such as Container Nursery Production and Business Management Manual; Integrated Pest Management for Floriculture and Nurseries; Retail Garden Center Manual; and Water, Root Media, and Nutrient Management for Greenhouse Crops. Minimize pesticide use, and employ only nonchemical practices when these are sufficiently effective. Avoid tank mixes (applications of multiple insecticides simultaneously); this advice is for insects and mites and differs from management of plant pathogens and weeds where mixes commonly are recommended. Avoid and minimize use of persistent insecticides and miticides; residues persist and a single application can expose multiple generations of pests, which favors resistance development. Rotate applications among different modes of action.

Minimize pesticide use

Minimizing pesticide use is fundamental to resistance management. Periodic, informed monitoring (scouting) and keeping good records of pest presence, relative abundance, and damage will help you determine the need and best timing for pesticide application, improve management effectiveness, and help to reduce the total number of applications. Use nonchemical strategies, such as biological control, crop rotation, host-free periods, pest exclusion (e.g., screening), and weed control to reduce the need to frequently apply pesticides.

Avoid tank mixes

Avoid combinations (mixes) of two insecticides or miticides in a single application. Especially avoid mixing two with the same mode of action, such as the organophosphates acephate and malathion; this increases selection for resistant pests. Mixing pesticides with different modes of action sometimes provides increased control, but this result is temporary and has the longer-term effect of increasing resistance development, and in multiple species of pest arthropods.

Applying two different types of insecticides may be warranted in particular situations, but consider avoiding unnecessary tank mixes. For example, insect growth regulators (IGRs) that control only immature stages might be combined with a product that affects adults. It might be possible to deliver these in different ways, such as applying the IGR as a foliar spray and the insecticide for adults as an aerosol; this provides rapid kill of adults but also little residual control. Using the adulticide in a different and minimally persistent way reduces its effect on immature stages targeted by the IGR. This approach reduces the selective pressure in comparison with the two pesticides applied together the same way.

Avoid persistent insecticides and miticides

Select pesticides with little or no residual activity when feasible. The longer a pesticide persists the more it favors development of resistant pest populations.

When application of a persistent pesticide is warranted, minimize the number of its applications. If monitoring and threshold levels suggest it is necessary, before reapplication consider other pesticides with different modes of action and little or no persistence. Also carefully consider the order and timing of applications. For example, if applying a neonicotinoid (e.g., acetamiprid, imidacloprid) to control whiteflies, avoid such application during early crop production because this prolongs exposure of whitefly populations to these persistent insecticides. Early in the crop cycle, rely on nonchemical management methods such as exclusion and sanitation, natural enemies, and selective and non-persistent pesticides such as botanicals, insect growth regulators, insecticidal soaps, microbials, and oils. Delay application of the neonicotinoid and other nonselective, persistent products until later during the crop. Note that the effectiveness of systemics applied to growing media or soil can be delayed while the pesticide is absorbed and moved (translocated) by plants, so applying too late can reduce effective control. For example imidacloprid (soil treatment) for whitely control in poinsettia should be applied no later than 3 weeks before color initiation.

Use long-term rotations

Resistance management for insects and mites, fungal pathogens, and weeds includes rotating applications among pesticide classes or modes of action. However, rotation strategies differ according to the targeted pest types. With fungicides, modes of action commonly are rotated every application, and more than one mode of action can be mixed together. Generally to help maintain pesticide efficacy with insecticides and miticides, rotate through different modes of action in successive generations and avoid more than two consecutive applications of the same mode of action. Some pesticide labels are even more restrictive.

Sometimes only one or very few pesticides are effective against a particular pest, and other available products are only marginally effective. In this case one strategy is to use less effective pesticides when marketed portions of the crop are less susceptible to damage or not present while pest numbers are relatively low. Reserve the more effective products for times when control must be most effective.

Text Updated: 01/22