• Topics: Entomology and Nematology  |  Price, James F  | McCord, Elzie Jr.  |  Nagle, Curtis A  |  North Florida REC-Quincy  | Strawberry Pest Management 

Causes and Management of Insect and Mite Resistance in Strawberry Production¹

Resistance of arthropods to crop management chemicals has been problematic since the early era of synthetic organic pesticides. During the late 1990s, the twospotted spider mite (Tetranychus urticae Koch) became resistant to abamectin, the miticide used in strawberry culture. Since then, several new miticides including acequinocyl, bifenazate, etoxazole, hexythiazox, and spiromesifen have been integrated into strawberry production, overuse of the abamectin has ceased, and it once again is effective in rotation with the new materials. This latter development could prove temporary, especially if growers again use abamectin regularly.

Poor performance of pesticides does not always indicate pest resistance. Such factors as pesticide degradation in storage, hydrolysis in acid or alkaline preparations, applications to an incorrect life stage, or other inadequate application procedures may contribute to poor control.

A Definition of Resistance

Pest populations can be susceptible or resistant to a pesticide. Resistance occurs when a formerly susceptible pest population becomes significantly less susceptible to a pesticide and degradation of the pesticide or improper application is not a factor. Pesticide resistance is a population-based phenomenon in which the genetic composition shifts and the population becomes dominated by individuals possessing genes that confer resistance.

Establishment of Resistance

Resistant populations are protected from formerly effective pesticides through one or more means. For example, resistant pests may: (1) deactivate (break down), (2) sequester (safely store within their bodies), (3) avoid, or (4) excrete the toxin from their bodies more effectively, (5) have an altered target site that will not accumulate the toxin, or (6) reduce the permeability by the toxin through their exoskeletons (“shells”).

Individuals within a susceptible pest population often vary in their level of susceptibility; however, the non-susceptible type occurs only very rarely. When a pesticide is applied repeatedly, the susceptible pests die and the resistant ones survive, mate with other survivors and reproduce. Some of their offspring inherit the parents' characteristic for survival. Upon additional applications, the susceptible offspring within the remaining population die and the resistant ones survive, mate with other survivors and produce more offspring. Further applications additionally select for the resistant individuals until that form (genotype) is common. The population then is regarded as resistant and the effectiveness of the pesticide is lost.

Resistance Management

Resistance can develop rapidly with pests that have many generations per year, when multiple generations are exposed to a pesticide, and when new individuals do not move into a treated area to dilute the frequency of the resistant genes. Some of these factors occurred on strawberry farms in the 1990s, contributing to development of abamectin resistance in spider mites.

The main objectives of on-farm resistance management programs should be to minimize the number of exposures of pests to pesticides with a similar mode of action and to use non-chemical approaches to arthropod management. (Mode of action is the specific activity of the toxin that results in the death of the pest. For instance, one mode of action is to inhibit mitochondrial complex I electron transport. This causes a failure of the pest to produce energy in affected cells and to die.)

Repeated exposures to a pesticide are the primary drivers of resistance but much can be done to manage pests by means other than chemicals. Care can be taken to rotate strawberries with other crops, use pest resistant varieties, plant pest-free transplants, conserve and release natural enemies, etc. Pest-specific tactics are available for particular situations such as removal of all ripe strawberries from the field to eliminate reproductive sites for sap beetles.

Strawberry fields should be scouted weekly and pesticide applications made only when pest densities approach economic injury levels. When pesticide use is required, products should be rotated among the different modes of action indicated on many modern product labels. A list of modes of action can be found by selecting “MoA Classification Scheme” at the Insecticide Resistance Action Committee Website: http://www.irac-online.org/Crop_Protection/MoA. asp .

Tables 1-3 present insecticide and miticide modes of action summaries for Florida strawberry production. Sound rotation plans often recommend pesticides of one mode of action for one pest generation and a pesticide of a different mode of action for another generation. If multiple pesticide applications are required, rotations should continue through all practical modes of action before returning to a previously used one. The use of certain unique products with known general modes of action (such as soaps and oils) is unlikely to result in pest resistance and no codes are assigned. These can be used without regard to a rotation plan for resistance management.

When pesticides are used, it is important to assure that fresh, fully potent pesticides are prepared and applied in accordance with label directions. Aqueous pesticidal preparations should be adjusted to near neutral pH (pH 7.0) or as specified by the label. Sprayer calibration, nozzle condition and pressure, and spray placement must be correct. Applications also should be timed and directed to contact the most susceptible life stage of the pest.

Conclusion

Episodes of pest resistance to popular pesticides can cause yield losses, reduction of fruit quality, added control costs, environmental degradation, and emotional stress among farmers. These consequences can be alleviated if resistance management is practiced throughout the strawberry industry. If growers minimize pesticide application by depending more on biological and cultural pest control measures, and reduce pest exposure to pesticides with identical modes of action, then resistance can become a rare phenomenon.

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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