• Topics: Entomology and Nematology | Price, James F | McCord, Elzie Jr. | Nagle, Curtis A | Buss, Eileen A | North Florida REC-Quincy | Insecticide Resistance Management | Landscape Plant Pest Insects and Mites

Managing Insecticide and Miticide Resistance in Florida Landscapes¹

Landscape managers in Florida are becoming more aware of pesticide resistance development in key turfgrass and ornamental plant pests. Pesticide resistance is no longer just a greenhouse or agricultural problem. Southern chinch bugs (Blissus insularis) became resistant to chlordane used in St. Augustinegrass in 1953, and have since become resistant to other chlorinated hydrocarbon, and organophosphate, carbamate, and pyrethroid insecticides (Cherry and Nagata 2005). The leafminer Liriomyza trifolii caused significant damage to annual bedding plants in the 1970s and early 1980s, which resulted in considerable insecticide use on infested plants, and subsequent resistance development to several chemical classes (Leibee 1981). With the limited development of new cost-effective pesticide chemistries, landscape managers need to be good stewards of existing products to try to mitigate or delay further resistance development.

A perceived product failure or poor pesticide performance does not always indicate pest resistance. Poor control may be the result of several factors, including pesticide degradation in storage, hydrolysis in acid or alkaline preparations, applications against an incorrect life stage, poor coverage or equipment calibration, or other inadequate application procedures. Consider which formulation is most appropriate for each pest and/or plant situation. For example, a granular insecticide used during a drought and watering restrictions may not work until enough moisture is present to release the toxicant from the carrier and move it into the soil. On the other hand, if a product is sprayed just before a rain storm, it could easily be washed out of the target zone before eliminating the pests.

Definitions

Resistance involves inherited (genetic) physiological and/or behavioral adaptations that confer a selective advantage in the presence of a pesticide and that lead to control failures. Cross-resistance occurs when resistance to one insecticide confers resistance to another insecticide, even where the insect has not been exposed to the latter product. Tolerance is when physiological and/or behavioral adaptations lead to increased survivorship relative to some toxicity baseline (not genetic). Mode of action is the specific physiological activity of a toxin that results in the death of a pest.

Resistance Development

Resistance development is affected by the frequency of application rate or dose of pesticide, and certain pest characteristics. Some arthropods are more likely to develop resistance to pesticides than others. Arthropods like mites, aphids, whiteflies, and thrips have similar traits that contribute to resistance development, such as having many generations per year, exposure of multiple generations to a pesticide, having a lot of offspring, limited dispersal, and exposure to sublethal (less than optimal) pesticide doses.

The pests within a population often vary in their level of susceptibility to a pesticide. But before exposure to a new pesticide, resistant individuals are rare. After an application, the more susceptible pests die and the less susceptible ones survive, mate with other survivors, and reproduce. Most of their offspring then inherit the parental trait that allowed them to survive the pesticide application. Further applications kill the most susceptible individuals, so the survivors mate and produce more similar offspring. Continued applications within one chemical class or mode of action further select the population until the resistant genotype is the most abundant or dominant. Pure resistance in a population is probably not present before product failures begin.

Functionally, pest populations may become insensitive to formerly effective pesticides through one or more means. For example, resistant pests may deactivate (break down), sequester (safely store within their bodies), avoid or excrete the toxin from their bodies more effectively, have an altered target site in the nervous system that will not respond to the toxin, or reduce the permeability of their exoskeletons (“shells”) to the toxin.

Resistance Management

The best solution to pesticide resistance development is to diversify your plant protection techniques and not rely solely on pesticides. Integrated pest management (combining cultural, mechanical, physical, biological, and chemical controls) is a good long-term philosophy and set of practices (Pedigo 2002, Scherer et al. 2006). For example, practice good sanitation. Completely remove all plant debris from a flower bed before installing new plants. Exclude pests by inspecting new plants and only buy pest-free plants. Use plant species and varieties that are resistant to key pests. Rotate plant species in annual flower beds. Conserve natural enemies by spot treating and leaving untreated refuges for them to live in. Most plants can tolerate minor pest damage for a while before pesticidal intervention may be needed. Periodically scout landscapes and use pesticides only when pest densities approach economic or aesthetic injury levels.

To maximize product efficacy, follow the label instructions and consider these tips. Use fresh, fully potent pesticides that are prepared and applied according to label directions. Aqueous pesticidal solutions should be adjusted to near neutral pH (pH 7.0) or as specified by the label. Sprayer/spreader calibration, nozzle condition and pressure, and treatment placement must be correct. Applications also should be timed and directed to contact the most susceptible life stage of the pest.

We try to delay or reverse resistance by avoiding use of the pesticide, mode of action, or chemical class for some time. The hope is that the resistant pest populations may lose their resistance traits and become susceptible in the absence of repeated exposure. However, if those pesticides are used again, resistance may return. The market life of key pesticides may be extended using several strategies, which include mixtures, rotations, and mosaics (Hoy 1999).

Mixtures

Applying mixtures of products exposes pests simultaneously to more than one toxicant. This strategy depends on several assumptions, including that resistance to each product is “monogenic” (only has one gene), that no cross-resistance occurs between both products, that resistant individuals are rare and the resistance gene is recessive, the products have similar residuals, and that some of the population remains untreated (Tabashnik 1990). Synergists, such as piperonyl butoxide (PBO), are often added to pyrethroids or pyrethrins to boost their efficacy. Examples of registered mixtures include Allectus (bifenthrin plus imidacloprid) for use in turfgrass, and Discus (cyfluthrin plus imidacloprid) in ornamentals. However, pesticide applicators could make their own mixtures if the pesticides are compatible and legal/label restrictions don't prohibit mixtures.

Rotations

The object is to alternate pesticides with different modes of action or chemical classes over time so each pest generation is exposed to only one product, but the population experiences more than one product over time. This strategy assumes that “negative cross-resistance” occurs, such that the number of individuals that are resistant to one product (e.g., a pyrethroid) will decline when exposed to a different product (e.g., an organophosphate). This may reduce the selection pressure as compared to repeatedly using the same product, mode of action, or chemical class. If multiple applications are required, use a different mode of action each time before returning to a previously-used one. An example of a rotation could be Talstar (pyrethroid), Sevin (carbamate), Merit (neonicotinoid), and Malathion (organophosphate). Rotation does not mean using different products in sequence (e.g., pyrethroids: Talstar/Onyx, Tempo, DeltaGard, Astro, Scimitar).

A list of modes of action can be found at the Insecticide Resistance Action Committee Website: http://www.irac-online.org/teams/mode-of-action [February 2012]. Tables 1-3 present a mode of action summary for insecticides and miticides intended for Florida landscape maintenance. Use pesticides with different IRAC numbers in a rotation - these numbers are listed on many new pesticide labels so you might not have to remember the different chemical class names or other specifics. The use of certain unique products with general modes of action (such as soaps and oils) is not expected to result in pest resistance, so no IRAC codes have been assigned and rotation of these products for resistance management is unnecessary.

Mosaics

Different locations within an area (e.g., lawns within a city) are treated with different pesticides to create a mosiac pattern in an area. This strategy may already occur in the landscape, given the variety of pest management companies and homeowners who treat plant pests differently in time and space. However, when only one mode of action or chemical class is used against a pest (e.g., pyrethroids against southern chinch bug) in both the homeowner and professional markets, and multiple pest control companies use the same products, then no real mosaic potential exists. A cooperative, area-wide pest management program would be necessary to truly implement this strategy.

Conclusions

Cases of pest resistance to popular pesticides increase control costs, the frequency of pesticide applications, the desire to use “fringe” or illegal methods of control, exposure of people and animals to toxins, and likely the amount of regulation needed to keep the environment safe. Implementing resistance management (e.g., integrated pest management) practices into the landscape maintenance industry will help keep current products on the market longer and allow pest managers to keep landscapes looking great.

References Cited

Cherry, R. and R. Nagata. 2005. Development of resistance in southern chinch bugs (Hemiptera: Lygaeidae) to the insecticide bifenthrin. Florida Entomologist 88(2): 219-221.

Hoy, M. A. 1999. Myths, models and mitigation of resistance to pesticides. In Insecticide Resistance: From Mechanisms to Management (ed. I. Denholm, J.A. Pickett, and A.L. Devonshire), pp. 111-119. New York: CABI Publishing.

Leibee, G. L. 1981. Insecticidal control of Liriomyza spp. in vegetables. In Proceedings of the IFAS-Industry Conference on Biology and Control of Liriomyza leafminers (ed. D. J. Schuster), pp. 216-220. Gainesville, FL: University of Florida, IFAS.

Pedigo, L. P. 2002. Entomology and Pest Management, 4th edition. Prentice Hall, Upper Saddle River, NJ. 742 pp.

Scherer, C. W., P. G. Koehler, D. E. Short, and E. A. Buss. 2006. Landscape Integrated Pest Management. ENY-298 (http://edis.ifas.ufl.edu/ IN109) EDIS, University of Florida, Gainesville.

Tabashnik, B. E. 1990. Modeling and evaluation of resistance management tactics. In Pesticide resistance in arthropods (ed. R. T. Roush and B. E. Tabashnik), pp. 153-182. New York: Chapman and Hall.

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. Nick T. Place, Dean.

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