When a Pesticide Doesn't Work1
Frederick M. Fishel2
Pesticides are valuable additions to the box of tools available to pest managers. However, they should be considered one part of the total integrated pest management (IPM) plan rather than the only solution. Pesticide failure can occur for a variety of reasons:
Improper pest identification (incorrect pesticide selection)
Incorrect pesticide dosage
Improper application timing
Pesticide does not reach target pest
Unfavorable environmental conditions
State of poor pesticide condition
Improper Pest Identification—Incorrect Pesticide Selection
Accurate pest identification should be the first step. Being able to accurately identify pests requires patience and practice. Subtle differences among pest species may often lead to a false identification. For example, control methods vary for different species of grassy weeds. Although they may have common features, such as parallel veins and round stems, crabgrass and bermudagrass control tactics are not always the same. Crabgrass is an annual, while bermudagrass is a tougher-to-control perennial with vegetative rhizomes and stolons. Although some postemergence herbicides may control both species, preemergence herbicides will only reliably control crabgrass.
Likewise, different species of mites (Figure 1) can be difficult to distinguish from one another because of their extremely small bodies. However, the pesticides selected to control different mite species can vary. An example would be pesticide selection for the control of spider mites and rust mites in citrus.
Regardless of the pest class, making an accurate identification is critical. UF/IFAS offers a variety of services to help determine the cause of plant problems and to provide pest identification through the UF/IFAS Plant Diagnostic Center (http://plantpath.ifas.ufl.edu/extension/plant-diagnostic-center/).
Incorrect Pesticide Dosage
Several reasons may account for this problem. Application equipment should be properly calibrated to deliver a known volume. Underdosing can be expensive because retreatment may be necessary. On the other hand, overdosing is a violation of the product's label wording. Keep in mind that the rate listed on a product label as controlling one specific pest will not necessarily be the amount needed to control other species (Figure 2).
Improper Application Timing
Apply the pesticide to the life stage of the pest that is most susceptible to the effects of the pesticide. Generally, herbicides are most effective on small, early stages of weed growth. Many insecticides are effective on insect larvae or nymphs but not on adults. Some pesticide labels will list their rates based upon growth stage or size (Figure 3).
Another potential problem involving timing is an application that takes place after the infiltration or departure of a pest. An application of a protectant fungicide will provide little or no control of a plant pathogen that has already invaded its host plant. Many labels will instruct that applications should begin prior to the onset of infection.
Pesticide Does Not Reach Target Pest
Sometimes pesticide applications are not effective because the pest is in a location that is difficult to reach. Many insects are located on the underside of leaves, under bark or soil, or within stems and fruits (Figure 4). When insects are on leaf undersides, applicator sprays must be directed at those areas to have an effect.
After application, some pesticides must be watered, by either rainfall or irrigation, into the soil zone where underground insects are feeding. Read the label for maximum product efficacy (Figure 5).
Unfavorable Environmental Conditions
Aside from the examples above, most pesticides should not be applied just before or during rainfall. Rain washes pesticides off foliage before they have time to take effect. High temperatures, lack of moisture, and both acid and alkaline soil pH produce thicker cuticle formation on the surface of weeds. Thick cuticles prevent herbicide uptake; thus, weed control is not maximized. Windy conditions can cause pesticides to drift from their intended sites and can also result in damage to desirable plants. (Figure 6). Injuries of this sort are subject to legal penalties
State of Poor Pesticide Condition
Under some conditions, some pesticides can change into a form that is not effective. The age of the pesticide, moisture, and temperature extremes are the primary factors responsible for chemical reactions that alter the formulation's active ingredient, rendering them ineffective. Moisture is generally a problem when dry products are stored in bags or containers that have not been adequately sealed. Statements on the product's label often instruct the user not to store the product in extreme heat (Figure 7). Heat may also volatilize some pesticides if their containers are not adequately sealed. Such statements are found in the "Storage and Disposal" section of the product labels.
Using mix water that is alkaline (pH > 7) is known to degrade some pesticides relatively quickly. Historically, this has been the case with carbamate and organophosphate insecticides; however, it is not strictly limited to those classes. Likewise, some pesticides lose their effectiveness when mixed with water that contains suspended or dissolved solids. Product labels will carry statements cautioning the applicator of such problems.
Pesticide resistance does not occur as often as some applicators may think. Although there are documented cases of more than 500 arthropod and 200 fungi and weed species, this is not the most common reason pesticides fail. Resistance often develops in pest populations that have been repeatedly treated with a single pesticide. Development of resistance in pest populations may sometimes be averted or delayed by avoiding the use of persistent pesticides, reducing the number of treatments and alternating pesticide modes of action.
There are many factors responsible for a pesticide's failure to controlling a pest. Although there are additional reasons, most often some type of human error is involved. Some failures can be avoided by simply following the product's label directions. Use common sense; read and follow all labels.
Additional Information and References
Ferrell, J.A., B.A. Sellers, and R. Leon. 2015. Calibration of Herbicide Applicators. Gainesville: University of Florida Institute of Food and Agricultural Sciences. http://edis.ifas.ufl.edu/wg013
Fishel, F.M. 2006. Storage Limitation Statements: Temperature - Fungicides. Gainesville: University of Florida Institute of Food and Agricultural Sciences. http://edis.ifas.ufl.edu/pi159
Fishel, F.M. 2006. Storage Limitation Statements: Temperature - Herbicides. Gainesville: University of Florida Institute of Food and Agricultural Sciences. http://edis.ifas.ufl.edu/pi160
Fishel, F.M. 2013. Water Quality and the Effectiveness of Pesticides. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/pi245
Fishel, F.M. and J.A. Ferrell. 2007. Water pH and the Effectiveness of Pesticides. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/pi193
Peres, N., P. F. Harmon, and C. L. Harmon. 2017. Sample Submission Guide for Plant Diagnostic Clinics of the Florida Plant Diagnostic Network. Florida Extension Plant Disease Clinic Network. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/sr007
Rogers, M.E., L.W. Timmer, and C.W. McCoy. 2007. 2008 Citrus Pest Management Guide: Pesticide Resistance and Resistance Management. Gainesville: University of Florida Institute of Food and Agricultural Sciences. http://edis.ifas.ufl.edu/cg026
This document is PI-163, one of a series of the Agronomy Department, UF/IFAS Extension. Original publication date June 2008. Reviewed February 2017. Visit the EDIS website at http://edis.ifas.ufl.edu.
Frederick M. Fishel, professor, Agronomy Department, and director, Pesticide Information Office; UF/IFAS Extension, Gainesville, FL 32611.
Use pesticides safely. Read and follow directions on the manufacturer's label.
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 UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.
U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.