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10 practical tips for preventing insecticide control failures


Although chemical products currently available for pest control are extremely effective when used correctly, pesticide applications sometimes fail to achieve the desired result. These control failures commonly lead to callbacks that cost additional time and money. Control failures can have many causes — the categories listed on the following pages describe the most common reasons why apparently good applications of pesticides result in unacceptable control.

Category 1.

Application was done correctly.
This category could be called “failure of product performance.” One can seemingly do everything correctly but still have living pests following application. What happened?

Possibility #1
Pest species is on the label but not controlled by the dose applied. Many labels of concentrate products have language stating that low doses should be used for “maintenance” applications, with relatively higher concentrations needed for “clean- outs.” In reality, cleanouts may involve a large population of a very susceptible species that is easily controlled by a low label concentration. In contrast, low populations of non-susceptible species often will not be killed, or even much inconvenienced, by application of the so-called maintenance concentration.

Even with direct insecticidal contact, the queen ant and 90 percent of the colony often remain unaffected.

Possibility #2
Pest species is not on the label and is not controlled by the application dose. In this case, the pest species is either unidentified or is misidentified and was not a species susceptible to the insecticide-dose. The general problem of misidentified pests was reviewed by Sims, Suiter and Ames (PCT, Dec. 2008). They found that identification problems are most common for small species belonging to the large insect orders such as beetles (Coleoptera), flies (Diptera) and wasps (Hymenoptera). Small occasional invaders such as Collembola also present problems. Even if a species is properly identified and listed on the label, control might be difficult. For example, millipedes are notoriously difficult to kill quickly because of their low metabolism and hard, thick cuticle.

A variation on the misidentification of the pest involves pesticide application to an incorrect life stage. Insect stages (eggs, larvae or nymphs, and adults) may differ greatly in their susceptibility to pesticide active ingredients. In addition, the different stages often occupy different locations of the residential or commercial habitat. This stage separation, in addition to differential susceptibility, makes thorough inspections and complete application coverage particularly important.

Possibility #3
Active ingredient has a slow mode of action that requires several days to produce results. Here, the treatment will be effective but slow, giving the impression that the application was ineffective. The active ingredients involved may not be neurotoxins but have a variety of other modes of action. Ironically, the actives in this category are often valuable for use in IPM (Integrated Pest Management) programs and are in the reduced risk category for humans and other mammals. Examples include insect growth regulators (IGRs) and metabolic inhibitors. If using these types of active ingredients, customers should be informed that several days may be required before results are obvious.

Possibility #4
The product doesn’t kill all social insects. Social insects, such as ants, are often treated by direct contact with an insecticide that only kills/repels a small percentage of the population (see Tschinkel, W. 2011. J. Insect Sci. 11:26). The queen, and typically more than 90 percent of foragers, may remain unaffected. The colony is not killed and serves as a source for future ant production and potential structure invasion.

Perceived failures also can occur with termite baiting systems. Elimination of a colony by baiting might be followed by rapid re-invasion of the evacuated foraging territory by a second unrelated colony or even another species. This could be seen as a treatment failure. Only colony identification using genetic techniques not readily available to the PMP could resolve the issue.

Possibility #5
The application doesn’t solve delusory parasitosis. If you make a treatment based on a “delusory parasitosis” claim, it could well be a misapplication. Arthropods may not be the cause of a customer’s itching, so a pesticide treatment is not the solution. Following treatment, the customer often continues to be “bitten” and to itch. Callbacks happen because the application did not work, and could indeed never work, on the “ghost” arthropod (see Hinkel, NC, 2000, Amer. Entomologist, 46(1), 17-25).

Category 2.

Application was done incorrectly.
Application errors may not be the easiest to avoid but they can be minimized with careful attention to the details of equipment and product-use instructions. For example, an applicator should be properly trained for crack and crevice applications, as well as for the delicate application of dusts.

Possibility #1
Problems with spray equipment or making incorrect dilutions. When was the last time you calibrated your spray equipment? Problems often arise from improper spray equipment calibration and worn nozzles. This can be important if it affects delivering the proper volume of spray needed, for example, to penetrate mulch or other debris during perimeter applications. Inaccurate application rates, spray patterns and droplet size all can reduce effectiveness of the pesticide. Failure to make the correct dilution is sometimes the problem, especially if too little of the concentrate and, thus, too little active ingredient is used.

Possibility #2
Wrong pesticide, inappropriate use sites or poor application timing. Use of the wrong pesticide product against a non-label pest species or on a non-label use site can result in control failure. Humidity, temperature and sunlight can reduce the effectiveness of outdoor applications. Outdoor applications just before, or during, stormy (rain) weather is clearly a waste of time and materials. Low temperatures can reduce the activity of some active ingredients and can make pests less active and less susceptible to pesticides. Applications to surfaces in direct sunlight during good weather can dramatically shorten the residual duration of the treatment. Residual deposits break down more rapidly under the heat and UV-exposure conditions of direct sunlight, especially during the summer months. Some surfaces, such as concrete sidewalks and driveways, also result in more rapid loss of residual effectiveness.

Possibility #3
Wrong formulation for the surface being treated. Porous surfaces such as untreated wood may affect the efficacy of spray treatments by absorbing the insecticide and making it unavailable to the target pest. Formulations such as wettable powders and microencapsulates are designed to hold the insecticide on the surface where it can be easily contacted by the pest. Most spray formulations are emulsifiable concentrates that spread and spray easily, but are most effective on nonporous surfaces.

Possibility #4
Spraying where you shouldn’t. Many PMPs use bait formulations to control ants and cockroaches. Research has shown that contamination of the bait, a bait station or even the area immediately around bait can significantly reduce effectiveness (Appel, AG., 2004. J. Econ. Entomol. 97: 2035-2042). Never get spray formulations, even if they are marketed as non-repellent, on or near bait deposits. Also, never contaminate bait deposits with flushing agents used for inspection because flushing is just another type of repellency.

Possibility #5
Sequential use of different products with one sprayer and tank mixes. Was more than one pesticide product sequentially used in the spray apparatus? If so, was the spray tank cleaned out after consecutive applications? What was the pH of the dilution water? High pH water can sometimes reduce product efficacy, especially when the dilution remains in the spray tank overnight or longer.

Were potentially incompatible products mixed in same container (bad tank mix)? (See Cloyd, RA, 2009. HortTechnology, vol. 19 (3): 638-646.) Sometimes it is convenient, and labor saving, to mix two or more active ingredients in the same spray equipment for application. However, it is not always possible to get good information from the manufacturer or the Internet, on formulation compatibility. Often, these mixes will not present a problem and can be applied simultaneously with good results, but there is the potential for physical incompatibility of the formulations that can lead to clogged nozzles on the spray equipment. Also, the possibility exists that two active ingredients are antagonistic (the opposite of synergistic), so the mixture of the two produces less efficacy than the expected sum of their combination.

Category 3.

Customer Cooperation.
The customer is often an integral part of the pest control process. Is he/she responsible for removing clutter and garbage in the kitchen? Will the customer caulk entry points for pests on the outside walls of the house? Will the carpet be vacuumed and shampooed? Will open containers of cereals and other grains be placed into jars with tight-fitting lids? Has the customer made landscaping “improvements” to areas that were treated with a termiticide, thereby compromising the termite protection barrier? If the customer does not do his or her part of the “team effort,” guess who gets the callback?

Category 4.

Insect Resistance.
If you have eliminated all the obvious reasons for control failure, then resistance may be involved. Pesticide resistance is a change in the sensitivity of a pest population to a pesticide, resulting in the failure of the pesticide application to control the pest. Resistance can develop when the same or similar pesticides are used over and over again. Many think that pests change or mutate to become resistant, but the truth is that all cases of resistance are driven by the process of selection. If you suspect resistance, the best approach is to consult with the manufacturer’s technical director or a local or state extension entomologist for advice on confirming the degree of resistance and recommendations on alternative control strategies.


Steven R. Sims, Ph.D., is a senior research scientist in entomology for BASF Corporation, St. Louis, Mo. Arthur G. Appel is professor and chair of the Department of Entomology and Plant Pathology at Auburn University, Auburn, Ala.