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Resistance involves the process of natural selection at work. It can occur with any organism in the environment. In the case of mosquitoes, insecticide resistance refers to differences in susceptibility that develop among populations of the same species exposed to a particular insecticide. Individual mosquitoes that are less susceptible (more resistant) to a specific insecticide have a better chance to survive exposure and reproduce. They pass their genetic make-up on to their offspring who inherit that resistance gene. Over time this can cause shifts in the mosquito population's susceptibility to a particular pesticide. Regular resistance testing is vital for mosquito control operations because resistant populations of mosquitoes reduce the effectiveness of control procedures.
 Mosquitoes can develop resistance through several different mechanisms. One is behavioral resistance. Behavioral resistance results from actions that have evolved in response to selective pressures presented by a toxicant. This type of resistance occurs in the form of actions that increase a population's ability to avoid the lethal effects of a pesticide. This response can be stimulus-dependent or stimulus-independent. For example, a stimulus-dependent response would involve the mosquitoes sensing the insecticide and then avoiding it. Such resistance occurs with insecticides that are in baits, or insecticide treated surfaces. A stimulus-independent response occurs when mosquitoes start occupying an area, or microhabitat, that is not treated with pesticide.
Physiological resistance is another way mosquitoes can become immune to insecticides. For example, reduced penetration through the cuticle has been noted in several species of mosquito. The cuticle is a waxy layer on the outside of the mosquito used for protection. Normally, the cuticle can be penetrated by contact with an insecticide. However, there have been some cases in which the insecticide penetration through the cuticle is slowed. This gives the detoxification mechanisms (immune system) in the mosquito more time to deal with uptake of the toxicant. Typically, as the insecticide penetrates the mosquito its defense systems become overloaded and the mosquito dies. The more time the mosquito has to deal with the insecticide, the greater its chances for survival.
Another form of resistance is metabolic. Metabolic resistance occurs when detoxification enzymes are used to break down the insecticide so it is no longer lethal. The enzymes break the insecticide down into compounds that can be metabolized by the mosquito, such as amino acids and sugars. In the absence of insecticide, however, the population reverts back to the original condition.
Cross-resistance is another resistance mechanism mosquitoes can develop. This form of resistance occurs because many insecticides attack similar target sites in mosquitoes. Cross- resistance develops when a single resistant form enables resistance against multiple insecticides. For example, studies have shown that DDT and synthetic pyrethroids work to attack the same target sites on some insects. Mosquitoes and houseflies have been recognized to develop resistance through one mechanism (genetic) for multiple insecticides.
Insecticide resistance is determined through genetic tests or through bioassay tests. Genetic testing involves studying the inheritance of resistance in a population. This process is time-consuming and rarely used by mosquito control districts. Most mosquito control operations rely on bioassay tests to determine resistance in mosquito populations.
Bioassay tests can be based on dose response or time response. Both are important in testing the efficacy of an insecticide. Dose response tests determine the amount of chemical necessary to achieve a certain percentage of mosquito mortality. The test involves topical application of insecticide using exposure to aerosol droplets or a treated surface, such as filter paper. The contact duration can vary for each test. For example, adults could be in contact with filter paper for one hour while larvae would be in contact with the insecticide continuously. After a pre-determined amount of time has passed, generally 24 hours after exposure, the number of dead mosquitoes is recorded and the lethal dose is determined. Resistance is noted if it takes progressively larger quantities of insecticide to kill the majority of the mosquitoes.
The Collier Mosquito Control District uses time response tests to determine resistance in mosquito populations. This is a new test method that has been developed by the U.S. Centers for Disease Control and Prevention. This test procedure uses glass bottles that are treated on the inside with a known amount of insecticide. Groups of mosquitoes are placed into the treated bottles, exposing them continuously to the insecticide. At predetermined times after initial exposure, the number of dead and alive mosquitoes is recorded. Once all the mosquitoes are dead, the time is logged and the percentage mortality is recorded for each time interval. Resistance is noted if it takes a progressively longer period of time for the mosquitoes to die.
It is important to note that both resistance tests must be replicated in order to compare the results. Resistance cannot be determined by one set of tests. Each test should be compared with previous tests in order to determine whether a mosquito population is developing resistance over time. Mosquito control districts should try to perform resistance tests throughout the mosquito season to compare the results from the beginning of the season through the end of the season. This will help districts analyze their operational procedures.
Besides testing, it is important for mosquito control districts to implement a resistance management plan to ensure optimal operation. There are many activities in which mosquito control districts engage that can contribute to resistant populations of mosquitoes. If districts can isolate these actions and attempt to minimize them, it will make a tremendous difference in their ability to control mosquito populations.
One way to achieve this is through integrated pest management (IPM). This practice involves utilizing several control methods to manage mosquito populations. By using various control methods, it does not give mosquito populations time to adapt to an individual method or insecticide.
There are other activities districts can do on a regular basis to minimize the chance of resistance. These include not using excessively low dosages of insecticide, applying insecticide less frequently, using chemicals that do not persist in the environment for long periods of time, alternating insecticide types, and avoiding insecticide formulations that release slowly into the environment.
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