Evaluating Mosquitoes for Insecticide Resistance
Centers for Disease Control and Prevention
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 (Glossary Explained)This term is defined in the glossary.
Beaker/Bottle Calibration & Interpretation

To determine what concentration of insecticide to use when running a bioassay, it will be necessary to calibrate the beaker or bottle. This must be done for each insecticide and species of mosquito.

The principle of calibration is the same whether you are preparing to test the mosquito larvae or adult mosquitoes. However, the procedure itself differs in one respect: when you test larvae you will calibrate beakers, while when you test adults you will calibrate bottles.

Data are likely available on the vector species and insecticide to be tested, but it is desirable for the user to develop their own data. Ideally, susceptible target are used to establish a baseline. If these are not available, use mosquitoes from the population to be controlled and use these data as a benchmark for interpreting future changes in the population. A range of insecticide concentrations is used.

  1. Prepare beakers/bottles as explained in the Bottle bioassay or Larval bioassay. Make several sets of beakers or bottles with a range of different concentrations.
  2. Run assays on susceptible mosquitoes/larvae.
  3. When the results are plotted as time-mortality data on a graph, you should see that with increasing concentration the time-mortality line becomes straighter, steeper and moves toward the Y axis. If you are in the correct range, the line will reach a point where increasing the concentration does not change the line's appearance. This is called the "saturation" point for the insecticide against the insecticide target site in the mosquitoes. At the saturation point, increasing the concentration any further will not increase the rate at which the insecticide penetrates the mosquito and gets to the target site. This is the diagnostic concentration that you will use durring the bioassays.

Sample Calibration Graphs

  1. To find the right saturation point, you may have to repeat the assay using different concentrations (larger or smaller), or smaller increments between concentrations. For example you may start out with increments of 100 ug, beginning with 100 ug/beaker and ending at 1000 ug/beaker. If after these assays you do not see a clear saturation point, you may need to run more beakers with <100 ug/beaker or >1000 ug/beaker.
    On the other hand, if you do see a clear saturation point, you can refine what that value is by running more beakers at smaller increments near where you see the break. You can go back and rerun beakers using an increment of 100 fold concentrations.
    CDC typically use insecticide solutions of 100 mg/ml, 10 mg/ml, 1 mg/ml, and 0.1mg/ml.

Interpreting data:

Like all resistance tests, bioassay data need to be compared to data from susceptible or base line sources. A resistance threshold for each insecticide can be determined by drawing a straight line down from the point at which all of the susceptible mosquitoes have died. Mosquitoes surviving at or beyond this threshold are interpreted as being less susceptible to the insecticide. Something in the mosquito is delaying the insecticide from reaching the target site and acting. In other words, they have some degree of resistance.

Learn more about Interprating data and graphs.

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This page last reviewed July 3, 2001

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