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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.
- 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.
- Run assays
on susceptible mosquitoes/larvae.
- 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 during the bioassays.
- 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 Interpreting data and graphs.
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