In this video, we'll discuss insecticide resistance
and how to use the CDC's Bottled Bioassay to test mosquitoes for resistance.
The use of insecticides to kill mosquitoes
is one part of an integrated mosquito management program.
Over time and with repeated use, insecticide
resistance can occur in mosquito populations.
Mosquitoes develop resistance to insecticides in different ways,
which are referred to as a resistance mechanism.
CDC is bottle bio assays can detect the strength of that mechanism.
Insecticide resistance is an overall reduction
in the ability of an insecticide to kill mosquitoes.
This means that when used as directed, a product no longer works
or only partially works to control mosquitoes.
Insecticide resistance can be product specific
or it can develop to a certain class of product.
CDC has developed a test or assay
to determine if a particular active ingredient in an insecticide is able to kill mosquitoes.
In particular, it detects changes in the time it takes an insecticide to kill the mosquito.
The technique referred to as the CDC Bottle Bioassay is simple,
fast and cost effective compared with alternatives.
Detecting insecticide resistance early is important
to monitoring the effectiveness of mosquito control strategies
and can help guide programs on which insecticide to use for spraying.
Mosquito control programs
in the United States can order CDC Bottle Bioassay kits.
Each kit includes a manual,
lab mat, measuring tube, insecticides,
disposable pipettes, aspirator, tube holder and bottles.
Programs using CDC Bottle Bioassay Kit will also need to supply mosquitoes for testing,
a timer, markers, tape, acetone or technical grade absolute ethanol and safety equipment such as
gloves, goggles and lab coat.
To perform the CDC Bottle Bioassay
a bottle is coated with a known amount of insecticide.
This is called the diagnostic dose.
Mosquitoes are then put into the bottle and observed for 2 hours.
At selected time points during those 2 hours, the number of dead mosquitoes are counted.
The test should be run for the entire 2 hours
unless all mosquitoes have died before the 2 hours is ended.
Resistance is determined by the percentage of mosquitoes that die
at the predetermined threshold time during those 2 hours.
This is referred to as the mortality rate.
Diagnostic doses and threshold times are determined using susceptible mosquitoes
or those mosquitoes that haven't yet developed resistance to insecticides.
For example, in this table.
45 minutes is the threshold time for Culex pipiens
exposed to 2.25 micrograms of Naled.
Mosquitoes that have developed resistance to insecticides will take longer to die.
The shape of the mortality curve beyond the threshold time gives
valuable information on how strong the expressed resistance is.
Let's look at an example.
Say you log the number of mosquitoes that had died at 15 minute intervals.
Your mortality curve might look like this.
The shape of the mortality curve beyond the 45
minute threshold time or the time at which point all susceptible mosquitoes should have died,
gives valuable information on how strong the resistance mechanism is.
In this case, we were able to see the wild mosquitoes are resistant to the insecticide tested.
In Wild Population 1, they die soon after the threshold time.
This means that the resistance mechanism is weak
in Wild Population 2, some mosquitoes never died in the two hour period.
This means that the survivors have a strong resistance mechanism.
Routine testing using the CDC Bottle Bioassay will help you monitor changes over time.
Programs in the continental United States and its territories can order free
insecticide resistance kits by sending an email to USBottleAssaykit@cdc.gov
and requesting an order form. Kits include: bottles, insecticide, and manual.
You can access the CDC Bottle Bioassay manual at any time
by visiting www.cdc.gov/mosquitoes.