Aspirate
15 to 25 mosquito larvae into each beaker. Use a number you can
easily count with accuracy as the mosquito larvae swim about.
With practice you can test many more mosquitoes than in your first
few assays. It is best if you collect all of the mosquitoes together
in a pipette and introduce them into the bottle at once; this
will reduce the amount of additional water that is added to the
beaker and minimize possible contaminates/organic material from
the larval water. In the case of especially organic rearing water,
larvae may first be transferred into clean water before to sampling.
Examine
the beaker to be sure all mosquitoes survived the transfer process.
Especially avoid exuviae, which can be easily confused with dead
larvae.
Remove all exuvae and pupae as necessary.
Start
a timer and record how many mosquitoes are dead or alive (whichever
is easier to count) every 15 minutes until all are dead or 1.5
- 2 hours have elapsed. Tapping the beaker or gently touching
the larvae with a stirring rod or other small instrument works
well to identify larvae that are still alive. You may want to
continue for a total of 2 hours when working with Malathion or
Fenthion as these chemicals tend to take longer to kill mosquitoes
than pyrethroids, for example.
* Dead larvae will either sink to the bottom of the beaker or
float to the top.
After
all larvae are dead or 2 hours have passed, recount the total
number of mosquitoes in each beaker. To get an accurate count,
empty the beaker through a screen to collect the larvae and count
the total.
Calculate
the percent mortality for each 15-minute interval.
Plot percent mortality (y axis) against time (x axis) using
a probability scale for the percent mortality. If probability
paper or a suitable statistical computer program is unavailable,
a standard graphical plot is a good substitute.
