Aspirate 15 to 25 mosquito larvae into each beaker. Use a number
you can easily count with accuracy as the mosquitoe 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 prior
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.
