2.3. Preparation of insecticide impregnated papers
Insecticide-impregnated papers were prepared according to the
standard World Health Organization (WHO) method [8]. WHO recommended
discriminating dosages of insecticides for Aedes mosquitoes
i.e. 4% DDT (an organochlorine), 0.75% malathion (an
organophosphate), 0.1% propoxur (a carbamate) and 0.25% permethrin
(a pyrethroid) [9] were prepared by mixing the technical
grade insecticide with the spreading agent. DDT, malathion and
propoxur solutions were prepared in olive oil. Permethrin solutions
were made in Dow-Corning 556 silicone fluid. Rectangles of
Whatman no. 1 filter papers (12 cm 15 cm) were used for insecticide
impregnation. Insecticide/oil solutions (0.7 ml) were mixed
with an equal volume of acetone and the mixture was spread uniformly
on the filter paper. Acetone was used to ensure an even distribution
of the solution on the paper. Insecticide impregnated
papers were left overnight at room temperature to allow complete
evaporation of acetone. Papers were then foil wrapped and stored
at 20 C until use.