Four-day-old female adult offsprings from the following hybridization groups were used in topical application bioassays of several
insecticides: male transgenic flies crossed with female Gal4 lines and
male transgenic flies crossed with female w1118. Four OP insecticides (i.e.,
chlorfenvinphos, malathion, parathion, and monocrotophos) and one
pyrethroid insecticide (i.e., deltamethrin) were dissolved in acetone and
diluted to obtain the final concentrations. The flies were anesthetized
with carbon dioxide; afterward, 0.25 μL of insecticide solution was
applied on the thoracic dorsum of flies by using a Burkard hand
microapplicator (Burkard Manufacturing Co., Hertfordshire, England).
Control flies were treated with 0.25 μL of acetone. Five doses of each
insecticide with six replicates for each dose (20 flies per replicate) were
administered. Mortality was determined at 24 h after treatment, and
data were analyzed using the log-probit program [28,29]to obtain LD50
values, slope of each mortality line, Chi-square values, and resistance
ratios with 95% confidence intervals.