A dipping technique for exposing large numbers of house-flies (Musca domestica L.) to measured doses of insecticide is described. It is suitable for selecting resistant populations and, while giving consistent results, is more rapid than other techniques used for this purpose.
Up to 2,000 flies of both sexes, less than 24 hr. old, are immersed for three minutes in 100 ml. of a 70 per cent, mixture of acetone and water containing the required concentration of insecticide, using a 9-cm. sintered glass Büchner funnel as the immersion chamber. The liquid is then removed by suction, the sides of the funnel are wiped with filter paper, and the flies are allowed to drain for three minutes; they are then transferred in small batches to plastic recovery chambers containing food. Mortality is recorded next day, and the survivors are released into breeding cages.
Experiments showed that immersion for three minutes in 70 per cent, acetone was virtually harmless to the flies and that the amount of insecticide deposited on individual flies was reasonably uniform (coefficient of variation about 20%). Batches of 2,000 flies, but not more, could be treated at one time.
When the dipping technique was compared with topical application of measured drops of insecticide, using a susceptible strain of house-flies and two other strains that were resistant to DDT and diazinon, dipping gave steeper log-probit regression lines than topical application, and the LD50's and resistance factors of the resistant strains were smaller. With flies resistant to DDT, dipping gave straight regression lines whereas topical application gave compound lines.