Changes in the susceptibility of normal and resistant house-flies (Musca domestica L.) to diazinon with age

In both a susceptible and a diazinon-resistant strain of house-flies (Musca domestica L.), adults of both sexes were most susceptible to diazinon immediately after they had emerged from the pupae. Their resistance increased for two days after emergence and remained steady for the next three days, and the increase was proportionately larger in the resistant strain. Taking flies 2 ± 2 hr. old as a basis, the increases in resistance by the third day were 1−2½ and 3½−6½ times for susceptible and resistant males, respectively, and 2−5½ and 9−18½ for susceptible and resistant females. In both strains, the two sexes were equally susceptible to diazinon during the first day, but later the females were more resistant than the males. The resistance factor (ratio of LD50's of resistant and susceptible strains) was least immediately after emergence and increased with age; it was 15–30 in flies 2 ± 2 hr. old and 50–90 three days later. There was no direct relation between changes in resistance and changes in the weight of the flies on aging. The resistance factor to diazinon should be measured not earlier than three days after emergence to obtain reproducible values. Removal of the ovaries in flies less than one day old did not affect the increase in resistance with age.