Susceptibility of DDT-resistant Houseflies to Other Insecticidal Sprays

Abstract

Five insecticides (chlordane, rotenone, chlorinated camphene, pyrethrum extract mixed with piperonyl cyclonene, and Thanite) in addition to DDT were tested as space sprays against the 15th, 16th and 17th generation of a special stock of flies in comparison to flies from a regular colony. The special strain was developed by rearing for several generations the progeny of flies that had recovered from DDT sprays. The tests were conducted in 100-cu. ft. cabinets. For each test, 1 ml. of soln. in cyclohexanone was atomized into the cabinet and allowed to settle for 20 sec. Special and regular flies were introduced into the cabinet in juxtaposition, and swung back and forth for 2 min. They were transferred to clean cages, fed, and held at 80[degree]F until mortality counts were made. Replications were run on 5 days. Mortality in untreated control cages did not exceed 1.5%. The special stock was distinctly more resistant to all materials than the regular colony. In further expts. each sex of the special stock was found to be more resistant to DDT residues than the corresponding sex of the regular flies, and the increased resistance was about equal to that shown in the space-spray tests. Twice as long an exposure, or more, was needed to cause mortalities among the special stock equal to those of the regular stock.