Selection of Houseflies (Diptera: Muscidae) with a Pyrethroid Space Spray Using a Large-Scale Laboratory Method

A strain of houseflies, Musca domestica L., with high levels of resistance to pyrethroids (resistance factor 15.2 at the LD50 to pyrethrins + piperonyl butoxide [PB] when collected from the field in 1988) had a resistance factor of 4.2 to pyrethrins + PB after 3 yr in laboratory culture. This strain subsequently was selected for resistance to cypermethrin in the laboratory. Large numbers of flies (>7,000) were released into disposable polythene cubicles (≍10 m3) shortly after adult emergence to ensure that mating did not occur before selection. A microemulsion formulation of cypermethrin was sprayed into the cubicle at a concentration intended to achieve >90% kill. Survivors were collected and allowed to mate; progeny of these flies and the flies from three subsequent generations were treated similarly but with increasing amounts of cypennethrin. Mortality was >95% at all selections except selection 2, in which 80% kill was achieved. Resistance levels to cypermethrin and natural pyrethrins synergized with PH were measured after each selection by topical application experiments. Resistance at the LD50 to pyrethrins + PB rose from 4.3 to 19.6; for cypermethrin treatments, the LD50 rose from 11.9 to 503.0 after five selections.