A pyrethrins-resistant strain of housepies, 213ab, previously selected with a 1:lO (by wt.) mixture of natural pyrethrins andpiperonyl butoxide, was further selected either with naturalpyrethrins alone (strain NPR) or with resmethrin (strain 104). Afrer 50generations the two populations differedin their resistance to the natural andsyntheticesters. Both wereresistant to allpyrethroids. Part of strain NPR was immune and very much more resistant than strain 104 to the natural pyrethrins and allethrin, but it was only 2-3 times more resistant than strain 104 against the new synthetic esters resmethrin (5-benzyl-3-furylmethyl ( f )-cis-trans-chrysanthemate), bio-resmethrin (5-benzyl-3-furylmethyl ( +)-trans-chrysanthemate), pyresmethrin (5-benzyl-3furylmethyl pyrethrate) and 5B2Me3FC (5-benzyl-2-methyl-3-furylmethyl (+)-cis-trans-chrysanthemate). Pretreatment of both strains with sesamex diminished but did not eliminate resistance. Synergism was greater in strain NPR, especially with natural pyrethrins and allethrin. Both strains had great resistance to DDT indicating that resistance to DDT andpyrethroids is linked.Diyerences in resistance to different compounds suggest that at least three factors can confer resistance, one of which, pen, delays penetration and two others involve detoxication, one py a on the acid side of the ester linkage and the other, py b, on the alcohol side. Naturalpyrethrins and resmethrin select for different groupings of these factors. Treatment with resmethrin does not select for py b presumably because this mechanism cannot attack the resmethrin molecule. Similarly when piperonyl butoxide is added to the natural pyrethrins py b is inhibited and so removed from selection pressure. Under these conditions, the strain produced contains the same factors as one selected by resmethrin and so shows the same small resistance to natural pyrethrins alone.
