Penetration and Metabolism of Bicyclic Phosphorus Esters Which Affect Their Insecticidal Activity against the Housefly

Abstract

Pretreatmentwith the microsomal oxidase inhibitor piperonyl butoxide (pb) greatly improved the topical insecticidal activity of the GABA antagonist, 4-n-propyl-2,6,7-trioxa-lphosphabicyclo[2.2.2]octane 1-oxide (4-n-propyl BP),* against the housefly, Musca domestica L., when the synergistic ratio was 313. The insecticidal activity was also enhanced by bypassing the integument with injection without pb pretreatment.When tested by four different methods, the order of the 24-hr LD50 values (ug/fly) for 4-n-propyl BP was: injection, pretreated with pb (0.169)<topical application with pb (0.212)<injection (0.449) <topical application (66.4). Penetration studies indicated that 4-n-propyl BP caused marked mortality at the internal level of 0.2-0.3ag per fly, although the penetration was slow. The topically applied thiono and phosphite analogs without pb were approximately as insecticidal as the oxon analog injected. The thiono analog had the lowest 24-hr LD50 value (with pb) among its analogs, which was 0.061ug per fly. The lethal level of the thionate ester in the housefly body was estimated to be one-tenth of that of 4-n-propyl BP. The difference in the internal lethal level between 4-n-propyl BP and its thiono analog seems to be related to the difference in their insecticidal activity when they are topically applied without pb.