Toxicity of Three Isomeric Mixtures of Resmethrin to Resistant and Susceptible House Flies (Diptera: Muscidae)

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Abstract
The topical toxicity of three isomeric mixtures of resmethrin (30:70, 22:78, and 11:89 [±] cis: trans isomers) to susceptible (S) and resmethrin-selected resistant (R) house flies, Musca domestica (L.), was determined. The mixture containing the greatest proportion of cis isomers (30%) was the most toxic to R flies (LC50 = 5.91 µg per fly) while that containing the lowest proportion of cis isomers (11%) was the least toxic (LC50 = 490 µg per fly). In the case of S flies, the LC50 and LC95 values for all three isomeric mixtures were not significantly different, indicating that the cis and trans isomers were equally toxic. This suggested that the R strain had developed a much higher degree of resistance to the trans isomers than to the cis. While both piperonyl butoxide (PB) and (S,S,S-tributyl phosphorotrithionate (DEF) increased the toxicity of resmethrin (30:70 cis: trans isomers) to R flies, PB was a better synergist than was DEF, suggesting that oxidative metabolism contributed more to the resistance in the strain than did hydrolytic mechanisms synergized by DEF. However, there were other factors involved, because R flies were considerably more tolerant of resmethrin: PB combinations than were S flies.