Resistance Levels in Diazinon-Pressured and Nonpressured Polyresistant House Flies12

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Abstract
The effect of diazinon selection pressure on polyresistance was followed with 4 strains of Musca domestica L., which originated from diazinon-treated wild populations. In many instances resistance reached a peak but declined after several generations, and then stabilized at a lower level. The resistances of the 4 strains stabilized in the following ranges: diazinon 50–75 times; ronnel 15–20 times; Isolan® (1-isopropyl-3-methyl-5-pyrazolyl dimethylcarbamate) 8 times; dimetilan 6–9 times; malathion 5–6 times; and pyrethrins, Lethane 384® (2-(2-butoxyethoxy)-ethyl thiocyanate), dimethoate, and naled 2–6 times. DDT resistance remained immeasurably high. In 2 strains lindane resistance reached peaks of 350 and 700 times but then declined rapidly after several generations and appeared to be regressing to a low level, as in the other 2 strains. There was also a regression of cyclodiene resistance despite diazinon pressure. Regression of polyresistance was followed with 3 strains (U and G for 95 generations and D for 45) which had been released from diazinon selection pressure. In general, there was an initial period of rapid decrease in resistance, followed by a much longer period of gradual decline. Tolerances to most of the insecticides tested appeared to stabilize, after 20–40 generations, at levels above those of the susceptible reference strain. The tolerances of U and G after 95 generations were: diazinon 5–8 times; ronnel 3–5 times; Isolan® 4–8 times; lindane 5–90 times; dimetilan 1.5 times; malathion 2 times; pyrethrin and dimethoate, essentially normal susceptibility. The resistance level to DDT fluctuated between 30 and 160 times. In general, the regression patterns of the D strain appeared to be following those of the U and G.