Mechanisms of Resistance in Diazinon-Selected Multi-Resistant Musca domestica1

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Abstract
Studies were made in three strains (a susceptible, a low-resistant, and a high-resistant) of house flies (Musca domestica L.) to determine whether resistance to Diazinon®(O,O-diethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate), parathion, and DDT is attributable to differences in absorption rates. Penetration studies with Diazinon (P32) showed less rapid absorption by resistant flies and it was concluded that reduction in permeability of the cuticle is a factor in resistance to Diazinon. This is not the only defense mechanism, however, since flies which were 125-fold resistant by topical measurement still showed some tolerance (14-fold) when the insecticide was injected. Similarly, the low-resistant strain (15-fold topically) was 3-fold resistant to injected Diazinon. More rapid detoxication is precluded as the additional resistance mechanism, since the susceptibles contained higher amounts of water-soluble metabolites than resistant flies. Resistance to Diazinon was higher by oral application than by either injection or topical treatment. Ali-esterase activity of the strains was inversely proportional to the level of Diazinon resistance; this fact confirms previous reports on other o-p resistant house flies. There were only slight differences among the strains in cholinesterase level and in sensitivity to in vitro inhibition. In parathion- and DDT-resistance, penetration appears not to be an important factor, since the differences between the tolerances measured by injection and by topical application were small compared with the levels of resistance. DDT-dehydrochlorinase activity was high in resistant flies and it appears that this activity may constitute at least the primary defense mechanism against DDT-type compounds, since the highly resistant strain was only slightly (2-to 3-fold) tolerant of DDT analogs in which dehydrohalogenation is sterically hindered.