INTERACTION OF INSECTICIDE RESISTANCE GENES IN FIELD POPULATIONS OF CULEX-PIPIENS (DIPTERA, CULICIDAE) FROM ITALY IN RESPONSE TO CHANGING INSECTICIDE SELECTION PRESSURE

Culex pipiens Linnaeus larvae were collected from various locations in Italy and colonized as separate strains. These were analysed for elevated non-specific esterase activity and frequency of altered acetylcholinesterase (AChE) mechanisms of insecticide resistance, and bioassayed, to define the cross-resistance spectra conferred by these to organophosphorus and carbamate insecticides. These mechanisms were furthe characterized by polyacrylamide gel electrophoresis. Elevated esterase A1 (formerly known as Est 3A) which predominated in C. pipiens from Italy in 1985 had been replaced by two esterases, A2 and B2. Altered acetylcholinesterase was still present at high frequencies. Altered and normal acetylcholinesterase were distinguished by differential mobility on polyacrylamide gel electrophoresis. Levels of insecticide resistance were higher in the Lucca region of Italy than in other areas sampled, in response to intensive use of temephos and to a lesser extent chlorpyrifos, employed to reduce mosquito biting nuisance to tourists in this area.