The detection and interaction of multiple organophosphorus and carbamate insecticide resistance genes in field populations of Culex pipiens from Italy

Collection of field populations of Culex pipiens from Lucca, Italy in 1984 and 1985 showed consistently high levels of resistance to a range of organophosphorus (OP) insecticides. Bioassays on the 1985 collection revealed that this population also had high levels of resistance to carbamates, even though these compounds had not been used in the field for Culex control. Biochemical analysis showed that the 1985 population contained the high esterase, Est-3A, and altered acetylcholinesterase (AChE) resistance mechanisms. The first OP resistance mechanism selected in Italian Culex was the Est-3A, and the AChE was then selected subsequently. Since September 1984 the altered AChE gene has been selected to a very high frequency in the Lucca population (P = 0.98), while there has been a significant decline in the frequency of the Est 3A gene from 0.70 to 0.36. The reason for these changes in gene frequency are considered.