Search

Esterase amplification is the major organophosphorus (OP) insecticide resistance mechanism in Culex mosquitoes. The amplified Estalpha2(1)¿Estbeta2(1) esterases are found in > 90% of resistant populations worldwide, whereas amplified DNAs (amplicons) containing Estbeta1s are much rarer. Individuals with the Estbeta1 amplicons appear to be at a selective disadvantage in competition with those carrying the Estalpha2(1)¿Estbeta2(1) amplicons. To test the hypothesis that this is because Estbeta1 is less able to bind insecticide than the common amplified esterases, Estbeta1(2) was purified from the multi-resistant Habana strain of Culex quinquefasciatus, from Cuba. In its native form Estbeta1 is a monomeric enzyme of 66 kDa, with a pI of 4.8. The bimolecular rate constants for interaction of Estbeta1(2) with several OP insecticides were similar to those for the commonly elevated esterases Estalpha2(1) and Estbeta2(1), and much higher than for the electrophoretically identical non-elevated Estbeta1(3) and Estalpha3. Hence the apparent selective advantage of the Estalpha2(1)¿Estbeta2(1) amplicon is not due to its greater efficiency of insecticide binding, as OP insecticides are significantly better inhibitors of all the amplified esterases than of their non-amplified counterparts and therefore should be equally effective at conferring resistance.

Use of malathion for mosquito control in Cuba for 7 years up to 1986 has selected for elevated non-specific esterase and altered acetylcholinesterase (AChE) resistance mechanisms in populations of the pest mosquito Culex quinquefasciatus Say. These mechanisms are still present in relatively high frequencies in the Havana area, despite the replacement of malathion by pyrethroid insecticides for the last 3 years in the mosquito control programme. Samples of Culex quinquefasciatus populations from within a 100 km radius of Havana had high levels of resistance to malathion and lower levels of resistance to propoxur, but there was little or no cross-resistance to the organophosphorus insecticide pirimiphos-methyl. Selection with malathion for twenty-two consecutive generations in the laboratory increased the level of malathion resistance to 1208-fold and propoxur level to 1002-fold, but the maximum level of pirimiphos-methyl resistance was only 11-fold. Pirimiphos-methyl is still operationally effective, despite the resistance mechanisms segregating, so this insecticide if used for control is unlikely to select either of the known resistance factors directly in the field population. Since 1986, pyrethroids have been used extensively, and low levels of pyrethroid resistance were detected in two of five field population samples tested. Malathion selection did not increase the level of pyrethroid resistance, which indicates that one or more distinct pyrethroid resistance factors are now being selected in the field populations of Culex quinquefasciatus.