RESUMO
The use of malathion to control mosquitoes in Cuba during 7 years until 1986 selected 2 resistance mechanisms: that of elevated activity of nonspecific esterases and that of altered acetylcholinesterase (Ache) in Culex quinquefasciatus (Say). In Havana, specifically in the area under study (Quibú River), malathion was replaced by cypermethrin in 1987 and cycles of treatment with cypermethrin have been intensively used since 1987 up to now when the populations of Aedes or Culex increase. In Culex quinquefasciatus (Say) from the Quibú River the resistance levels, mainly to malathion, declined significantly from 1986 to 1997. An increase of resistance to pyrethroid was observed during that period of 11 years. The mechanism of elevated esterases rose to a frequency of 1 and there was also an increase in the frequency of the mechanism of Ache. The esterase B1, responsible for the resistance to malathion, but not to pyrethroid, was selected in this population until 1986. Starting from the use of pyrethroid for the control in this area, 2 new phenotypes of esterases named A6 and B6, apparently related to pyrethroid resistance, were selected.