Preimaginal development of Aedes aegypti L. (Diptera: Culicidae) in brackish water gives rise to adult mosquitoes with thicker cuticles and greater insecticide resistance.

Aedes aegypti L. and Aedes albopictus Skuse mosquitoes (Diptera: Culicidae), the principal vectors of many human arboviral diseases, lay eggs and undergo preimaginal development in fresh water. They have recently been shown to also develop in brackish water in coastal areas. Previous findings showed that Ae. aegypti larvae developing in brackish water possessed thicker cuticles and greater larvicide resistance than larvae developing in fresh water. The present study compared cuticle ultrastructure, resistance to adulticides, and the activities of adulticide detoxifying enzymes in female mosquitoes emerging from fresh and brackish water-developing Ae. aegypti preimaginal stages. The results showed that brackish water-derived females possessed significantly thicker tarsal and abdominal cuticles compared to fresh water-derived females. Brackish water-derived Ae. aegypti females were also significantly more resistant to three different types of pyrethroids and malathion compared to fresh water-derived females. Corresponding reversal of cuticle changes and adulticide resistance when preimaginal salinity was reversed showed that preimaginal salinity determined both procuticle structure and adulticide resistance in brackish water-derived females. Compared with fresh water-derived Ae. aegypti females, brackish water-derived females had similar activities of the adulticide-detoxifying enzyme families of esterases and glutathione S-transferases and a modest increase in the activity of monooxygenases, all of which were lower than the threshold values attributed to resistance in field populations of Ae. aegypti. Reduced permeability of the thicker and remodelled cuticles in brackish water-derived Ae. aegypti females to adulticides is proposed to be mainly responsible for their greater resistance to different types of adulticides. Greater salinity tolerance of preimaginal stages, adult cuticle changes and higher larvicide and adulticide resistance are inherited properties of brackish water-developing Ae. aegypti that reverse in a few generations after transfer to fresh water. This is compatible with a role for epigenetic changes in the adaptation of Ae. aegypti to brackish water. Greater resistance of salinity-tolerant Ae. aegypti to adulticides and larvicides poses a hitherto unappreciated problem for controlling arboviral diseases, with attendant implications also for other mosquito-borne diseases.