Insecticide resistance mechanisms in Aedes aegypti (L.) from Merida, Yucatan, Mexico, related with two dengue transmission seasons

The city of Merida, Yucatan has well-defined annual seasons, rainy and dry, facilitating the development of Aedes aegypti (L), vector of dengue virus. The increase in number of mosquitoes result in extensive use of chemical control over a long period of time against larvae and adult stages, favouring the development of insecticide resistance. We determined the susceptibility of adults of Ae. aegypti to permethrin, deltamethrin, phenothrin, and larvae to temephos. Enzymatic mechanisms of resistance were characterized in Ae. aegypti from Merida, Yucatan, collected in rainy season of 2007 and 2009, and dry season  of 2007, 2008, 2009 and 2010. Larval bioassays were carried out using the WHO (1981) method. Bottle bioassays were done to determinate adult susceptibility (Brogdon and McAllister 1998), calculating: median knockdown concentration (CK₅₀), median lethal concentration (LC₅₀), median knockdown time (TK₅₀) and median lethal time (TL50). Brogdon et. al. (1997a, 1998b) technique was used to determine detoxificative enzymes. Larval populations of Ae. aegypti showed high resistance levels to temephos, in both dry season of 2007 (38X), 2008 (18.4X), 2009 (20X), 2010 (19X) and rainy season of 2007 (18X) and 2009 (19.6X). Adult populations of Ae. aegypti, showed high levels of knockdown resistance to permethrin (dry season 2007-12.5X, 2008-18.3X, 2009-19.6X y 2010-30.4X, rainy season 2007-20.2X y 2009-20.8X) and phenothrin (dry season 2007-37.5X, 2008-182.2X, 2009-173.6X y 2010-178.4X, rainy season  2007-43.9X y 2009-180.1X). All populations showed high post-recovery resistance to permethrin (dry season 2007-25X, 2008-30X, 2009-30.8X, 2010-44X, rainy season 2007-31X y 2009-31.4X) and phenothrin (dry season 2007-41X, 2008-158X, 2009-220X y 2010-229X, rainy season 2007-50X y 2009-226X). Populations showed levels of knock-down resistance to deltamethrin high to moderate (dry season 2007-21.6X, 2008-8.2X, 2009-12X, 2010-9.7X, rainy season 2007-10.2X, 2009-10.2X). The same with post-recovery resistance (dry season 2007-33X, 2008-8X, 2009-9.5X, 2010-8.5X, and rainy season 2007 9.5X, 2009-9X. Knock-down and post-recovery resistance levels exhibited by Ae. aegypti adult populations corresponding to rainy season were significantly different from  dry season. α-esterases enzymes were significantly elevated in mosquitoes exposed to permethrin and temephos, α and β-esterases in mosquitoes exposed to deltamethrin, α-esterases and multiple function oxidases in mosquitoes exposed to phenothrin. In examining the association between the LC₅₀ values and over-expressed enzyme levels in surviving populations from one season to another, there were no significant correlation values thereby no fulfilling the criteria proposed to associate an enzyme mechanism with resistance to insecticide. The results suggest that the studied populations have been subject to strong selection pressure in the field, result of continuous application of temephos for over 30 years and permethrin for over 10 years, coupled with the effect of insecticides applied to agricultural and urban pests.