208 - Insecticide Switch Alters Resistance Patterns in Aedes aegypti in Tapachula, Mexico

Abstract: In Tapachula, Mexico, the long-term reliance on pyrethroids (PYRs) for controlling Aedes aegypti over 15 years led to widespread resistance, raising alarms over the declining efficacy of dengue prevention efforts. In 2013, vector control programs pivoted to organophosphates (OPs) due to their different mode of action. We conducted a five-year longitudinal study (2016-2021) to evaluate how Aedes aegypti populations responded to this strategic insecticide switch. Using bottle bioassays, we tracked changes in susceptibility across 24 collection sites, assessing the lethal concentration 50 (LC50) for two OPs—malathion and chlorpyrifos—and two PYRs—permethrin and deltamethrin. Our findings revealed a gradual but significant rise in OP resistance, with mosquitoes displaying moderate to high resistance to chlorpyrifos and low resistance to malathion. Notably, pyrethroid resistance decreased sharply after their withdrawal from use. Operational testing with local mosquito populations further confirmed that pyrethroid resistance dramatically reduces the efficacy of the two tested PYRs, whereas mosquitoes remain susceptible to OPs. With OPs and PYRs representing key tools in public health insecticide strategies, ongoing resistance monitoring is essential to prevent further loss of insecticide effectiveness and safeguard mosquito control efforts.