Failure of pyriproxyfen at recommended application frequency and doses to control Aedes mosquitoes in Thailand

Mosquito-borne diseases like malaria, dengue fever, and Zika virus remain global health concerns. Pyriproxyfen is effective in controlling mosquitoes by disrupting their development. This study seeks to assess pyriproxyfen's ability to prevent Aedes aegypti emergence from water sources. It is part of a trial evaluating pyriproxyfen's impact on reducing mosquito infestation and dengue transmission, verifying its persistence and effectiveness in real-world and laboratory conditions.

The study was conducted in Khon Kaen province (northeastern region) and Prachuap Khiri Khan province (western region) of Thailand. We assessed pyriproxyfen residual effectiveness, inhibition of mosquito larval emergence and active ingredients among batches in a pyriproxyfen-based mosquito control trial in Khon Kaen. In Prachuap Khiri Khan we evaluated pyriproxyfen effectiveness across various water sources. The active ingredients in two pyriproxyfen batches were analyzed in a Sumitomo laboratory and in an independent laboratory.

Thirty days after field water containers were treated with pyriproxyfen the inhibition of mosquito larval emergence declined to ~60% and 60 days post-treatment the inhibition of emergence was just ~10%. Two batches of pyriproxyfen tested in the laboratory had > 85% inhibition of emergence and the active ingredient concentrations varied from 0.45-0.52%, close to the manufacturer's specifications of 0.5%. In laboratory experiments, the inhibition of mosquito emergence of pyriproxyfen in different water sources started declining after 42 days. Rain- and groundwater had higher inhibition rates (20-30%) than tap water (~10%) after 98 days. Emergence inhibition rates correlated negatively with water pH (F(1,118) = 5.626, p < 0.001) and positively with total dissolved solids, conductivity, and salinity of the water (F(1,118) = 48.302, p < 0.001), (F(1,118) = 37.022, p < 0.001), and (F(1,118) = 36.699, p < 0.001), respectively.

Pyriproxyfen failed to control Aedes mosquitoes at the recommended application frequency and doses in the field. The potential reasons for lack of effectiveness may be caused by environmental factors, such as pH, water source, and other water characteristics or social factors, such as homeowners' behaviors and water storage practices. The study underscores the importance of understanding environmental and social factors to tailor application strategies and ensuring sustained efficacy through regular monitoring, particularly in diverse contexts.