Discriminating Lethal Concentrations for Transfluthrin, a Volatile Pyrethroid Compound for Mosquito Control in Thailand

Abstract

Pyrethroids are commonly used to control malaria and dengue vectors in Thailand. The lack of specific lethal discriminating concentrations (DCs) for specific mosquito species has possibly compromised more accurate assessments of physiological susceptibility to various chemicals over time. Previous studies have established DCs of various residual pyrethroids against specific mosquitoes in Thailand. However, DCs for transfluthrin (TFT), a highly volatile pyrethroid compound, against mosquito vectors in Thailand has been lacking. The aim of this study was to determine the DCs and susceptibility baselines of TFT against pyrethroid-susceptible laboratory strains of Aedes aegypti, Anopheles minimus, and An. dirus using the World Health Organization adult susceptibility tube method. Final DCs of TFT of each species were determined based on doubling the 99% lethal concentration at the following percentages: Ae. aegypti (0.06824%), An. minimus (0.06382%), and An. dirus (0.01508%). Subsequently, the respective TFT DCs were used to test field-collected populations of Ae. aegypti, An. harrisoni (Minimus Complex species), and An. dirus. Anopheles harrisoni and An. dirus were found completely susceptible (100% mortality), whereas Ae. aegypti from Nonthaburi Province was resistant to TFT. The suitability of the testing system and procedures is discussed. Routine assessment of insecticide susceptibility should include pyrethroids with high-vapor-pressure characteristics for informing control programs and consumers of product and chemical effectiveness.

Behavioral responses to transfluthrin by Aedes aegypti, Anopheles minimus, Anopheles harrisoni, and Anopheles dirus (Diptera: Culicidae)

“…aegypti (NON) were found resistant to TFT (< 56% mortality at 24-h) using the previously established DC (0.06824%); whereas, An . harrisoni (KAN) was fully susceptible to TFT using the DC 0.06382% [ 40 ].…”

“… Escape responses recorded at 1-min intervals during 30-min exposure to TFT at LC 50 established for each laboratory strain [ 40 ]. Concentration for An .…”

“…Previously, the standard WHO susceptibility assay, using lethal and discriminating concentrations of TFT on cellulose filter papers, was used to measure phenotypic response of mosquitoes with survival or mortality as endpoints [ 40 ]. Given that TFT is highly volatile and its bioavailability on cellulose papers is time-dependent, we acknowledge that the treatment of filter papers followed by a 24-h period of air drying at room temperature before testing poses a potential, if not significant, problem by not reporting the actual concentrations used at time of testing [ 40 ]. This study reports the initial concentration applied on the papers; however, we did not attempt to chemically determine the amount of active ingredient remaining in the papers.…”

“…This study reports the initial concentration applied on the papers; however, we did not attempt to chemically determine the amount of active ingredient remaining in the papers. The treatment of filter paper with TFT strictly followed procedures of a previous study that established the initial concentrations [ 40 ]. Assuming the rate of volatility is constant regardless of concentration applied and the remaining residual concentrations at time of testing elicited 'sublethal' effects, the ER actions were comparable as a ‘relative’ dose-response between the four different concentrations.…”

“…minimus and An . dirus [ 40 ], respectively (Tables 1 – 3 ). Untreated control papers were prepared with acetone and silicone oil only.…”