Discriminating lethal concentrations for pyrethroid compounds used in susceptibility monitoring of Anopheles epiroticus, a malaria vector in Thailand.

Anopheles epiroticus is a malaria vector in Thailand found primarily along coastal areas with brackish water habitats. Insecticides, particularly pyrethroid class compounds, are commonly used to control malaria vectors in Thailand. The lack of specific discriminating lethal concentrations for An. epiroticus has possibly compromised a more accurate assessment of physiological susceptibility to various chemicals. The routine assessment of vector response to insecticides is a key program management component to prevent or mitigate the development of resistance. The purpose of this study was to determine the discriminating (diagnostic) lethal concentrations of five common synthetic pyrethroids (deltamethrin, permethrin, bifenthrin, lambda(λ)-cyhalothrin, and alpha(α)-cypermethrin) used in Thailand for malaria control, against a susceptible colonized population of An. epiroticus. Final discriminating concentrations were 0.006% deltamethrin, 0.349% permethrin, 0.033% bifenthrin, 0.012% λ-cyhalothrin, and 0.0009% α-cypermethrin. Using concentrations established for each chemical, a field population of An. epiroticus from southern Thailand was found completely susceptible to each concentration. Periodic monitoring of insecticide susceptibility of An. epiroticus and other malaria vector species is needed to assess the efficacy of chemicals and guide insecticide policy and control programs.