Selection for pyrethroid resistance in a colony of Anopheles minimus species A, a malaria vector in Thailand.

This study tested susceptibilities of Anopheles minimus mosquitoes to deltamethrin during each of 19 generations (although technical problems excluded selective pressure experiments during generations 11-13). The ultimate goal was to establish a pyrethroid resistant colony of this important malaria vector in Thailand. Resistance was selected for by exposing, using the World Health Organization test protocol, sequential generations of An. minimus females to LD50 and LT50 values of deltamethrin. The LD50 and LD90 values were determined for populations from each subsequent generation by probit analysis and significant increases (chi-square test, P>0.01) occurred from one generation to the next. There was approximately a 22-fold increase in the LD50 and a 27-fold increase in LD90 when the F10 generation was compared to the parent colony (F1). Similarly, the LT50 and LT90 values were also increased during selection experiments during generations 14-19. There was roughly a 3-fold increase in susceptibility of F19 females compared to F14 females. In addition, deltamethrin conferred a cross-resistance to DDT in the selected colony. Baseline information from these experiments will serve as a guide for future studies on susceptibilities of wild An. minimus populations in Thailand.