Enhanced mortality in deltamethrin-resistant Aedes aegypti in Thailand using a piperonyl butoxide synergist.

Aedes aegypti is the primary vector of dengue viruses in Thailand. Control of this mosquito continues to rely heavily on use of insecticides in various forms and applications. The synergistic effect of piperonyl butoxide (PBO), combined with deltamethrin against eight populations of Ae. aegypti collected from different regions in Thailand is presented. The standard WHO adult contact bioassays found all populations with low to moderate levels of resistance to deltamethrin alone (using a 0.05% discriminating concentration), with final mortalities ranging from 15.6 to 70%, while a laboratory strain was fully susceptible (100% mortality). Pre-exposure of female mosquitoes to 4% PBO for 1 h, followed immediately by exposure to deltamethrin for 1 h, significantly increased mortality in seven populations (64.8-98.1%) with the exception of mosquitoes derived from Lampang Province. The knockdown time (KDT) synergist ratios between deltamethrin only and PBO + deltamethrin ranged from 1.7 to 2.8 for KDT50 and 1.9 to 4.0 for KDT95. Between deltamethrin alone and mosquitoes exposed to PBO + deltamethrin, all resistant populations produced significant differences (P < 0.05) in final 24-h mortality, except marginally for Lampang (P = 0.053). The synergistic effects of PBO with deltamethrin-resistant Ae. aegypti suggest a combination of this synergist with deltamethrin or other pyrethroid compounds can significantly enhance the effectiveness of these insecticides against pyrethroid-resistant Ae. aegypti found commonly in Thailand.