Resistance to deltamethrin in Culex pipiens pallens (Diptera: Culicidae) from Zhejiang, China

Resistance to deltamethrin, a pyrethroid insecticide, was found in 7 Culex pipiens pallens L, populations collected in Zhejiang (southeast China). Larval populations exhibited resistance ratios of 5.9-24.73 at LC50 and of 95.8-294.5 at LC95. Mortality ratios between field-collected adults and susceptible laboratory adults exposed to the discriminating dosage (0.01%/20 min) were 44.7-87.3. Resistance levels varied among populations as follows: Wenzhou>Ningbo>Hangzhou>Zhoushan>Jiaxing>Jinhua>Taizhou. Heavy use of pyrethroid insecticides for controlling mosquitoes in some areas has increased pyrethroid resistance in their regional populations. After selecting larvae from each population with deltamethrin for 12-15 generations, the resistance ratio increased by 103.3-342.5 at LC50 and by 79.7-203.1 at LC95. These results indicated that a large scale Cx. p, pallens control with pyrethroid insecticides would induce a further increase of pyrethroid resistance in Zhejiang.