Journal of the American Mosquito Control Association, 16(l):9-12,2OOO Copyright O 2OOOby the American Mosquito Control Association, Inc. RESISTANCE AND RESPONSE TO SELECTION TO DELTAMETHRIN TN ANOPHELES S/NENS/S FROM ZHEJIANG. CHINA JINFU WANG College of Life Sciences, Zhejiang University, No. 232, Wen San Road, Hangzhou, Zhejiang 3IOOI2, People's Republic of China ABSTRACT Resistance levels to deltamethrin were measured in 5 natural populations of Anopheles sinensis. The median lethal concentrations (LC.os) of deltamethrin in these populations-were higher than those in suscep- tible strains originating from the same populations, especially in thi Wenzhou population, which had a resistance (RR.o) ratio of 11 relative to its susceptible strain. Resistant strains were ielected with deltamethrin for 12 generations. Resistance levels in resistant strains were 130 to 190-fold higher than in susceptible strains, and l0 to 4o-fold higher than in natural populations. Response of selection (R) i; the resistant strain from the Wenzhou population was less than 0.1, and those in resistant strains from other natural populations were more than 0.1. This suggests that a resistant strain from a natural population with higher resistance has a lower increase in RR than a resistant strain from a natural population with low resistance under identical insecticide selection. These results are discussed in relation to mosquito control strategies. KEY woRDs. Anopheles sinensis, deltamethrin, resistance, response of selection INTRODUCTION plied for oviposition. Larvae from each population were reared in an enamel washbowl containing Anopheles sinensis Wiedemann, an important 2,000 ml of water. Emerging adults were again vector of malaria, is widely distributed in rurai ar- held in the screen cages as described above. eas of Zhejiang, China. Malaria vector control was Larvae and adults were held at 27 -+ l"C with + started using organophosphate insecticides in the 8O lOVo rel- ative humidity, and a photoperiod middle 1960s. The disease has been absent from of 12:12 h (light: dark). When needed, most of areas in Zhejiang since the early 1980s, but females were bloodfed on mice. portion is still present in a few areas, such as in Wenzhou, A of the 4th-stage larvae from each natural population Zhejiang. In 1991, mosquito-curtains soaked with was used for bioassays, with the deltamethrin were used to control An. sinensis in 4th-stage larva of its susceptible strain as the con- the rural area of Wenzhou. This is the lst report of trol. Another portion of the 4th-stage larvae was mosquito conrrol with pyrethroids in Zhejiang, al_ selected with deltamethrin. though pyrethroids have been used in agriculture A resistant strain from each natural population since the early l98os. This work was carried out was selected with deltamethrin for each generation for 2 years and resulted in a sharp decrease of ma- at a dose resulting in 60--70Vo mortality. Each re- laria in this area (Yao 1992). sistant strain was selected for 12 generations and To coordinate research on mosquito-curtain soak- the lethal concentration of the 4th-stage larvae was ing in Wenzhou, we carried out a study of delta- measured in each generation. methrin resistance in mosquitoes. This study was Susceptible strains of An. sinensis with the same undertaken to determine the resistance level in the genetic backgrounds as their respective natural pop- lenzhou population of An. sinensls by comparing ulations were provided by the epidemic prevention the lethal concentration of pyrethroids to its sus- stations in Wenzhou, Jinhua, Ningbo, Hangzhou, ceptible strain. Because pyrethroid resistance is and Jiaxing. These strains had been held under re- now widespread in many crop pest species in Zhe- laxation of insecticide selection for more than l0 jiang, possible resistance of mosquitoes to delta- years (Wang et al. 1996a). methrin in other rural areas of Zhejiang was also Insecticides: Deltamethrin (982o powder) sup- determined. A resistant strain from each natural plied by the Chemical Laboratory, Zhejiang Acad- population was selected with deltamethrin, and the emy of Agricultural Sciences, was diluted to I and increases in their ratios of resistance were analvzed. O.lVo stock solutions with acetone. l,arval bioassay: Resistance of larvae to delta- methdn was measured MATERIALS AND METHODS in a total volume of 2OO ml using the standard World Health Orsanization Mosquitoes: Adult Anopheles sinensis were col- (WHO) rest merhod (WHO 1975, Liu ti85). Sets lected from breeding sites in 5 rural areas (Wen_ of 25 larvae were placed in enamel bowls contain- zhou, Jinhua, Ningbo, Hangzhou, and Jiaxing) dur- ing 20O ml reactive volume. For the range-finding ing the summer of 1995-96. Each adult population test, the concentration gradient points were 2-2.5, collected was held in a screen cage (ZgxZZxZ3 and each regular test contained 6-7 concentrations cm) and fed l5%o glucose solurion. A silk fabric (i.e., the regular test for the Wenzhou natural pop- laid on moistened cotton in a culture plate was sup- ulation contained 7 concentrations from 0.04 to Joumll or rns ArraenrceN Moseuno Con"rnol AssocIATIoN VoL. 16, No. I Table 1. Log-dosage probit mortality data analysis for deltamethrin tested against 5 natural populations (N) and susceptible strains (S)t. Population Strain LCrot LC,, RRro RR,, x2 Wenzhou N 0.3327+ O.O9t2 3.7204 10.91 20.20 16.72*1 S 0.0305 + 0.0012 o.1842 l.Jf Jinhua N 0.1672+ 0.0614 2.9073 6.58 tt.62 13.24* S 0.0254 + 0.0009 o.2501 r.37 Ningbo N 0.1355+ 0.0317 3.0556 4.22 t4.79 10.37* S 0.0321+ 0.0011 o.2066 l.2l Hangzhou N o.to37 + o.o725 2.8254 4.96 12.92 10.04* S 0.0209 + 0.0017 0.2187 2.O9 Jiaxing N o.t44'7 + O.O5'14 2.9526 4.54 14.81 11.73* S 0.0319+ 0.0008 o.1994 1.14 t LC.u, medim lethal concentration; LC,,,,, 99Vo lethal concentration; RR.o, resistance ratio to deltamethrin at LC5o relative to susceptible strains; RRon, resistance ratio to deltamethrin at LC* relative to susccptible strains. 2 12 values lbr the fit-goodness of data to straight lines are given. Includes standtrd enor for the LC.o. 3 * indicates a significant deviation from a linear response. 2.56 ppm with the gradient point : 2) with 3 rep- that the Wenzhou population had the highest resis- licates per concentration. Mortality 20 min after tance level among these natural populations (RR o treatment was recorded. Relative concentrations of : ll, RRs - 20), and other natural populations acetone were used as controls. showed <lO-fold resistance at the LCro (Jinhua: Statistical analysis: Mortality data were ana- 6.58, Ningbo: 4.22, Hangzhort: 4.96, and Jiaxing: Iyzed using the log-probit program of Raymond 4.54) and <I5-fold resistance at the LCee (Jinhua: (1993), based on Finney (1971). This program tests 11.62, Ningbo: 14.79, Hangzhou: 12.92, and Jiax- for the linearity of mortality curves, provides lethal ing: 14.81). The Wenzhou population was signifi- concentrations (LCs), tests whether mortality lines cantly (P < O.O5) more resistant than all other nat- are parallel, and computes resistance ratios (RR). ural populations. Response of selection (R) in each resistant strain was calculated with the equation suggested by Ta- bashnik and McGaughey (1994) as follows: Development of resistance under deltamethrin selection R : log(RR.no,r)/n, Development of resistance to deltamethrin for where RR.oFr2was the resistance ratio at the median each resistant strain under insecticide selection is lethal concentration (LC.o) in the 12th generation shown in Fig. 1. Under selection for 12 generations, of each resistant strain relative to its natural popu- LC.os in resistant strains were more than 4.0 ppm lation, and n was the number of generations under (4.07 ppm for the strain from the Wenzhou popu- insecticide selection. This parameter indicates the lation, 4.34 ppm for the strain from the Jinhua pop- increase in RR in each resistant strain under insec- ulation, 5.01 for the strain from the Ningbo popu- ticide selection, Resistance differences among mos- lation, 4.01 ppm for the strain from the Hangzhou quito populations were analyzed with Student's t- population, and 4.73 ppm for the strain from the test (Hu and Zhang 1985). Jiaxing populations). Compared with the suscepti- ble strain, resistance levels in these resistant strains RESULTS increased approximately 130-, 170-, 150-, 190-, and 150-fold, respectively. These resistant strains did Sensitivity status of 5 natural populations not differ significantly (P > 0.05) in resistance to Thble 1 gives the results of log-dosage probit deltamethrin. mortality analysis of 4th-stage larvae in 5 natural Resistance rations (RR.o) in resistant strains, rel- populations and their susceptible strains. All sus- ative to natural populations, are shown in Table 2. ceptible strains were homogeneous in their re- Under deltamethrin selection for 12 generations at sponse to deltamethrin, with the data points for all 6O-70Vo mortality, resistance levels increased 12- natural populations deviating significantly from a to -4O-fold over natural populations (12.25-told straight line. Parallelism of the mortality lines ob- for the stain from the Wenzhou population, 25.95- tained with 5 susceptible strains was not rejected at fold for the Jinhua population, 36.99-fold for the the 5Vo confidence level; they were not significantly Ningbo population, 38.67-fold for the Hangzhou (P > 0.05) different for susceptibility to deltameth- population, and 32.70-fold for the Jiaxing popula- rin. All natural populations were significantly (P < tion). The RR for the resistant strain from the Wen- 0.05) more resistant than their susceptible strains to zhou population was significantly (P < O.O5) lower deltamethrin.