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
Table 2. Change in resistance ratios (RR.o = LCror,/ LCro*t) to deltamethrin at the median lethal +, Ienzhou concentration (LC.o) under selection pressure with '+ Jinhua deltamethrin relative to the natural population (Fn "r ilingbo generation) for each resistant strains. + llangzou RR- - Jiaring Genera- Hang- tion (F) Wenzhou Jinhua Ningbo zhou Jiaxing 0 111ll 2.14 1.78 1.75 2.06 3.59 4 4.73 3.87 4.98 8.92 5.68 6 6.49 8.04 14.24 13.10 14.45 8 8.48 13.33 23.t7 23.07 16.14 l0 13.05 22.46 31.43 31.05 23.67 t2 12.25 25.95 36.99 38.67 32.70 'LCron, = LC.o for the F, (i : O, 2, . . -, 12) generation; LCrn* : LCro for the Fo generation (the natural population).
0245810t2 resistance between resistant strains, it could be in- Gener*ion (F) ferred that these 5 populations should have a sim- ilar potential of resistance to deltamethrin and that Fig. l. Median lethal concentrations(LCros) of each differences in resistance generationfor 5 strainsunder deltamethrinselection at the between the Wenzhou nat- doseof 6O-7OVomortality. ural population and other natural populations may arise from differences in local control measures. Resistance to deltamethrin in other natural popula- tions from the Wenzhou population, 0.1178 for the Jin- may be produced by use of pyrethroids in ag- hua population, O.l3O7 for the Ningbo population, riculture and personal protection because pyre- 0.1323 for the Hangzhou population, and O.1262 throids have not been used intensively for the for the Jiaxing population. The R value in the re- purpose of controlling mosquitoes in these areas. sistant strain from the Wenzhou population was sig- This hypothesis should be further tested. nificantly (P < 0.05) less than that other resisrant Nerve insensitivity plays an important role in the strains. resistance of insects to pyrethroids (Omer et al. 1980, Bloomquist and Miller 1985, Shono 19g5. Zhai 1995). DISCUSSION Pyrethroid-resistant strains of mosqui- toes were selected under laboratory conditions and ln Zhejiang, organophosphate insecticides have were detected by cross-resistance to DDT (WHO been used to control mosquitoes for more than 2O 1981, Chen 1990). Deltamethrin-resistant strains years and organophosphate resistance is now wide_ with high RRs (>100, relative to susceptible spread in most mosquito species (Wang et al. strains) have been selected in my laboratory. Under 1996b). Therefore, replacement of organophosphate the same selection pressure, the response to selec- insecticides with other classes of insecticides, such tion in the resistant strain from the Wenzhou pop- as pyrethroids, is an important means of increasing ulation was less sensitive than that of other resistant mosquito control. Soaking mosquito-curtains with strains. This may have occurred because the resis- deltamethrin in Wenzhou played an important role tance level of the Wenzhou natural population in controlling tlJe An. sinensis population, and the was higher than that of other natural incidence of malaria in this area decreased greatly populations. How- ever, resistance levels (Yao 1992). Howeve4 as use of insecticides iontin- of the resistant strains were not significantly ues, susceptible individuals in populations decrease different. To and resistant individuals in populations increase, re_ decrease resistance of mosquitoes, different sulting in a resistant population. The population of classes of insecticides should be rotated in those An. sinensis in the area of Wenzhou. which used natural populations with a high resistance to a given mosquito-curtains soaked with deltamethrin for the class of insecticides. In Wenzhou, pyrethroids purpose of controlling mosquitoes, had a high fre_ should be replaced with other classes of insecticides quency of resistance genes. Resistance levels of or used with synergists so as to increase the effect otier natural populations were higher than those of of pyrethroids. For other natural populations, py- their susceptible strains, but lower than that of the rethroids can be used, but the development of py- Wenzhou population. From the similarity of sensi- rethroid resistance in these natural populations tivity between susceptible strains and similariry of should be monitored yearly. 12 JounNA.r-or rnp AvrnrcnN Mosqurro Con"rnol Assocnrrox VoL. 16, No. I
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