| Objective：Culex tritaeniorhynchus is an important vector of Japanese encephalitis (JE) virus.It also carries Chikungunya virus, banna virus, Kadipiro virus, ring virus and West Nilevirus, etc[1]. China is a high prevalence area of JE, and national pandemics had occurredin the1960s and early1970s. With the widespread immunization of JE vaccine since1970s, there was a significant decline in JE incidence. Monitoring data of JE from1998to2002showed that fatality rate of the disease was5-35%, and30-50%of the survivingcases got serious sequelae[2]. A national immunization program of Japanese encephalitisvaccine began in Yunnan Province in2007, which lowered down the prevalence of thedisease by some degree. However, the mortality was maintained at a relatively high levelin2008(5.9%) and2009(3.79%). So, the situation remains serious now. Control andsurveillance of mosquito prevalence is still an important job. Chemical pesticides arebeing applied widely to eradicate Culex tritaeniorhynchus now. But long-term heavy useof chemical pesticides resulted in high resistance of Culex tritaeniorhynchus againstdichlorvos and deltamethrin in many places in China. The mosquitoes also showedresistance to other commonly used chemical pesticides[4-8]. So, finding new nontoxic,pollution-free pesticides is an important task in vector control. It was found for the firsttime that Bacillus thuringiensis israelensis (Bti) has a strong insecticidal activity onmosquito larvae. It has a high selectivity to target organisms, is nontoxic to human andanimal, not to pollute environment and hurt predators. Besides, the target organismsdon’t obtain resistance easily to Bti. Furthermore, the cost is much lower than chemicalpesticide. It is easy to produce Bti with simple technology and convenient to use. Withall these features, Bti has good application and development prospects. Yingjiang County belongs to Dehong, Yunnan Province. It is located in thesouthwest frontier in China, which borders Myanmar. The area is width surroundedby mountains and abundant in rice, sugar cane. The moist, warmness, paddy fields andcattle breeding create a good environment for mosquito breeding. As a result, there is ahigh population density of mosquitoes in the area. Culex tritaeniorhynchus is the mostdominant mosquito in there. In2008and2009, the incidence of JE in Yingjiang countyranked the top five of Yunnan Province[3]. Our previous research showed that Culextritaeniorhynchus in this area had very high resistance to deltamethrin. Therefore,looking for new pesticides has become a key problem to be solved currently. In thepresent study, Culex tritaeniorhynchus larvae were captured in paddy water in TaipingTown, the china-myanmar border region, and taken back to the laboratory. Then, thekilling effect of Bti on Culex tritaeniorhynchus larvae was carried out by bioassay.Besides, the impact factors such as instar, temperature, water quality and density oflarvae on the killing effect were also investigated. And, the effect of Bti treatment oneclosion rate was studied at last. This study will be helpful to provide scientific basis forreasonable using of Bti in control of Culex tritaeniorhynchus.Methods：Firstly, bioassay was carried out to test the killing effect of Bti on different instar ofCulex tritaeniorhynchus larvae by probit analysis method. Then, the killing effect underdifferent temperature, water quality and density of larvae were investigated. After beingtreated with sublethal dose of Bti, eclosion rate was compared with the untreated group.Result：1. Culex tritaeniorhynchus larvae were quickly killed by Bti within12hours.2. Twenty four hours after treatment with Bti on the second and fourth instar oflarvae, linear-regression analysis was carried out with GraphPad Prism software. Goodlinear relation was got between the log of concentration and probit value based onmortality. Formulae were gotten as y=2.588x-0.2524, y=2.03x+0.3654. According to theformulae, LC50and LC95were calculated. As a result, LC50of the second and fourthinstar larvae were107IU/L and192IU/L respectively, LC95were465IU/L and9091IU/L respectively. The result indicated that the second instar larvae were moresusceptible to Bti than the fourth instar larvae. 3. According to the results of bioassay on effect of water quality on Bti killing, theformulae of groups in well water and rice paddies water were y=2.03+0.3654x, y=1.115+1.306x, respectively. And the LC50of the2kinds of water were192IU/L and2056IU/L. The LC95of the2groups were9091IU/L and62348IU/L，respectively. Theresults indicated that Culex tritaeniorhynchus larvae were more susceptible in well waterthan in rice paddy water.4. Bti treatment was carried out under16℃、28℃、37℃, respectively. The mortalitywas recorded at6h,12h and24h post treatment. There was a significant difference of theaccumulated mortality of larvae between groups under three different temperatures at6h,12h and24h post treatment. At a same time point, the mortality is higher under hightemperature than that under low temperature (chi-square test, P <0.01). Comparativeanalysis between two groups showed that there was no statistically significant differencebetween the groups on accumulated mortality of larvae under28℃and37℃at12h and24h post treatment (chi-square test, p>0.05), but both were significantly higher thanthose under16℃(chi-square test, p <0.01).5. It was showed that the killing effect kept dropping with the increase of larvadensity，no matter in Bti low-dose groups or high-dose groups. The killing effect in highdose groups was in a straight line down with the increase of larva density. The killingeffect was not obvious in low dose group when the density of larva had reached120/L.6. The eclosion rate of larvae after pupation without treatment with Bti was72.46%.And eclosion rate of the group treated with sublethal Bti was45.15%only. Thedifference was significant (chi-square test, P <0.01).Conclusions：Culex tritaeniorhynchus larvae are sensitive to Bti. The killing effect of Bti on C.tritaeniorhynchus larvae could be accelerated and improved by enhancement of thetemperature in a limited range. The killing effect is better to treat the young larvae thanthe old one. Doses need to be increased in turbid water and high larvae density situation.Bti treatment can decrease eclosion rate of larvae after pupation. The results indicate thatBti is a kind of ideal larvicide to control Culex tritaeniorhynchus. |
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