Impact of pesticide use on the density of Anopheles anthropophagus and malaria incidence

Objective To investigate the impact of the application of chemical pesticides on the distribution of Anopheles anthropophagus in rice fields and the malaria incidence.Method Twenty-four villages from 16 counties in the provinces of Zhejiang,Sichuan and Guangxi were chosen for the surveys in the period of 1983-1987.For the survey of An.anthropophagus,indoor human bait trapping until midnight and catching the mosquitoes in all the nets in early morning were carried out to get the density and population ratio of the mosquitoes.Historical data on Anopheles spp.,malaria incidence,acreage of the single or double season cropping of rice and on the quantity of chemical pesticide used in rice fields were collected from the study areas.Results In Hang-jia-hu region of Zhejiang Province,double season rice cropping was performed at that time,the quantity of pesticides used in 1973 was 45 kg/hm2,which was as high as 50 times than that in the 1950s.The density of An.anthropophagus decreased yearly,no An.anthropophagus could be found at 11 survey points in late 1980s.The malaria incidence dropped to less than 1 per 10 000.In Leshan and Yibin areas of Sichuan Province,the major cultivation was single cropping,pesticides were applied in paddy fields since 1960s,and the average quantity of pesticides used was 8.6 kg/hm2 during 1970s-1980s.No significant difference on the density of An.anthropophagus was revealed between 1980s(86.2%) and 1960s(82.2%)(χ2=0.63,P0.05).After mid-1980s,pesticide use gradually increased,and reached to 18.18 kg/hm2 in average in the years after 2000.The density of An.anthropophagus decreased,no An.anthropophagus was found in 2010 in the area surveyed and no malaria cases were reported as well.With double season cropping in Huanjiang County of Guangxi,the pesticide amount consumed was 1.79 kg/hm2,25.13 kg/hm2 and 7.68 kg/hm2 in paddy fields in 1960s,1970s,and 1980s,respectively.The proportion of An.anthropophagus in anophelines was 52%(1 747/3 392) in the beginning of the 1980s.After the year 2000,the average pesticide use increased to 20.38 kg/hm2 in paddy fields.It was difficult to find An.anthropophagus in human dwellings after 2008.The average annual malaria incidence dropped to 0.14 per 10 000.Conclusion Change of farming activities and especially use of chemical pesticides in high quantity at the rice fields undermine the breeding environments of An.anthropophagus,greatly reduce the mosquito population and therefore the malaria incidence.