<P> The role of DDT in combating mosquitoes has been the subject of considerable controversy . Although DDT has been proven to affect biodiversity and cause eggshell thinning in birds such as the bald eagle, some say that DDT is the most effective weapon in combating mosquitoes, and hence malaria . While some of this disagreement is based on differences in the extent to which disease control is valued as opposed to the value of biodiversity, there is also genuine disagreement amongst experts about the costs and benefits of using DDT . </P> <P> Notwithstanding, DDT - resistant mosquitoes have started to increase in numbers, especially in tropics due to mutations, reducing the effectiveness of this chemical; these mutations can rapidly spread over vast areas if pesticides are applied indiscriminately (Chevillon et al. 1999). In areas where DDT resistance is encountered, malathion, propoxur or lindane is used . </P> <Table> <Tr> <Th> Toxicant </Th> <Th> Dosage in g / m </Th> <Th> Average duration of effectiveness in months </Th> </Tr> <Tr> <Td> DDT </Td> <Td> 1 to 2 </Td> <Td> 6 to 12 </Td> </Tr> <Tr> <Td> Lindane </Td> <Td> 0.5 </Td> <Td> </Td> </Tr> <Tr> <Td> Malathion </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Propoxur </Td> <Td> </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Toxicant </Th> <Th> Dosage in g / m </Th> <Th> Average duration of effectiveness in months </Th> </Tr>

Conclusion about adding a layer of oil in water with wrigglers