In many places mosquitoes became resistant to DDT. This meant that DDT did not work anymore in these areas. Scientists worried that DDT was making people and animals sick. It killed a lot of wildlife too. DDT also stays in the environment for a long time. For these reasons, people mostly use other chemicals for vector control. "Organophosphate" or "carbonate" pesticides are used, like "malathion" or "bendiocarb".