ddt was banned in the us, not the world. other countries can use it to control malaria as they like, and some do. due to evolved resistance, it's not as effective as it was when the us eradicated malaria (not principally by using ddt), and other insecticides have been developed that are about as effective, have fewer known environmental and health impacts, and are similar in cost. so, most other countries use it sparingly or not at all.