The mosquitoes that land on the net are prevented from biting the person sleeping under the net, and mosquitoes often avoid the net altogether. In many instances, malaria mosquitoes rest on the walls before or after biting people. Residual treatment of the walls inside a house repels or kills the mosquitoes. This malaria control strategy is very effective where houses have four walls. An insecticide with residual activity needs to be used so that the treatment lasts for some months.
Some insecticides are more expensive than others.
Also, malaria mosquitoes in some places are resistant to some insecticides. Thus, a large variety of possible insecticides need to be available for this purpose.