Insecticides kill insects and they can be very effective. In agriculture the use of insecticides has been enormously beneficial to farmers and has contributed to greatly increased food quantity and quality.
In health, the situation is different. Insecticides can only kill insects they touch. In a town or city a very high percentage of mosquito breeding sites are in and around people’s homes. (Aedes aegypti actually flies short distances (up to 200 meters) in their lifetime). So unless a public health authority can gain access to people’s homes on a very regular basis they are not able to control this mosquito. Even the best funded and most organised public health authorities cannot successfully control Aedes aegypti in urban environments so there is a pressing need to develop new tools to control this mosquito species.
Another issue is that mosquitoes can develop resistance to insecticides but it is a lack of access to the home that is the greatest challenge.