DDT

From the NCPA:

DDT was an important factor in the virtual extermination of malaria in several developed countries including the United States, and it was spectacularly successful in developing countries such as India and Sri Lanka.

But it was a victim of its own success; no longer fearful of malaria, industrialized nations prohibited DDT’s manufacture and use because of its adverse effects on birds of prey -- and fears of its potential but unverified long-term impacts on human health.

However, DDT is still the most cost-effective insecticide available for use against many mosquitoes that spread malaria, which mostly affects developing countries.

DDT is relatively inexpensive and much more effective than the next best alternative insecticide.