Here's an interesting article by Donald Roberts, an expert in tropical diseases, about the use of DDT to help control malaria in Africa. It turns out that using the insecticide indoors has a repellent effect even on mosquitoes that have developed an immunity to DDT.
In our studies, in which we sprayed DDT on the walls of huts in Thailand, three out of every five test mosquitoes sensed the presence of DDT molecules and would not enter the huts. Many of those that did enter and made contact with DDT became irritated and quickly flew out.
The mosquitoes we used were the kind that carry dengue and yellow fever, not malaria. But there is ample evidence that malaria-carrying mosquitoes respond similarly to DDT. Several malaria-carrying species are even more sensitive to DDT’s repellent effects.
When we sprayed the huts with either dieldrin or alphacypermethrin, in contrast, all the test mosquitoes entered. Alphacypermethrin acted as a contact irritant, and it killed others that lingered on a treated surface. Dieldrin worked only as a poison — a powerful one, killing 92 percent of mosquitoes that made contact with it, far more than alphacypermethrin or DDT.
But dieldrin’s strong toxicity means that mosquitoes quickly develop resistance to it. Its use against malaria was short-lived, ending in the 1950s, because it so quickly became powerless.
But the mosquitoes hate the DDT even if it doesn't kill them. That is really good news for malaria control.
