Genetic researchers on Tuesday said they had discovered how the parasite that causes malaria becomes resistant to DDT and to insecticides used in anti-malaria bednets.
The secret lies in just one change in the DNA code on a single gene, they said.
A singe mutation changes a normal gene for metabolism, known as GsTe2, into one that helps the mosquito break down the insecticide molecule so that it is no longer toxic.
Insecticide resistance is a major worry in the fight against malaria.
DDT, banned decades ago in many countries because of its damage to the environment, remains an important mosquito-killing tool in poor economies.
Chemicals called pyrethroids are also used to treat bednets, shielding infants against the insect.
Researchers led by Charles Wondji at the Liverpool School of Tropical Medicine in England found a population of resistant Anopheles funestus mosquitoes in the western African state of Benin.
They unravelled the genome of the insects and compared it against a non-resistant strain of mosquitoes, to see what made things so different.
The answer: a mutation called L119F -- which was confirmed by looking at resistant mosquitoes in other parts of the world.
The team then introduced the mutant gene into fruit flies, a widely used laboratory tool. The insects themselves became resistant to both pyrethroids and DDT.
The work, reported in the journal Genome Biology, has opened the way to a test to spot emerging insecticide resistance in mosquitoes.
"Such tools will allow control programmes to detect and track resistance at an early stage in the field," said Wondji. "This significant progress opens the door for us to do this with other forms of resistance as well, and in other... species" that transmit malaria.