This discovery overturns current theories that resistance to pesticides burdens insects with a genetic disadvantage that would stop them from competing with non-resistant insects once farmers stop using that pesticide.

Instead, researchers now believe that fruit flies that develop resistance to DDT gain a two-fold advantage: not only can they survive being sprayed with pesticide, which other insects cannot, but in doing so they develop a genetic advantage that makes them and their offspring more likely to thrive even when spraying is abandoned.

Researchers warn that the same process may be going on when doctors across the world prescribe antibiotics to cure infections.

Antibiotic resistance may potentially confer the same kind of genetic advantage to 'superbug' bacteria, and measures such as preventing certain antibiotics from being prescribed may not halt the spread of antibiotic resistance in bacteria.

"We found that DDT resistance in fruit flies not only carries no cost but in fact confers an advantage when inherited through the female," said Richard ffrench-Constant (correct), from the University of Bath, who led the study.

"This suggests that by becoming DDT resistant the female flies are passing on some unknown advantage to their progeny, presumably associated with the single metabolic enzyme (cytochrome P450) that they over express.''

"These results are important for the use of any drug, pesticide or antibiotic as they suggest that resistance will not always go away when we do not spray or prescribe antibiotics."

Scientists had previously believed that the genetic 'cost' of resistance would mean that DDT resistance would dwindle once the pesticide taken out of use and DDT-susceptible insects would regain dominance.

"Although this assumption is widespread, data to support this contention is actually thin," said Professor ffrench-Constant. He believes previous work may not have looked at genetically related strains and that 'costs' may therefore be associated with the differing genetic backgrounds of insects examined, and not the resistance genes themselves.

"Experimenters looking at genetic fitness in resistant insects often only look at single character traits such as number of eggs laid, and often compare resistant and susceptible lines that are genetically unrelated.

"Differences in fitness therefore often correspond to differences in genetic background rather and are not due to the resistance gene itself."

Using DDT-resistant fruit flies (Drosophila melanogaster) in state-of-the-art controlled temperature rooms provided by the Wolfson Trust, Caroline McCart, a PhD student in the Department of Biology and Biochemistry at the University, went to great lengths to make sure that DDT resistant and susceptible strains differed only by the resistance gene itself.

Using antibiotics they also 'cured' the flies of the microbes that are known to affect their ability to reproduce and could affect the results.

In order to assess the genetic fitness of both the resistant and susceptible strains, the researchers monitored the survival and development rate of all life stages of their offspring.

They found that DDT resistance in fruit flies not only carries no cost but in fact confers an advantage when inherited through the female.

This discovery comes at a time when a number of developing nations, including South Africa, are considering re-introducing (or continuing the use of) DDT in an attempt to reduce the major health problems caused by malaria.

Use of DDT (Dichloro-diphenyl-trichloroethane) increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus.

DDT-resistant mosquitoes were first detected in India in 1959, and they have increased so rapidly that when a local spray program is begun now, most mosquitoes become resistant in a matter of months rather than years.

Worryingly, some resistant strains also show 'cross-resistance' to a number of different compounds, so spraying with one insecticide can unexpectedly increase resistance to newer compounds subsequently introduced to try and overcome resistance.

The World Health Organization estimates that during the period of DDT use, approximately 25 million human lives have been saved. Today pyrethroids are most commonly used in mosquito control but they act on the same target in the nervous system as DDT and ironically spraying with DDT may therefore have pre-selected for resistance to the newer pyrethroids.

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