Now banned in most of the world, the insecticide DDT was used widely throughout the mid-20th century as a way of combatting diseases like malaria and protecting crops and livestock. It didn't come without a response, however. In 1938, just seven species were resistant to DDT, but by 1984, that number has ballooned to 447, according to figures from Duke University. To learn more about the evolution of this resistance, researchers have examined its effects on the common fruit fly, discovering that it leaves them facing an uphill battle to reproduce.

To further understand the evolution and spread of insecticide resistance, scientists at the University of Exeter say that learning about the fitness of resistant organisms is a big step forward. Enter drosophila melanogaster, known as the common fruit fly, which has provided scientists with a tool to study the development of insecticide resistance for more than half a century.

For the latest study, the researchers set out to investigate the difference a single genetic change – which bestows DDT-resistance – had on the average male fruit fly's ability to reproduce. This involved breeding males of various sizes that were resistant to DDT, and some that were not, and then pitting them against one another in a series of mating trials, some competitive and some non-competitive.

Of 187 competitive trials, males without DDT resistance won out 120 times. The researchers say the non-resistant flies were more likely to win even when the resistant flies were larger in size.

The other findings include observations that resistant males were more likely to "decamp" halfway through a mating attempt (give up and hop off), that they were less likely to perform courtship songs by flapping their wings and were less likely to chase females. If they did perform a courtship song, they were less likely to follow it with an attempt to mate and spent twice as much time courting a female before gaining her acceptance. These wide-ranging behavioral effects of DDT resistance came as a surprise to the researchers.

"It is amazing that even if all the genes are exactly the same, having this one gene expressed at a higher level has all these effects," said the University of Exeter's Professor Nina Wedell. "The expression level of one gene responsible for detoxifying DDT also makes males smaller, less aggressive and rubbish at courting. We don't yet know how this comes about."

The research was published in the journal Behavior Genetics.