Scientists have found that the single genetic change which protects flies from the pesticide DDT also makes males smaller, less aggressive and "rubbish" at courting females.
Resistant males are also more prone to "decamping" - the technical term for giving up midway through a mating attempt.
The University of Exeter researchers were "astounded" that a single allele - a different version of the same gene - could have such a dramatic impact.
"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 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 researchers studied common fruit flies and found that DDT-resistant males also performed courtship songs through wing vibrations and chased females at lower rates and had a two-fold increase in the time spent courting before a female accepted them as a mate.
:: The paper, Pleiotropic effects of DDT resistance on male size and behaviour, is published in the journal Behavior Genetics.