Nine in 10 of London's ladybirds suffer from a sexually transmitted disease.
The figure is well above that of the ladybird population outside the UK capital and scientists have been looking for an explanation.
"We think it's the car pollution," Dr Greg Hurst of University College London told the BBC. Ladybirds feed on greenfly and greenfly thrive in polluted conditions.
It seems that in London, the greenfly do so well that the ladybirds breed very quickly and their generations get mixed up; one generation may easily breed with the next or previous one.
This increases the transmission of the sexually transmitted disease.
"When they come out from over-wintering, they are remarkable promiscuous," Dr Hurst said. "They actually get the highest prevalence of sexually transmitted infections of virtually any insect.
"By the end of May, 80-95% of adult ladybirds will have acquired a sexually transmitted infection."
Fortunately for the ladybirds, the effects of the disease are not severe.
"This disease is very mild and benign. In Eastern Europe, the ladybirds get a more serious disease which makes them sterile; so once they've got the disease, they've got about 17 days' breeding left," said Dr Hurst.
UK ladybirds suffer from a fungal STD. The affliction is one of many subjects addressed at an international symposium on insects at the University of Reading, UK.