LADYBIRDS - those apparently benign insects that control aphids in gardens and feature endearingly in children's storybooks - have a dark secret: they are cannibals, obsessed with promiscuous sex, and they pass on venereal disease to each other.

Researchers at Cambridge University have discovered that ladybirds suffer from the insect equivalent of pubic lice. The discovery could open the way to new methods of controlling insect populations and is the first case of a sexually transmitted disease (STD) in insects, and only the second in invertebrates.

Dr Greg Hurst, a research student working with Dr Michael Majerus of the university's Department of Genetics, discovered the 'lice' - ecoparasitic mites - on Russian two-spot ladybirds from a Moscow laboratory. The researchers are collaborating with Moscow's Vavilov Institute of General Genetics.

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Dr Majerus said: 'The ladybird is notoriously promiscuous so it's a good candidate for STD. They will stay copulating for three hours or more . . females will feed and bonk at the same time . . .'

The mite lives and reproduces on the underside of the hardened wing cases, sucking blood and producing eggs. The larvae that hatch migrate to the back tip of the insect and then pop on to the next ladybird during sex.

Dr Hurst and Dr Majerus are trying to work out if there is a connection between the pubic lice and the fact that many females produce only female offspring; male eggs fail to hatch and newly emergent females eat the eggs containing potential brothers.

Dr Hurst discovered the strange dearth of male eggs was due to a bacterial infection in the eggs. 'We took the females with the trait and fed them on antibiotics,' he said. 'We ground up tetracycline pills, chucked them into golden syrup, fed it to the ladybirds and that cured them almost immediately - the sex ratio went to normal'.

The researchers identified the bacterium by looking for its DNA. In conjunction with scientists at Rochester University in the United States, they realised that the DNA was similar to that from rickettsia bacteria. Typhus is one rickettsia disease which affects humans and most diseases are found in vertebrates, although they are spread by arthropods.

There is a further puzzle: it is all but impossible to tell the gender of an adult ladybird, so 'how does the rickettsia bacterium identify it is inside a male egg and then zap it, without doing the same to the female?' Dr Majerus asked.