Female spiders are famous for their ruthless reproductive strategies โ€” but in fact, they're at the mercy of a parasitic bug that forces them to have only female offspring.

Wolbachia bacteria infect up to two-thirds of insect species. They are transmitted through eggs, and so need their host to have as many females as possible. As a result, the bacteria have evolved a bag of dirty tricks to maximise their chances of making it to the next generation.

The parasite can drive their hosts to virgin births, which result in females only, gender-bend lads into ladies or simply kill males early in development. But until now nobody knew if Wolbachia could wreak havoc on spiders too.

Bram Vanthournout at Ghent University in Belgium and colleagues have found that infected female dwarf spiders produce more females than those without Wolbachia. Giving the spiders antibiotics restored a normal sex ratio, proving that the previous female bias really was Wolbachia's work (BMC Evolutionary Biology, DOI: 10.1186/1471-2148-11-15).

Because female-dominated clutches were smaller, the researchers suspect that the bugs kill off males long before birth.

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Vanthournout reckons that the bacteria may also be behind virgin births, which could even give spiders an evolutionary advantage. "Some social spiders need very few males, so male competition is high," he says. Wolbachia might spare females the trouble of producing males who will never mate.

This article originally appeared in New Scientist