The parasitic bacterium Wolbachia is common in insects around the world, which makes it perhaps the most common reproductive parasite on Earth. And it doesn’t like males.
Wolbachia bacteria infect the reproductive organs (ovaries and testes) of insects. They infect other organs too, but their effect on insect reproduction is to me the most interesting aspect of this little parasite. You see, the bacteria travel in insect eggs but not insect sperm: sperm is just too small for it to get around. As a result of this, Wolbachia has had some extraordinary effects on insect reproduction.
First, and most simply, Wolbachia kills male insects – often before they’re born. For one species of butterfly, the great eggfly / blue moon butterfly, that meant 100 times more female butterflies than male butterflies. But Wolbachia may also lead to male insects turning into females at the larval stage, resulting in the same distorted sex ratios. The bacteria in some species can even prevent infected males from reproducing with uninfected females – which means that infected females have more offspring than uninfected ones and thus the bacteria is spread further faster. It all sounds a lot like the comic book series Y, The Last Man…
Evolution, as always, fights back against the challenge. Some wasp species have evolved parthogenesis: reproduction without males. The butterfly I mentioned earlier actually mutated a genetic immunity to Wolbachia, at least in the Samoan population. This was one of the fastest recorded species changes by natural selection (it took five years) and you can read all about it in the linked article below.
For most species, there is little defence. Because of this, Wolbachia is being investigated as a tool for mass insect control: release a bunch of mosquitoes infected with the bacteria and watch the population crash.
The Generalist Academy 
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Wolbachia creates a key asymmetry: infected males are sterile with uninfected females, but not the converse.
There are two different ways of using Wolbachia with mosquitoes. One, to reduce population, release infected males, who are effectively sterile. Wolbachia doesn’t spread or continue in this use.
The other, release infected females and males, who reproduce both among themselves and by infected females with uninfected males — creating infected offspring. The asymmetry creates a “gene drive” effect promoting Wolbachia infection. This does not reduce mosquito population; the point of this is that Wolbachia interferes with multiplication and transmission of dengue and malaria.
Thank you for the clarification eub; you’re right, it’s a great multi-purpose bacterium 🙂