Why would a sterile male cricket mate with an infertile female? On the surface, this behaviour makes no sense: sex takes energy and effort, and there’s nothing in it for either of these partners. Neither one can foster the next generation.

Shelley Adamo from Dalhousie University has the answer. Her team have shown that one particular insect virus can sterilise crickets, but also change their behaviour so they continue to mate with each other. By doing so, they pass the virus on to uninfected hosts.

This virus is the latest example of parasitic mind control—a topic that I’ve covered regularly on this blog, and that I spoke about at the recent TED2014 conference.

And there are viruses. The rabies virus is a classic manipulator, which occasionally makes its hosts more aggressive so they spread the virus through bites. There’s a type of baculovirus that compels caterpillars to climb into trees, so their disintegrating bodies send a rain of viral particles onto plants below. And now we have the less-than-catchily named IIV-6/CrIV—a sexually transmitted virus that acts as an aphrodisiac for infertile crickets.

Adamo’s team first noticed the virus when some of the female crickets in their lab stopped laying eggs. They dissected the uncooperative insects and found that their fat bodies—organs that store fat and make proteins—were swollen and blue. Under a microscope, these organs were loaded with viruses, which were packed into dense crystals that gave off a blue sheen.

The team identified the viruses as IIV-6/CrIV, which had been discovered in crickets just over a decade ago. It effectively sterilises any cricket that it infects. A typical female carries more than a hundred eggs in her reproductive system, but these infected ones had fewer than ten. Meanwhile, the males had plenty of sperm, but these cells couldn’t swim.

Still, the males actually became quicker to court nearby females and the females continued to mate with them. These continuing hook-ups don’t benefit the crickets. But the virus, which turned out to be sexually transmitted, gets an easy ride into fresh, uninfected hosts.

Adamo found that IIV-6/CrIV also changes the crickets’ behaviour in other subtle ways. A sick cricket will typically try to ward off an invading virus by releasing immune chemicals from its fat body. While these protective substances do their work, the cricket stops eating and trying to attract mates.

But a cricket that’s infected by IIV-6/CrIV doesn’t show these “sickness behaviours”. The virus targets the fat bodies, preventing them from firing off their defensive salvos. It also seems to compel the insect to continue eating and courting. That makes sense: as Adamo writes, “a host that behaves as if it were sick will not attract mates” and a cricket that doesn’t mate is a dead-end for the virus.

There are almost certainly many more examples of sexually-transmitted parasites that subvert their hosts’ behaviour. There’s another virus called Hz-2v, which infects and sterilises the corn earworm moth, but also ensures that its hosts carry on mating. And Toxoplasma gondii, besides turning rats into cat-seeking missiles, also makes male rats more attractive to females.