The bracoviruses can't independently reproduce because they lack genes for making the protein coats that give them form and structure. Those coat genes didn't vanish. In 2009, Anne Bezier and Jean-Michel Drezen from Francois Rabelais University showed that they exist within the wasp genomes. The bracoviruses aren’t just allies for the wasps: They are part of the wasps.

Based on the diversity of these viral genes within different species of braconid wasps, Bezier and Drezen estimated that they must have entered the wasp genome around 100 million years ago, before the braconid dynasty expanded into its current lush state. Back then, an ancient virus infected an ancient wasp, inserted its genes among those of its host, and created a partnership that has been dooming caterpillars ever since.

More recently, Gaelen Burke and Michael Strand from University of Georgia showed that the wasp genomes contain two separate clusters of viral genes. The first is a replication set, which the wasps use to turn their ovaries into virus-making factories. The second is a virulence set, which attacks the caterpillars. But when the wasps build the viruses, they fill them only with the virulence genes, not the replication ones. That’s why the resulting particles can attack caterpillars, but can’t reproduce or spread to new hosts. They are fully domesticated.

The caterpillars aren't just helpless victims in this drama. Sometimes, they successfully fight off the wasp-and-virus tag team. Other times, the wasps screw up, by attacking caterpillars of the wrong species, against whom their particular viruses are useless. Either way, caterpillars occasionally survive their encounters with braconids, but still end up with swarms of bracoviruses in their bodies. What happens then? Since those viruses were originally part of one insect genome (the wasp’s), could they find their way into another (the caterpillar’s)?

The answer is yes. Last year, Sean Schneider and James Thomas from the University of Washington found evidence of bracovirus genes in the genomes of the silk moth and the monarch butterfly. The duo described the wasps as “accidental genetic engineers,” implanting the genomes of their caterpillar victims with their own (viral) DNA. In other words, one insect was genetically modifying another with viral genes, via a sting.

“What’s kind of funny is that such a species as iconic as the monarch has been genetically modified by the parasitic wasp virus and can thus be considered as a natural GMO,” says Drezen, in an email. He, together with Salvador Herrero from the University of Valencia, has now found similar genes in a wider range of butterfly and moth species, including important pests like the beet armyworm and fall armyworm. And they’ve found that these sequences may not just be passive hitchhikers.