"The insect world is full of species of parasitic wasps that spend their infancy eating other insects alive. And for reasons that scientists don’t fully understand, they have repeatedly adopted and tamed wild, disease-causing viruses and turned them into biological weapons. Half a dozen examples already are described, and new research hints at many more."
Some wasps are called 'parasitoids' because they lay their eggs in still-living caterpillars. The eggs develop into larvae that eat the caterpillar from the inside.
But turnabout is fair play. Sometimes, other wasps called 'hyperparasitoids' lay their eggs in the larvae of these parasitoids!
The caterpillars also fight back. Their immune system detects the wasp's eggs, and they will do things like surround the eggs in a layer of tissue that chokes them.
But many parasitoid wasps have a trick to stop this. They deploy viruses that infect the caterpillar and affect its behavior in various ways - for example, slowing its immune response to the implanted eggs.
These viruses can become so deeply symbiotic with the wasps that their genetic code becomes part of the wasp's DNA. So every wasp comes born with the ability to produce these viruses. They're called 'polydnaviruses'.
In fact some wasps are symbiotic with two kinds of virus. One kind, on its own, would quickly kill the caterpillar - not good for the wasp. The other kind keeps the first kind under control.
And I'm immensely simplifying things here. There are over 25,000 species of parasitoid wasps, so there's a huge variety of things that happen, which scientists are just starting to understand! I had fun reading this:
Why such diversity? I think it's just that there are so many plants! So insect larvae like caterpillars naturally tend to feed on them... in turn providing a big food source for parasitoids, and so on.
Given that pretty much all insect species, and beyond into spiders and more, are attacked by parasitoid wasps, and that for most hosts there are both host-specific and generic parasitoid wasp species, it’s been estimated that there are more parasitoid wasps than all other insect species combined. Their usually cryptic larval life stages and often brief adult stages may be behind the severe undercounting.
See:
"Quantifying the unquantifiable: why Hymenoptera, not Coleoptera, is the most speciose animal order", Forbes et al. 2018 https://doi.org/10.1186/s12898-018-0176-x
X-ray microscopy of fossil parasitoid wasps to study the evolution of echolocation for finding hosts:
"†Kryptovelona carstengroehni gen. et sp. nov. and †Orussus juttagroehnae sp. nov. are the first female members of the parasitoid wasp family Orussidae recorded from Baltic amber. We describe them, including relevant parts of the internal anatomy examined with synchrotron scanning. The fossils display a number of modifications in the antennae and foreleg correlated with the specialized host-detection mechanism, and in the ovipositor apparatus, as well as in the thorax and abdomen for accommodating the internalized ovipositor."
"By comparing the new Baltic amber taxa with †Cretorussus, it is possible to trace the progressive refinement of the echolocation mechanism through reductions in the number of antennomeres and foreleg tarsomeres."