Love them or hate them, insects have mastered a myriad of strategies to ensure their permanence in our world. Even those with an affinity for bugs have to admit that some of these strategies are objectively horrifying. Take, for example, parasitoid wasps. Parasitoid wasps are true alien invaders, hijacking the bodies of living invertebrate hosts to rear their young, much like Ridley Scott’s iconic Xenomorph.
Parasitoid wasp larvae will feed on their host until they pupate and eventually emerge as adults, at which point their host, usually living until now, finally expires. There are thousands of species of parasitoid wasps, each one specializing on a different insect order, making them an excellent form of biological pest control. Some, like giant ichneumons, are large and conspicuous, with formidable-looking ovipositors capable of drilling through wood to reach their victim, ehrr… host. Others are only noticed by the what they leave behind. Take, for example, aphid mummies.
Aphid mummies are the empty, bloated husks of what were once living aphids. The braconid wasps in the Aphidius genus are aphid specialists, depositing their eggs into the living body of aphid nymphs.
The wasp larvae will hatch inside the aphid, feeding on the innards while avoiding vital organs; after all, a dead host cannot provide the sustenance that a live one can. Eventually, the wasp larvae will pupate inside the aphid, ultimately killing the host in the process. Since insects do not feed during the pupal stage, a living food source is no longer necessary. Once pupal development is complete, the wasp will cut an “escape hatch” in the aphid and emerge as an adult, leaving only the aphid mummy behind.
The type of parasitism that these wasps engage in depend entirely on the species; it’s not a one-size-fits-all strategy. Endoparasites (endo meaning inside) develop inside the host, leaving the host alive to feed, grow and develop until the wasp larvae emerge.
Endoparasitic wasps utilize a variety of strategies in order to bypass their hosts immune system, including venom, ovarian proteins, and viruses. We tend to think of viruses as invading pathogens that make us sick, like chickenpox, the seasonal flu, and of course, COVID-19 and other coronaviruses. But many endoparasticic wasps have a mutualistic relationship with a family of viruses known as polydnaviruses. The virus can only replicate in the oviducts of a female parasitoid wasp, so the virus needs the wasp. In turn, the wasp needs the virus to inject into a host to prevent the host immune system from attacking her offspring as they develop. These viruses evolved with parasitoid wasps so long ago that the viral DNA is incorporated into the wasp genome.
Ectoparasitic wasps, on the other hand, develop outside the host. In order to achieve this, the female wasp must paralyze her host immediately. There is no better example of this than the formidable tarantula hawk wasp (shown above).
The tarantula hawk wasp (Pepsis sp.) will drag her prey to a specially prepared burrow, where a single egg is laid on the spider’s abdomen, and the entrance is covered.
When the wasp larva hatches, it gnaws a small hole in the spider’s abdomen, then enters and feeds voraciously, avoiding vital organs for as long as possible to keep the spider alive. After several weeks, the larva pupates. Finally, the wasp becomes an adult and emerges from the spider’s abdomen to continue the life cycle.
Parasitoid wasps are fascinating and, let’s face it, objectively dreadful. There are even species of parasitoid wasp that parasitize other parasitoid wasps as those parasitoid wasps are parasitizing a host insect, creating a wasp inception! Their life cycle even horrified the likes of Charles Darwin, who wrote the following:
“I had no intention to write atheistically, but I own that I cannot see as plainly as others do, and as I should wish to do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars […]”
Take notes, David Cronenberg.