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Darwin's Classic Monster: The Parasitoid Wasp

"There seems to me too much misery in the world."
​Image: Rene Sylvestersen

Spoiler alert: Fortitude finale content.

​Have you been watching Fortitude? Probably not, huh. Despite being the best thing on TV this past winter/spring, the show hasn't gotten a whole lot of public appreciation/fanfare. It's among the the more uniquely creepy not-quite science-fictions I can remember and its frozen-over horror comes courtesy of one of nature's very real demons. While the Walking Dead chases after zombie mythology and metaphor, Fortitude goes straight to the real fucking walking dead, which exist in nature courtesy of what can only be described as actual monsters: parasitoid wasps.


In fairness, nature has a whole lot of genuine monsters: ​zombie ant fungus, ​ticks, hookworms, fire ants. I could go on. But only one of those monsters was sufficiently horrid enough for Charles Darwin to find it as evidence against the existence of god (well that and cats playing with mice, but wasps have grotesquerie going for them). This is Darwin's almost-full quote regarding the conflict between divinity and waspery, from ​a letter written to the pioneering botanist Asa Gray:

With respect to the theological view of the question: This is always painful to me. I am bewildered. 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 …

So, yes, that's the business of a parasitic wasp, which is actually a large collection of wasp families with somewhat different behaviors and preying on somewhat different victims (though all of them insects … so far). An adult female wasp will lay her eggs within a host through a process known as ovipositing. This will be done via her ovipositor, which is, yes, her stinger. The eggs will grow and develop into larvae, which will feed on their host from the inside-out. Somewhere along the way the host will actually die or be kept in a state very near death until, finally, the little wasp spins a cocoon around and-or within its host, eventually emerging as a proper, normal-seeming wasp.


During its normal adult life, the wasp will feed on nectar from flowers, and no one will really be the wiser until it comes time to reproduce.

That's just the rough sketch, however. The brutality is in the details of the process (which varies a bit wasp species to wasp species). Some wasps are a bit more advanced and, as they inject their eggs into a host, they inject a bunch of other stuff too, including ovarian proteins and venom, which help protect the wasp eggs from the immune system of the host. Some wasps have a mutually beneficial relationship with polydnaviruses, and as part of their whole thing infect the host with these bonus pathogens, which act to manipulate the host's biology to make it more wasp-favorable.

It gets worse.

"The emerald cockroach wasp, for example, turns its cockroach host into an ill-fated nanny," ​writes Christie Wilcox at Discovery. "With carefully placed stings which inject a venom cocktail into the roach's brain, the wasp puts the roach into a zombie-like state where it happily follows its attacker to a dark chamber underground. The wasp then lays her eggs in the complacent roach and seals it in its tomb. Soon enough, the eggs hatch, eat, pupate, and emerge while the cockroach sits and waits for its body to be consumed."

And worse.

Once the eggs of the parasitic wasp Cotesia glomerata hatch within a host caterpillar and the resulting larvae go to town on said caterpillar's internal organs, the host is kept alive, in a sense. Through a so-far unknown process, the caterpillar turns into an "undead bodyguard," protecting the wasp larvae and even spinning them a nice silk web. Only then is the caterpillar allowed the peace of sweet death.

Image: Franz Vanoosthuyse

A February ​study in the Proceedings of the Royal Society B. describes the life-cycle of the ladybug-preying Dinocampus coccinellae in never before seen detail. Its title bears repeating: "Who is the puppet master? Replication of a parasitic wasp-associated virus correlates with host behaviour manipulation."

"The female D. coccinellae lays its eggs in the ladybeetle and the parasitoid larvae develop inside the body of the coccinellid host," the paper explains. "After about 20 days, a single prepupa egresses and spins a cocoon between the ladybeetle's legs. At this time, the ladybeetle's behaviour is modified: it remains static and displays tremors. Throughout parasitoid pupation, the host remains alive and positioned on top of the parasitoid cocoon, serving as a bodyguard to protect the parasitoid cocoon from predation [8]. After a week, the adult parasitoid emerges from the cocoon. Some ladybeetles recover from the paralysis, resume feeding and can even reproduce."

I'm not sure the host's potential survival makes the process that much less horrid, but such a possibility reminds us that what we're dealing with here is the result of parasitism, which by no means demands death. Parasites are among the most well-adapted creatures known, which brings us back to Darwin's waning faith and slight reassurance, offered in the same god-questioning letter as above: "I am inclined to look at everything as resulting from designed laws, with the details, whether good or bad, left to the working out of what we may call chance." Or luck, really bad luck.