Parasitoid Wasps Are Walking Horror Stories

Love it or hate it, Halloween's upon us, and so is the proliferation of horror movies on many of your 800 cable channels. Most everyone seems to have a favorite horror genre, whether it's the slasher, psychological thriller, or classic monster movie. The boon of horror fiction is, of course, that you can turn off the movie or close the book if it gets too scary. Nature, however, has written a terrifying story we can't turn off at will, and it stars the parasitoid wasp.

Wasps of the Ichneumonidae family are parasitoids, meaning that unlike your run-of-the-mill parasites, their behavior often results in the sterilization or death of their host. Many ichneumonid species measure about a centimeter in length, so they often go unnoticed by humans. Despite their diminutive size, these creatures could be the inspiration behind a sci-fi or medical thriller flick.

First, many parasitoid wasps are parthenogenetic, meaning their species are entirely female and essentially clone themselves using unfertilized eggs. After a female's eggs develop, she uses her long stinger-like ovipositor to inject the eggs and a soupy substance into a caterpillar, insect, spider, or other suitable host. The eggs hatch into larvae, which infest the host and feed on its blood, internal tissues, and non-essential organs in order to keep the host alive long enough to ensure their successful growth. The larvae then cut through the host's body and exit, typically causing the host's eventual death.

The egg solution injected into the host also contains a polydnavirus-possibly developed from the wasp's genes-that hijacks the host's neurological functions while it's being slowly consumed. A virus' function seems to vary based on the wasp species. At the most basic level, the virus breaks down a host's immune system until the larvae have hatched and gained enough strength to overpower the immune system on their own. Some viruses compel a mortally wounded caterpillar to spin a protective web around the just-hatched larvae to grant an added level of protection. Wasp viruses injected into spiders force them to spin a web high above a forest floor; when the web is complete a wasp larva hatches, kills the spider, drains it of its blood, and spins its own pupa in the spider's web, protected from any threats on the ground below. (If you have a half hour or so, this presentation goes over a whole bunch of wasp virus scenarios as well as other strange parasites. Plus, who wouldn't want this guy's "I <3 Parasites" shirt!?)

The icing on this story is that parasitoid wasps are also hyperparasitic, meaning they are prone to parasitizing their own eggs or larvae. So, when Ichneumonid larvae build pupae to protect themselves while morphing into an adult wasp, they're really trying to protect themselves from being parasitized themselves as much as against predators.

Thankfully, all of this micro-activity goes on outside the scope of our normal days, but scientists have been fascinated by parasitoid wasps for centuries. Darwin was troubled by Ichneumonids and said their behavior dramatically shook his faith in a supposedly benevolent creator. Ichneumonids are also beneficial in many areas of the world and control the populations of crop pests like tobacco hornworm and tomato hornworm caterpillars.

I think we can all agree with the words of an annoying internet meme: Damn nature, you scary!

Image credits: Alex Popovkin / CC BY 2.0 | Christoph Rupprecht / CC BY-SA 2.0