PARASITOID WASPS

By Jonathan Wojcik


Source



  For all their bizarre, sometimes grotesque adaptations, parasitic organisms are seldom directly lethal to their hosts, having adapted to live in a careful balance with the animals they depend on as food and shelter. Parasitoids, however, are a very different story. These are defined as parasites which cannot complete their life cycle without the death of the host organism, transitioning at some point from parasite, to predator. While many varied life forms practice this sort of internal assassination, none are such devoted parasitoids as the insects we call wasps.



A tobacco hornworm covered in wasp cocoons - source



  While most humans are familiar with wasps as angry, colonial, pepsi-diving summer pests, the majority of species are tiny, barely noticeable parasites of other insects or arachnids, integral in the population management of their fellow arthropoda. For nearly any insect you can name, a specialized wasp exists to destroy it from within, including every insect we humans recognize as a crop pest. As you can read more about in my first ever article for cracked.com, many forms of plant life have even evolved chemical signals to communicate with parasitoid wasps as a natural extermination service.

  While these eerie and elegant creatures number in the tens of thousands, I've selected just ahandful of the most famous and unusual examples for a small peek into their beautifully chilling lives.



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Mutillidae: the flightless cuckoos



  Common in dry, sandy climates, "velvet ants" are so named for the ant-like appearance and dense fur of the larger, entirely wingless females, who pack a sting so painful they have also been dubbed "cow killers" in parts of the United States. A lack of wings and an exceptionally thick, rock-hard exoskeleton allows these brood parasitoids to withstand most attacks from fellow bees and wasps, marching straight into their nests and laying their own eggs alongside the host's. Velvet ant larvae will begin by feeding on host food provisions, moving on to parasitize and ultimately consume the host larvae. Many of their victims are parasites themselves, making some velvet ants hyperparasitic.

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Mymaridae: invisible fairies of death



   Also called fairy wasps or fairyflies, Mymaridae include the tiniest insects known to man, their wings not so much sustaining their flight as simply steering them as they float through the air like dust motes. Many species are even partially or fully aquatic, employing their wings as swimming paddles. The small size of these "fairies" allows them to parasitize other insects during the egg stage, laying several of their own within a single host egg. Young will mature and even mate (with their own siblings) before breaking free of the host's egg shell, and target such a diverse range of insects as beetles, grasshoppers, true bugs, flies and even lice.

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Hymenoepimecis argyraphaga's spider straight jacket



   While there are many wasps devoted to attacking spiders, this particular species boasts an exceptionally devious strategy. Targeting only the web-building spider Plesiometa argra, the adult female lays a single egg on the host's abdomen. After up to two weeks of feeding on the spider's blood, the larva administers a chemical to essentially reprogram the host's behavior. It's normal for a spider to spin an entire new web on an almost nightly basis, but when our parasitized spider begins its next construction project, something goes terribly, terribly awry; it stops after the first few steps of the process and starts back at the beginning, repeating the same actions until it has built only a tiny, dense, cocoon-like web. The spider traps itself within this small prison, and waits motionlessly as the wasp consumes it and pupates within the protective bag.

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Ampulex compressa & the cockroach lobotomy



   Also known as the emerald cockroach wasp, this mere invertebrate was administering delicate brain surgery before our kind even started to bang rocks together. With a single jab of her stinger, the female penetrates a cockroach's brain and destroys precisely the right area to disable its escape reflex. Next, she need only tug her victim's antenna like a dog leash, guiding it dumbly into her burrow. There, she lays a single egg on the host's body and seals the tunnel entrance behind her. Over the next two weeks, the larval wasp will slowly eat the apathetic cockroach alive, prolonging its life through the process by saving its most vital tissues for last.

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Glyptapanteles & the necropillar



   When larval Glyptapanteles wasps tunnel back out of their caterpillar host, they still need to cocoon themselves and undergo metamorphosis into adults, leaving them open to attack from any number of predatory insects - even fellow parasitoids looking to exact some ironic justice. To better survive in this vulnerable state, at least a single wasp larva will remain behind in the caterpillar's body, keeping their half-eaten, barely living victim under control as a sort of "zombie" bodyguard.

   Over the following weeks, the caterpillar will devote the last of its energy to protecting the very things that were previously drilling through its innards, perched over the cocoons like an otherworldly mother bird, blanketing them with its own silk and flailing madly to repel other insects. By the time the wasps emerge as adults, their undead servitor - and the siblings still controlling it - will finally perish. We still aren't certain how the wasps choose which larvae remain in the caterpillar, but I personally like to think that it may be determined from birth, making certain larvae a sort of specialized, zombie-piloting caste.

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Dinocampus coccinellae & the indentured ladybug

Source



   Another wasp employing a "zombie guard," D. coccinellae targets female ladybird beetles or "ladybugs," one of mankind's arbitrarily favored arthropods. A single larva will develop in the host's abdomen, breaking out through the posterior (ouch) and spinning a cocoon at the ladybird's feet. The beetle is left in a paralyzed state through the pupation process, tightly clutching the cocoon and convulsing at irregular intervals, discouraging not only predatory insects but larger vertebrates as well; a ladybird's famously endearing polka dots didn't evolve to make it cute, but to warn other animals that its contents taste like burning vomit.



   D. coccinellae is a tad less beneficial to us humans than other parasitoids, as ladybird beetles are themselves an important predator of crop-destroying aphids. Fortunately, almost a quarter of parasitized ladies make a surprising recovery from being eaten alive and enslaved.

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Ichneumon eumerus's inside inside job



   In my list of the coolest and strangest caterpillars, I described how the young Alcon blue butterfly (above) goes undercover in the nests of ants, deceiving them into feeding and protecting it as they would their own queen. While these crafty creatures are guaranteed protection from most of their predators, a certain wasp happens to be quite a bit sneakier.

BBC Video Here



   The female Ichneumon eumerus can smell the presence of blue butterfly larvae from well outside an ant colony, and once inside, releases a pheromone to confuse the ants into attacking one another. Slipping through the ensuing chaos, the wasp implants a single egg into each freeloading caterpillar, granting her own young the protection of an entire ant army; a parasite hidden in a parasite hidden in plain view.

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The Inner Conflicts of Trigonalids

Simon Van Noort - Waspweb.org - the bizarre abdomen shape functions like a hole punch on plant leaves.



   Easily the weirdest of them all, the rarely seen Trigonalids take a tapeworm-like approach to parasitism, planting their eggs along the edges of leaves for hosts such as caterpillars and sawfly larvae to mistakenly ingest. The purpose of such an indirect strategy is unclear, as it dooms the vast majority of young to slowly starve in their own eggs, and even the lucky few who do get swallowed aren't out of the woods yet, as it's not actually the caterpillars they're after at all; armed with sharp, oversized jaws, the Trigonalid larvae prey entirely upon the larvae of other parasitoids, including both their fellow wasps and parasitoid flies. In a host body with no other parasites, the Trigonalid larva is doomed to starvation, completely unequipped to draw nourishment from its living home.

Early instar Trigonalid larva (left) showing the predatory jaws.



   A few species of Trigonalid take a different but no less circuitous route; caterpillars are loaded with trigonalid eggs, but these hatch only if the caterpillar is captured by non-parasitic wasps who will feed its pre-chewed remains to larvae in their nests, who ingest the eggs with their meal and become hosts themselves, even further mirroring the habits of tapeworms.

I never would have known about these incredible insects if not for Scrubmuncher, a blog packed with some of the weirdest, most remarkable stories from the insect world and more!

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The Secret Weapon


   Perhaps the most bizarre and startling characteristic of the parasitoid wasps isn't just their gruesome method of childcare, their worldwide diversity, their symbiosis with plant life or even the cerebral rewiring we've seen from several species, but their deeply, deeply intimate relationship with another, far more ancient parasitic force.

  

Polydnaviruses are found in a significant portion of braconid and ichneumon wasps, themselves comprising the majority of parasitoid species. Far beyond your typical symbiosis, the wasp's own DNA contains the complete genome of its respective polydnavirus, information used by specialized cells in the wasp's reproductive system to assemble viral bodies from scratch. These are injected into hosts alongside the insect's eggs, weakening the victim's immune system and altering its metabolic processes in ways which benefit the parasite.

   Unlike independent viruses, polydnavirus particles lack the full genetic information necessary to duplicate themselves, existing only when the time comes for the wasp's body to construct them. The insect essentially retains exclusive rights to the complete viral "recipe," averting the potential destruction of a precious host (or even its own larvae) were the virus to multiply on its own.

   Theories vary on how this relationship may have come about. It seems likely that an ancient viral infection evolved a partnership with an early wasp host, while others have proposed that wasp DNA evolved its own viral aspect, independently or from "borrowed" viral genes.

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