Lessons from the Dementor Wasp


Save the Zombie-Makers!

Parasites may seem too gross or too wicked to be worth saving from extinction. Or they may just seem so skilled in their sinister arts that we don’t have to worry about them, since they’ll always find a new victim.

In fact, parasites warrant our concern, right along with their hosts. That’s not to say that we’d better off if smallpox or rinderpest were still running wild. But letting parasites hurtle into oblivion due to our ecological recklessness is a bad idea.

Here’s a case in point: The World Wildlife Fund has just drawn attention to a parasitic wasp, Ampulex dementor, that makes cockroaches its zombified victims. The wasp was found in 2007 in Thailand, and in 2014 a German museum held a contest to give it a species name. Museum goers voted to name it after the soul-sucking dementors in the Harry Potter series.

WWF highlighted A. dementor in a new report on the 139 new species from the Greater Mekong Region that were described in 2014 alone. This region, which includes Cambodia, Laos, Myanmar, Thailand, and Vietnam, is stunningly rich with species. It’s also incredibly productive, yielding a quarter of the world’s catch of freshwater fish. But it’s also under intense pressure, ranking in the top five threatened biodiversity hotspots on Earth. Dams, roads, logging, and hunting are all taking their toll on the species there. Climate change will only add to the threats the Mekong’s species face.

A species like A. dementor is caught in a special bind. We didn’t even know it existed until recently, so it’s hard to know precisely how well the species is faring. No one has a detailed map of its range before human pressure ramped up in the past century, and no one has a corresponding map of its current range.

On top of that, the published scientific literature–pretty much just a single paper published last year–doesn’t even tell us about the particular cockroaches the wasp parasitizes. Does it zombify several species of cockroaches? Does it zombify just one? These questions matter a lot to the survival of A. dementor. If it parasitizes a single rare species, it could become extinct if its host disappears. (While a few species of cockroaches have become global champions by adapting to our homes, the vast majority can only survive in wild forests.)

While we know little about this parasite, the ecological threats to the Greater Mekong Region should make us concerned about it. And losing a species of parasite can be a bad thing. Parasites, for example, are important players in food webs. If they disappear from an ecosystem, their hosts–and the species that are affected by those hosts–may undergo wild swings. If you don’t like cockroaches, the last thing you want is for the parasites that devour them from the inside out to vanish.

Parasites are also worth saving for what they have to teach us. And that’s especially true for wasps like A. dementor. It belongs to a lineage known as Ampulicidae or the cockroach wasps, which contains 200 named species–and probably many more waiting to be discovered. The best known of these species is Ampulex compressa, sometimes called the emerald cockroach wasp. Phenomena readers may be quite familiar with the emerald cockroach wasp, because fellow blogger Ed Yong and I just won’t shut up about it. (I also added an epilogue to my book Parasite Rex pretty much just to write about it.)

The reason we know so much about the emerald cockroach wasp is that a team of researchers led by Frederic Libersat at Ben-Gurion University in Israel have figured out how to rear the wasps in their lab, and for years now they’ve been observing its remarkable skills.

The female emerald cockroach wasp searches for roaches, probably scanning the ground while sniffing the air. The wasp swoops down on the roach and stings it in its abdomen, temporarily paralyzing it. It then delivers a second shot to the head–literally snaking its stinger into the recesses of the cockroach brain. Now the cockroach loses all motivation to do much of anything. You can even shock its leg and it won’t budge on its own. But the wasp can grab onto an antenna and lead it into a burrow.

There, the wasp lays an egg on the roach’s underside and then leaves, sealing the burrow behind it. The egg hatches and the larva sucks on the roach in tick-like fashion for a while, before squirming inside the host’s body to finish off its growth. To keep its host from dying of infections, it smears an antibiotic cocktail on the roach’s inner body wall. The wasp larva forms a cocoon inside the roach, which then finally dies. Later, the fully-grown wasp pokes its head out of the roach, wriggles entirely free, and leaves the burrow.

These wasps may have many lessons for us. Most of their antibiotics are new to science, for example, and so they may be worth investigating further for medicine. The wasps have also evolved a remarkable skill at manipulating the cockroach brain. Figuring out how they do it might tell us more about how the nervous systems of insects work. And it might provide some inspirations for ways to manipulate our own brains–not to turn ourselves into zombies, but to treat psychological disorders.

But almost all the insights we’ve got about cockroach wasps come from a single species. Far from being degenerates, as they were traditionally viewed, parasites can evolve rapidly, hitting on new strategies for conquering their hosts. So it’s entirely possible that A. dementor uses a soul-sucking arsenal that’s significantly different than its cousin species A. compressa. The only way we can enjoy discovering that arsenal is to make sure this species doesn’t vanish first.

This article was written by Carl Zimmer and appeared on his blog, The Loom

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