The Undead Walk, but Zombie Bugs Crawl

Body-melting bacteria. Parasites that infiltrate nervous systems. Deadly mind-controlling fungi. Enter the world of zombies...zombified arthropods, that is. An exploration of one of the natural world's most unsettling phenomena, Rise of the Zombie Bugs uncovers the mechanics of real-life invertebrate zombification and the horrifying infections that befall ants, snails, caterpillars, and other creepy-crawlies. The truth, in many cases, is stranger than fiction, and this macabre study of nature's most unnerving evolutionary quirk is no exception.

Join author Mindy Weisberger in a conversation on how nature's zombies are far scarier than those of Hollywood, the potential medical applications of parasitizing organisms, and why Train to Busan is a must-watch zombie flick.

You’re a self-described fan of horror - what was your first encounter with zombie media, and do you think that it had a meaningful impact on your career as a science writer and your decision to write this book?

The first zombies that caught my attention were in Michael Jackson's Thriller music video—those killer dance moves combined with oozing zombie makeup, prosthetics, and fresh-from-the-grave costumes really left a lasting impression! But at the time, I was much more interested in science fiction than I was in science. During film school and into my early career as a director and editor, I worked on music videos and documentaries but had little interest in science storytelling. It was only after I joined the American Museum of Natural History as a media producer that I began to realize that science stories were at least as fascinating and bizarre as the science fiction I knew and loved. Perhaps Thriller and all the other horror movies and books and comics that I devoured fertilized my brain so that later, an affinity for zombifying organisms and zombie science in the natural world could take root and grow. 

Real life is often stranger than fiction. Would you say that this holds true for the “zombies” we see in the natural world versus what Hollywood creates?

Absolutely! For example: In zombie movies, do uninfected people want to be sexually intimate with zombies? Generally, they don't. (With some exceptions. There are always exceptions.) The "classic" Hollywood zombies are gore-spattered reanimated corpses in varying stages of freshness, depending on how long ago they were zombified. They also tend to practice antisocial behaviors such as biting and brain-eating. Their decaying bodies and hyper-aggressiveness ensure (usually) that uninfected people aren't going to be attracted to a zombie. But some real-life zombifiers spread infection by boosting the sexual attractiveness of their zombified hosts. 

A periodical cicada infected by the fungus Massospora cicadina often ends up losing the back third of its abdomen, replacing it with a packed yellowish mass of fungal spores; the fungus then manipulates male zombies into enticing other males to come and mate with them by getting the infected males to flick their wings as female cicadas do. Flies infected by the fungus Entomophthora muscae—the "insect-destroyer"—die with their wings spread wide in a mating pose, and their corpses release volatiles containing compounds called alkanes, which female flies produce when they're ready to mate. The fungus Eryniopsis lampyridarum, which infects goldenrod soldier beetles, also makes beetle corpses open their wing cases and spread their wings wide in mating postures, to attract beetles looking for love.

Zombification as a sexually transmitted disease is a bizarre strategy that is woefully underexplored in movies, but it's highly effective for some real-world zombifiers.

Zombie-causing illnesses in media are often shown as spreading through bites. Can you briefly explain how real-world “zombification” occurs?

There are many different mechanisms for sharing a zombifying infection; it all depends on who's doing the zombifying! Baculoviruses make infected caterpillars climb plants where they expire while dangling head-down. After a caterpillar dies, the virus liquifies the corpse's tissues and the virus-laden goo dribbles down onto vegetation below, where it will be eaten by other caterpillars. The fungus Ophiocordyceps grows a stalk that emerges behind an infected ant's head, releasing spores that drift to the ground and then stick to foraging ants. Periodical cicadas can pick up Massospora fungus as they emerge from the ground as nymphs, or by mating with zombified cicadas, as adults. Snails snarf up zombifying Leucochloridium eggs in bird feces. Some types of wasps zombify cockroaches by stinging them in the brain; others lay their eggs inside caterpillars or glue them to the bodies of spiders, and zombification happens when the wasp larvae are ready to pupate.

Are organisms with zombifying qualities a common facet of the natural world, or are they relatively rare?

Parasitism in general is a lifestyle that is both ancient and widespread. The earliest direct fossil evidence of parasitism is kleptoparasitic worms that lived on the shells of brachiopods, as seen in fossils from a site in China dated to approximately 512 million years ago. Of the roughly 7.7 million known animal species, about 40 percent are estimated to be parasitic. But there are only a few hundred known associations of parasites and hosts where the parasites are actively zombifying the animals that they infect.

Deeply researched, deeply disturbing, and deeply fascinating.

—Ryan North, author of How to Take Over the World: Practical Schemes and Scientific Solutions for the Aspiring Supervillain

Could there ever be any threats to mammals (namely us humans) falling victim to any of these attackers?

It's unlikely that a bug-zombifying wasp, worm, fly, virus, or fungus could evolve to expand its behavior-manipulating repertoire to affect humans, which have more complex brains and behaviors than the parasites' usual victims. Zombifiers that infect invertebrates have had millions of years to develop tools for inhabiting hosts and nudging behavior in ways that make the most of existing invertebrate behaviors and neurochemistry. Some zombifying organisms—such as Ophiocordyceps fungi—are so attuned to host biology that they can only change behavior in one species, even though they can infect and kill multiple species. Infecting humans, evading our immune responses, and manipulating how we behave would require a substantial evolutionary leap for these types of zombifiers. Humans have more to fear from emerging zoonotic diseases caused by viruses that already are known to affect vertebrates, and which can mutate rapidly to infect mammals and then jump between mammal species.

Out of all the zombifying agents in the natural world—from parasitic wasps to mind-altering Ophiocordyceps—which do you find the most terrifying, and why?

There's something about horsehair worms that I find deeply unsettling. YouTube has plenty of videos that show these worms emerging from their hosts, and watching an exceptionally long, threadlike parasite wriggle out of the body of a living cricket as the insect struggles to stay afloat in a pool of water is a sight that you don't easily forget.

Horsehair worms in the phylum Nematomorpha have a life cycle that begins in water, where their eggs hatch. Many of these hatchlings are eaten by aquatic insect larvae, but their story doesn't end there. Swallowed worms hang out inside host larva in a protective cyst, waiting for that host to become an adult, venture onto land, and be eaten by a terrestrial arthropod, such as a cricket or mantid. Inside this definitive host, the worm grows to maturity. 

And it grows a lot. A horsehair worm can grow to be many times the length of its host, where it coils up inside the abdomen like a garden hose. The true horror begins when the worm is ready to come out. Horsehair worms mate in water, so they make their terrestrial hosts seek water and jump in. Once the host is in water, the worm starts to spool out from the host's backside. Its exit can take up to 10 minutes—though, it seems a whole lot longer if you're watching it happen in real time. I can only imagine how long it feels for the insect host. 

What do you think the future of science holds for the study of zombifying parasites? Might there be any useful applications for the properties of these organisms?

There's still so much that scientists are piecing together about the number of parasitic species that actively manipulate host behavior, the evolutionary pressures that shape zombifying abilities, and how zombifiers make their hosts do their bidding. Researchers are also investigating potential medical uses for zombifying compounds. Even simple behaviors require complex coordination of signals throughout the brain and nervous system—that's why it’s so challenging to pinpoint exactly what a zombifier is changing in its host, and how. 

One area of interest is a zombifier’s first target: the immune system. Many zombifying organisms first attack their hosts through immunosuppression. Identifying how they do that could help researchers develop medicines for immune system disorders or for treating neurodegenerative diseases such as Parkinson's disease and multiple sclerosis, in which inflammation from immune responses can amplify symptoms.

Another application for zombifiers is pest mitigation. People have used naturally-occurring pathogens as a form of insect pest control since the 19th century, and zombifiers—which are environmentally safer than chemical pesticides—are a handy weapon in this arsenal. Starting in the 1970s, the World Health Organization and the United Nations’ Food and Agriculture Organization recommended the use of caterpillar-manipulating baculoviruses for keeping populations of pest insects such as spongy moths (Lymantria dispar) in check. In parts of South America, zombifying phorid flies are known to help regulate populations of fire ants, and in the United States, where red imported fire ants are invasive, the US Department of Agriculture is using phorid flies to contain invasive ants and keep them from overwhelming native ant species.

Lastly, name your top three zombie movies.

Just three??? Fine, in no particular order …

Shaun of the Dead cracks me up every time I see it. It's a smart, funny story of a group of very regular people scrambling to survive an urban zombie apocalypse, using only their wits, improvised weapons, and buckets of snark. Plus, the movie answers a very important question: if your best friend becomes a zombie, can you still hang out?

The Girls With All the Gifts is a great adaptation of the M.R. Carey novel, in which an outbreak of a zombifying pathogen—a fungus, for a change!—has an unexpected wrinkle. The fungus forms a symbiotic relationship with certain human hosts, all of whom are children. It's a story of adaptation and evolution as well as a cautionary tale about what happens when society fails to contain a pandemic. Melanie, the half-zombie girl who just wants to do nice things for her teacher, will steal your heart (if she doesn't eat your brain first).

Train to Busan is a wild ride, with hordes of fast-moving zombies that roil like tsunami waves and are absolutely terrifying to watch. Flavoring the drama are nuances of class structure and personal integrity; some characters seek safety by looking out for each other (clearly the better choice for survival under adverse conditions) while others opt for saving their own skin at any cost. The pacing and special effects are outstanding, and you'll be kept on the edge of your seat by the pulse-pounding action sequences—by the end of which nearly everyone that you were rooting for will be dead.

Mindy Weisberger, a science writer and media producer, is a senior science editor at Scholastic. Her work has appeared in numerous outlets, including CNN.com, Live Science, and Scientific American.