The Biological Warfare of Wasps

All 186 wasp species recently discovered in Costa Rica are lethal killers—parasites that kill their caterpillar hosts in a macabre manner.

(Photos: Courtesy José L. Fernández-Triana)

Feb 28, 2014· 3 MIN READ
Richard Conniff is the author of House of Lost Worlds: Dinosaurs, Dynasties, and the Story of Life on Earth and other books.

Earlier this year, a study came out describing a new plant species in the Andes that is the sole home of an estimated 40 or 50 kinds of insects. I thought that had a certain “wow” factor. It also seemed like a chance to write about “keystone species”—the ones on which whole ecosystems depend—and the ripple effects when such a species goes extinct.

So I asked for a comment from evolutionary ecologist Dan Janzen at the University of Pennsylvania, and he responded with characteristic pith and vinegar. “You tell me what species on the planet is not an important part of the life cycle of many tens to hundreds of other species,” he demanded. “As for so-called keystone species, that simply means a species whose removal or other kind of perturbation happens to create a set of ripples big enough for a two-meter-tall, diurnal, nearly deaf, nearly dumb, nearly odor-incompetent, nearly taste-incompetent, urban invasive species”—that would be us, Homo sapiens—“to see, or bother to see, the ripple." I let that project slide.

But a new study out this week in the journal Zookeys gives me a better idea of what Janzen was getting at. It describes 186 new species in northwestern Costa Rica, all parasitic wasps, the largest of them about half the length of my pinkie nail, and most—at one to five millimeters—much smaller. They’re certainly too small for most people to notice and too obscure to care about—except perhaps that each is a deadly master of a macabre kind of biological warfare.

First, the background. For more than 30 years, ecologists and parataxonomists—the foot soldiers in the science of species discovery—have been methodically prowling the 1,200 square kilometers of Costa Rica’s Area de Conservación Guanacaste, picking caterpillars off vegetation. The caterpillars—500,000 of them so far—end up in individual plastic bags strung together on ropes, like Chinese lanterns, in rearing barns.

That may sound odd. But the idea—part of a long-term research project established by Janzen—is to see what emerges and thus how the Guanacaste ecosystem functions. Sometimes it’s a moth or butterfly, and the researchers have so far identified 10,000 species living in Guanacaste. But often what emerges instead are parasitic wasps, which have grown strong by eating the host caterpillar alive, from the inside out.

These parasites are the offspring of an adult female, which has diligently sought out a caterpillar of the right species and used her stinger to inject her victim with a virus. Each female wasp then lays her eggs on the body of her victim, says José Fernández-Triana, the lead author of the new study, and “the virus shuts down the immune system of the caterpillar to prevent it from encapsulating the eggs and killing them.”

When the wasp larvae emerge from the egg, they begin to eat the living caterpillar. In some cases, the virus will also manipulate the caterpillar’s behavior, so that when the time comes for the tiny wasp larvae to emerge and build their cocoons, the half-eaten caterpillar will actually protect them from ants or other predators.

“What we’re finding,” says Fernández-Triana, “is that 90 percent of the species of wasp were parasitizing either one species or a few species in the same genus. It’s chemical—no, biological—warfare. You’re injecting a virus that is only going to work on the one or two target caterpillar species.”

The new study represents only a fraction of the diversity at Guanacaste. It looks at just a single genus of wasps, Apanteles, which was known until recently from just three species in Costa Rica. The study brings the Apanteles total up to 205 species, including 19 described previously, as well as 186 new species.

Fernández-Triana and his coauthors combined three techniques to make their species identifications: DNA bar coding, using a small section of the mitochondrial genome, was accurate enough to detect a probable species difference 90 percent of the time. Details of body configuration, or morphology, were also essential, and Fernández-Triana had to make 49 body measurements on each of several hundred microscopic specimens. And ecological factors—particularly the kind of caterpillar species from which the wasp has emerged—helped triangulate the species diagnosis.

The study—one of many more such studies out of Guanacaste—suggests the incredible diversity of species living on other species. It puts flesh on the 18th-century verse by Jonathan Swift: “So nat'ralists observe, a flea / Hath smaller fleas that on him prey; / And these have smaller fleas to bite 'em. / And so proceeds Ad infinitum."

But let’s get back to Janzen, a coauthor of the new study, who fulminates, in Swiftian fashion, against human folly: We manage not to care, or we pretend not to notice, he says, that the “extinction, whether local or regional or total, of any species will impact the lives of a number of other species.” Humans have been doing that, he says, with “the attendant shrug of the shoulders,” since the Pleistocene. “We specialize in the elimination of species to make space (of all sorts) for us and our domesticates, and we are now busily polishing off the entire field to zero competition, with very few of ‘them,’ leaving ourselves as the last competitor standing. Kind of obvious how that is going to end.”

On a less misanthropic note, the new study takes the unusual step of naming all of the new species after the parataxonomists, by first and last name, who did the grunt work of collecting and rearing them. The new names can be unwieldy—there’s an Apanteles guadaluperodriguezae and an A. hazelcambroneroae, for instance. But they serve as a reminder to the Guanacaste staff members of how much their own fate—and perhaps also ours—may be tied up in the lives of the obscure species they have discovered.