Parasite Rex

8 How to Live in a Parasitic World

On my visit to Santa Barbara, after Kevin Lafferty had showed me how parasites hold sway over a salt marsh, I spent a morning with one of Armand Kuris’s graduate students, a young man named Mark Torchin. He led me through one of the marine biology labs to a blue door in the corner. A sign marked QUARANTINE plastered the door. When Torchin opened the door and we walked into the dark, I could hear what sounded like a flowing creek. Torchin found the switch to the cold fluorescent lights, which shone down on a high table running the length of the room. On the left side were aquarium tanks full of water, with crabs skittering around inside on broken pieces of white mesh. On the right were tubs with cups stacked in them, each holding a single crab in a scoop of water. The sound of a creek came from the system of pipes that pumped sea water in from the lagoon just outside, flowing into the tanks and dribbling onto the table before heading down a drain, to flow back to the Pacific.

The crabs were Carcinus maenas, the European green crab. Some were the size of teacups, some only of shot glasses. If you walk along the coast of northern California and the Pacific Northwest, you may find green crabs, and that’s a fact that has certain people terrified. Before 1991, there were no green crabs on the California coast. Its original range was along the beaches of Europe. There it was a voracious creature; in Great Britain, biologists have watched single crabs eat forty cockles, each half an inch long, in a single day. For thousands—perhaps millions—of years, the rest of the world was spared from the green crab’s hunger, but that changed when humans invented ships. The green crab sheds thousands of nearly invisible larvae into the water, which can be easily sucked into the holds of ships when they take on ballast water. Perhaps two hundred years ago, some ship traveling to the American colonies carried green crabs to the New World. They quickly began to spread along the coast of the eastern United States, devouring shellfish in northern New England and Canada. The softshell clam, once the basis of a whole fishing industry in New England, disappeared altogether.

The crabs traveled to South Africa and Australia as well, but for centuries the west coast of the United States was spared. Despite all the ships traveling there from Europe and the eastern United States, it wasn’t until 1991 that a fisherman near San Francisco pulled up a green crab in his nets for the first time. As soon as reports spread around marine biology circles, scientists became gloomy. Almost every species of shellfish around San Francisco was suitable prey, and if the green crab should spread along the coast in the ships that traveled down to Los Angeles or up to the northwest, it could spread to new habitats, feasting on oysters, Dungeness crabs, and other valuable creatures. The burrows it dug might destabilize dikes, levees, and channels, causing even more damage. “It’s a disaster,” says Armand Kuris. “It’s all the things you want in a worst-case scenario.”

The green crabs in the quarantined lab in Santa Barbara skittered in their tanks. Some had ghostly white claws growing in the place where they had lost a previous one. And some, as I could see when Torchin pulled them out of the water and turned them upside down, their legs and claws windmilling around helplessly, carried a sac on their abdomen the color of butterscotch. They looked like normal crabs, but they had been transformed into something else. They were filled with Sacculina carcini, that degenerate parasitic barnacle of Ray Lankester’s nightmares. Torchin, Lafferty, and Kuris were trying to use Sacculina to save the Pacific coast from the green crab.

In the late 1800s, scientists sometimes referred to parasitology as medical zoology. They were referring to the way they had to understand parasites as real organisms, with natural histories of their own, before they could try to fight the diseases the parasites caused. Now, a century later, the term has taken on a new life. Now the patient isn’t a person but the natural world. Alien species are spreading uncontrollably across continents and seas; native plants and animals are falling prey to new diseases; habitats are disappearing as forests turn to stumps and coastlines to condominiums. As ecosystems have faltered, scientists have come to recognize that parasites are important to their health. A healthy ecosystem is riddled with parasites, and in some cases, an ecosystem may even depend on parasites for its health. As humans alter the world, tipping the biosphere out of kilter, it may be possible to enlist parasites to help us undo some of our mistakes and perhaps keep us from making new ones.

Scientists first conceived of using parasites against pests in the 1880s. The original idea was simple. A parasite is a cheap, never-ending pest-killer. It can seek out its host and invade it, fighting off the host’s immune system and, in many cases, leaving the host dead. Farmers who use pesticides have to spray their plants at least once a year, but parasites keep regenerating and tracking down new hosts. Simply sow the parasite, the argument went, and your troubles are over. In the early part of this century, farmers were having exactly the sort of success that had been promised. Scales and beetles and other pests were destroyed by wasps and flies and other sorts of parasites. The parasites couldn’t eradicate the pests completely, but they no longer threatened to wipe out whole fields.

In the 1930s, the agrochemical industry was born. DDT arrived on the market, a powerful pesticide that came with the luster of modern science—a synthetic creation that humans could use to master nature. As a result, biological control withered away. A few biologists in California and Australia kept studying parasites in the hope of bringing back biological control. And over the next forty years, pesticides began to falter. Insects evolved resistance to DDT. The chemical worked its way into the food chain, causing birds to lay eggs with thin eggshells. An environmental movement opposed to pesticides started up, and the aging masters of biological control saw a chance for a comeback.

“I was a graduate student at Berkeley at the time,” says Armand Kuris. “It was so interesting. These were old guys, twenty years, thirty years my senior. They were old agricultural guys with string ties and stuff like that. And there they were in the sixties with all the hippies, and they found themselves in the same bed together. In the beginning it was weird, but then they realized they were on the same side. It was one of the sidebars to the history of the sixties.”

In its second incarnation, biological control with parasites had a much more solid scientific foundation. Insects can evolve resistance to DDT, but parasites can evolve as well. They can come up with new molecular formulas for attacking their hosts, canceling out any resistance the pests may evolve. A parasite could rein in a pest, some scientists argued, by bringing back at least some balance to nature. Most pests are alien species like the green crab, brought to a new land. One reason they are so harmful is that they have escaped their parasites and can breed unchecked, while native species have to struggle against their own parasites. Introducing a parasite from the invader’s homeland, the argument for biological control goes, is really just a way to reestablish some natural restraints.

The new biological control has in fact produced some spectacular triumphs over dangerous hosts. It may, for example, have saved much of Africa from starvation. What rice is to China, what potatoes once were to Ireland, cassava is to Africa. The plant grows three feet high, with broad green leaves that are as nutritious as spinach and far tastier. The roots of spinach don’t count for much, but cassava roots are thick slabs of starch. Cassava is rugged enough to grow where other roots would rot away, so for some villages in the wetter parts of Africa it’s the only thing poised between them and famine. From Senegal, on the Ivory Coast, to Mozambique, on the Indian Ocean, 200 million people depend on it. And in 1973 the cassava began to die.

On the little plots around Kinshasa, the capital of Zaire, leaves began to curl and shrivel, and without photosynthesis the roots became stunted. Within a few years there was so little cassava around the city that a family’s supply for a week cost more than a month’s wages. In the meantime, cassava began to die around other port cities along the Atlantic coast of Africa: Brazzaville, Cabinda, Lagos, Dakar.

When people uncurled the withered leaves, they found a white speckling, which resolved itself under a magnifying glass into thousands of pale flat insects. No one had ever seen the insects before in Africa; in fact, no one had ever seen this particular species before anywhere in the world. Known as cassava mealybugs, they are one of the many plant-eating parasites, tuned to the narrow frequency of their host-plant species. The insect stabs the cassava leaf with its proboscis, which anchors it in place. It sucks out the sap, at the same time injecting a poison that somehow stops the roots from growing, which probably lets the mealybug take up more food through the plant’s leaves. Cassava mealybugs are all female, and a single female can lay eight hundred eggs in its microscopic lifetime. By the end of a growing season a single shoot may sag with twenty thousand insects.

The curling of the leaves is also caused by the mealybug’s poison. It may be that the shriveling helps the insect spread from plant to plant. A healthy cassava field puts up a thick blanket of leaves to the wind, deflecting breezes up and over the plants. But when cassava becomes host to mealybugs, the blanket becomes tattered, letting the wind work its way among the shoots, carrying with it young larvae to colonize new plants. While this is only a theory, there’s no doubt that once a single cassava plant in a field falls to the mealybug, the rest are doomed. To make matters worse, cassava is a portable plant; a farmer can take a shoot and start a new field with it somewhere else. If even a single mealybug is hidden in the leaves, the new field, and the older fields around it, become infested.

The leaping of the mealybugs from port to port was probably brought about this way. Someone may have even taken a mealybug on a plane, because in 1985 it turned up several thousand miles away in Tanzania, where it began to spread from field to field. Wherever it went, it didn’t simply rob farmers of a single year’s crops. Since they needed cuttings to replant their fields, and none of their cuttings was free of the mealybugs, the farmers lost the crops for years to come.

In 1979, a Swiss scientist arrived in Ibadan, a Nigerian university town deep in cassava mealybug country. He was Hans Herren, an entomologist who had grown up working on his family’s farm outside Montreux. “As I was growing up, we were going from almost completely organic farming to a full pesticide thing,” Herren told me twenty years later when I visited him in Nairobi. His hair had gone gray, but he was still a live wire, able to tell a story rapid fire for an hour straight. “I can remember in ten years going from using almost no chemicals to using herbicides and pesticides. I was the one driving the tractor off hours from school, treating our potatoes, our tobacco, our wheat, and everything else with all these chemicals. I remember these guys coming around the farm selling chemicals to my father. I saw how we did it before, and then we went into this treadmill of more and more and more.”

Herren went to college hoping to find a way to jump off the treadmill without landing too painfully. He studied biological control, first in Switzerland, then at the home of its renaissance at the University of California at Berkeley. The International Institute of Tropical Agriculture offered him a job, or, more precisely, a challenge: Could he find a parasite for the cassava mealybug? He didn’t think twice before taking the job. “Going to Nigeria was a chance to practice on a very large scale what I had learned in Berkeley and Zurich.”

When Herren arrived at Ibadan, he discovered that most of the scientists there were sure he would fail. They were breeders, creating new cassava hybrids designed for fast growth and resistance to disease. They were sure they could handle the mealybug disaster. “They said, ‘Mealybug? No problem: breeding, that’s the solution.’ ” And when they met Herren, their thoughts ran in a different direction: “ ‘This guy from Berkeley—what does he know? This ecological freak.’ ” Herren himself had nothing against breeding, but for the crisis at hand there simply wasn’t enough time. The mealybug was catapulting from one city to another and then racing through the surrounding farm land “like a dust cloud,” says Herren. Breeding a resistant hybrid can take a decade, and in ten years there might not have been any cassava left to save.

In order to find a parasite for the cassava mealybug, Herren had to find where the mealybugs had come from. They had appeared out of nowhere around Kinshasa. They were not related to any known mealybug in Africa, but to a species that lived on cotton across the Atlantic, in the Yucatan. “Then I started to think, ‘Well, it’s from Central America—that’s interesting, because cassava is also from the Americas originally. The Portuguese brought it to Africa back in the slave trade. The voyage was a very long one, down in the ship, and the salty water killed whatever was on it, so they never brought any insects across. So the plants were really thriving for several hundred years until somebody brought in mealybugs.” No one had ever seen the cassava mealybug in the New World, Herren reasoned, because there was some parasite there keeping it at bay. “If it were not under control we would already know about it.”

Herren paged through entomological and agricultural journals, reading up on the insects that ate domesticated cassava. “Something didn’t make sense. The scientists in the Americas had been working on cassava for the last fifty years, breeding, all kinds of things, and nobody had seen that mealybug. Now wild cassava, a lot of them are used as ornamentals. They are the most beautiful plants. So I thought, maybe somebody carried a nice-looking plant. If nobody has found this mealybug in the cassava plants in so many years, why should it be there? I was going to have to look not only at cassava but at its wild relatives.”

Looking throughout Latin America for an insect no one had seen before would take even longer than trying to breed cassava out of its woes. But throughout the range of wild cassava, Herren recognized a few hot spots of cassava genetic diversity. They might also be where the most diverse of cassava-eating insects are. And one of those insects might turn out to be the one eating up Africa.

Herren set off for the Americas in March 1980. He started by visiting several museum collections of plants, looking at dried specimens of cassava. It was possible, he thought, that someone had already found what he was looking for. “But I could find nothing, so I said, let’s go look for the real thing. I went over to California and bought myself a big van. I established a lab in the back, a bed, everything, and I started driving through Central America, all the way to Panama, looking for wild cassava and cultivated ones.”

As Herren wandered down through Central America, a network of entomologists there was also on the lookout for the insects. Many new mealybugs turned up in the search, but none of them turned out to be the species blooming in Africa. “I decided, okay, let’s go away from Central America. Let’s go to South America. I parked my van in the Panama airport and flew down to Colombia to visit a friend of mine. We set off to Venezuela and looked at one of the centers of diversity, the northern part of Venezuela. We drove for weeks. We found a lot of cassava mealybugs, but never the right one. So I gave him pictures, good photographs of what I was looking for, what the plant looks like when the mealybug is on it, and I went back to Africa.”

His friend Tony Bilotti went to Paraguay not long after Herren went back to Ibadan. He was visiting some fellow Americans serving in the Peace Corps, and he knew that he was now in a cassava hot spot in Latin America, the only one that Herren hadn’t had time to visit. Driving one day past a field of cassava, he noticed a few plants that looked a little funny. He stopped and plucked the leaves. Inside them he saw Herren’s mealybug.

When Herren got word, he had Bilotti send the insects to the British Museum, where entomologists could identify them precisely. Although the insects were dead, the entomologists recognized them as the species in Africa. And as they dissected them they discovered inside their bodies the true end of Herren’s search: parasitic wasps. Now Herren had the parasite that kept the cassava mealybug a minor pest in one corner of Paraguay, and the parasite he needed for Africa. He had entomologists in Paraguay send live mealybugs to England, where they could be raised under quarantine and the parasites could be captured as they emerged from their hosts. He sent mealybugs and cassava plants from Africa to the same quarantine, where scientists were able to get the wasp to lay its eggs in them. Even more important, the experiments showed that the wasps could lay eggs only in the cassava mealybugs. They hadn’t tuned themselves to the immune systems of other mealybugs, which could choke the wasp eggs in suffocating capsules. The wasps, Herren decided, would be safe to bring to Africa. Three months later, Herren got his first shipment of the wasps.

He was ready for them. He and his students at Ibadan had been building greenhouses where they could grow cassava infected with mealybugs and capture the wasps that thrived on them, and they figured out how to mate the wasps. After they had collected a few hundred of the egg-laying females, they made their first release in the fields around the Ibadan campus in November 1981. “Within three months, the mealybug population crashed. Then we knew we had something good going. It was barely a year and a half that we had gone from not knowing anything about this to having something in the field that worked.”

Biological control, even in its renaissance, remained a modest enterprise. Entomologists would raise wasps in their labs and load them into small containers that they’d take with them when they drove to orchards or corn fields. But a great dream took possession of Herren: to spread the wasp across Africa. “What I didn’t like in biological control was the way it was done as a shoestring operation, in a cheap way, using a secondhand beaker, raising wasps in some small cages—not done in the best possible way. That’s why biological control lost to chemicals.”

He knew that the dream would be expensive: $30 million, in fact. “That was when I was called a megalomaniac. I said, ‘Look, when you guys over in America have a fruit fly outbreak in California, which is only the size of a pin compared to this whole thing over here, you spend $150 million in one year. We’re talking about 200 million people who are at risk, not a few businesses that make oranges. We are dealing with one and a half times the area of the United States. We’re not going to do this in cages and on donkeyback and bicycles. We’re going to do this with technology, machinery, electronics, aircraft.’”

Maybe it was the word aircraft that made people suspicious. Herren claimed that he would be able to spread his wasp across Africa by sowing it, crop-duster fashion, from a plane. The wasps were put to sleep with carbon dioxide and then lodged in cylinders of foam rubber, two hundred fifty in each, which were loaded into a magazine that had been custom-built for Herren at an Austrian camera factory. As the pilot passed over a field Herren intended for him to drop the wasps precisely. “It was like in fighter aircrafts. You know when to drop the bomb by looking at the crosshairs. We tried this over a swimming pool in Ibadan. We’d fly over and drop the wasps. At one hundred eighty miles per hour, we were able to get them in there.”

In the meantime, the wasps Herren had set free in the fields around Ibadan had been thriving. Two years after their release, he decided to see how far they had spread. “We went on foot. We thought, ‘Oh, no big deal, we’ll just walk.’ And we walked the whole day, and we kept finding them. We thought, there’s something wrong here. Nobody had ever seen this sort of wasp spread more than a few kilometers. And the next day we came back and we took the car and we drove. We drove one hundred and fifty kilometers before we finally found no more wasps.”

By 1985, thanks to these early successes, Herren had collected $3 million of start-up money, and his pilots were strafing the countryside with wasps. The parasites tumbled out of his plane and landed on fields in Nigeria, in Kenya, in Mozambique, in countries from the Atlantic Ocean to the Indian Ocean. His team was raising 150,000 wasps every month, and although many of the wasps died during the long journeys from Ibadan to the release sites, he really needed only a single viable female wasp to survive the flight and the fall and to start looking for hosts. Even among parasitic wasps, the host-hunting skill of the Paraguayan species was extraordinary. “The wasp has developed an ability to search which is fantastic,” Herren says, with a pride that is almost paternal. “If you have one plant with mealybugs on it in a field that’s a hundred meters by a hundred meters, the wasp will find it. We tested this. We had fields that were clean. We put mealybugs on one plant, and we released the wasps from a corner of the field. Within a day they were on the plant. Then we tried something else. We put the mealybugs on the plant and then took them off. Then we released the wasps and they ended up on the same plant. There’s something that the plant releases that attracts the wasps, a cry for help.”

Herren trained twelve hundred people from the countries where the wasps had been introduced to recognize it. A few months after the drops, they began to comb through the fields to see how fast the wasp was spreading and how the mealybugs were faring. “Everywhere the problem was gone twelve months after the release. We could hardly believe it ourselves, that it worked so fast.”

The last flight of the wasp duster was in 1991, but for the next few years entomologists still went on tracking its effects. In about 95 percent of the fields where the wasp had been released, the mealybug had virtually disappeared. As they lost their hosts the wasps had diminished to only a few survivors as well. In the remaining 5 percent of the farmland, the mealybugs still thrived, but Herren was able to show why: the farmers didn’t take good care of their fields. Their plants were scrawny, and the mealybugs that fed on them tended to be scrawny as well. The species of wasp that Herren used is a careful judge of the size of its host, able to use its antennae like a ruler to figure out how big a mealybug is. Only then do they decide which sex to make their offspring. (When a female wasp mates, it stores the male’s sperm in a gland, which it can use later to fertilize its eggs. Thanks to wasp genetics, an unfertilized egg will grow up to be male, while a fertilized one will grow up to be female.)

The wasps choose to lay only males in small mealybugs. Their logic lies in the cheapness of males. The chances of an egg successfully maturing to an adult are worse in a small mealybug because there’s less food for the parasite to eat. Because the wasps put males in small hosts, only a few of them may survive to adulthood. But that doesn’t matter because it takes only a few males to inseminate a lot of females.

Thanks to the wasp’s strategy, a field of badly farmed cassava will be filled with male wasps. Since males don’t lay eggs, they pose no threat to the mealybugs, which have a chance to quickly rebuild their population. “We’ve told the farmers, ‘Look, biocontrol can only work when everything else is in good shape. If you don’t weed your field, nothing will work.’”

Herren told me the story of the cassava mealybug one sparkling day in Nairobi. He had moved there in 1991 to become the director general of the International Center for Insect Physiology and Ecology, a massive complex on the outskirts of the capital with sculptures of dung beetles out front. The job is one of his many rewards for having saved the staple crop of 200 million people. The center is filled with entomologists trying to find ways to use insects to make human life better by producing honey and silk and by destroying pests. A stem borer has been chewing its way through the corn of East Africa, but Herren’s scientists have found a wasp from India that parasitizes it. When I visited, they had already set it loose in Kenya to see whether it would survive in the wild. It did, and now they had no idea how far it had spread. And that sort of ignorance was fine with them.

Lafferty and Kuris wanted to do for the green crab what Herren had done for the cassava mealybug. They knew that in Europe many green crabs were plagued by parasites such as Sacculina, but the crabs they dissected from San Francisco Bay were parasite-free. That might be one of the reasons why it could outcompete other crabs in its new home. So Lafferty and Kuris began to contemplate bringing Sacculina to California as well. Sacculina-infected green crabs could be dropped into the Pacific waters. They would act like miniature parasite crop dusters by spraying Sacculina larvae into the water. The larvae would seek out uninfected crabs, burrow into them, and spread their tendrils out. Bringing Sacculina to California wouldn’t have the same effect as the parasitic wasps had on cassava mealybugs, because the biology of the two parasites is very different. The wasp kills its hosts by devouring their innards and then chewing its way out of their bodies. Sacculina doesn’t kill its green crab hosts, but it does castrate them and then make them compete for food with uninfected crabs. Lafferty built mathematical models that suggested that if Sacculina came to the Pacific, it would make the crabs decline, but more slowly than the cassava mealybugs. It would be the missing crab eggs that would bring down their numbers, rather than dead crabs. So when Sacculina and the green crab finally reached an equilibrium with each other, the crabs would be reduced but not gone.

But to Lafferty and Kuris, it didn’t seem as if there were any other choices. “All other alternatives are way worse ecologically,” says Kuris. “Antibarnacle paint on boats is polluting our estuaries in a major way. Up in Oregon there’s someone in a crop duster spraying mud flats against ghost shrimp, to protect the goddamned introduced oyster production. It’s killing Dungeness crabs.”

For a few years, Lafferty and Kuris couldn’t drum up any funds to study Sacculina, but by 1998 the green crab had reached the shores of Washington State. It was poised to move into Puget Sound, with its huge Dungeness crab fishery. At last Kuris and Lafferty got the money they needed. They contacted the world’s expert on Sacculina and related parasitic barnacles, a scientist in Denmark named Jens Høeg. Høeg sent them coolers filled with infected green crabs packed in ice.

Mark Torchin, Kuris’s graduate student, set up the crabs in a quarantined room. He couldn’t simply seal off the room completely, though, because the crabs and the parasites needed circulating sea water to survive. Torchin built pipes that pumped sea water in from the ocean; the water poured into a group of tanks, and the overflow, which might carry the invisible parasite larvae, traveled through a series of filters and tubs of gravel before pouring into an outgoing pipe headed for a nearby lagoon.

For months, Torchin slowly got acquainted with Sacculina and its bizarre life cycle. He figured out how to recognize when a crab was getting ready to release a new batch of parasite larvae from the sac on its abdomen (the sac would turn from butterscotch-colored to a dull caramel). He would put the crabs in little plastic cups to collect the larvae, and then he’d siphon off some of the Sacculina-laden water. He would pour it into another cup with a healthy green crab and wait for the female Sacculina to get into its new host.

Each day he would grab a crab by the claw and pinch it with his fingers. To escape, the crab would sever its own limb from the inside and drop back into the water. Torchin would take the limb to his microscope and look for larvae grabbing onto the hairs of the crab’s claw and digging into the soft pits that anchored them. When a female Sacculina succeeded in infecting a crab, he’d let it develop into a knob on the crab’s abdomen, and then he’d try to get males into it.

After a few months, Torchin was able to shepherd Sacculina from larva to adult. Then, at the beginning of 1999, he applied what he had learned to native California crabs. He chose the common shore crab, Hemigrapsus oregonensis, and exposed it to Sacculina. This was probably the first time in the history of these two species that they had ever met—a crab from California and a parasitic barnacle from Europe. Torchin waited to see what would happen.

A female Sacculina, he discovered, had no trouble getting inside the shore crab. It could even send its tendrils out through its new host’s body. But then something went wrong. In a European green crab, the parasite can carefully wind its tendrils around the nerves, not only causing no damage to them but passing mind-altering signals to their host. In the shore crab, though, Sacculina’s tendrils just seemed to destroy its host’s nerves. Torchin would come in some mornings and find shore crabs on their backs, still breathing but completely paralyzed. Within a few days the infected shore crabs died, and Sacculina died with them.

The biologists had come up hard against the trouble with parasites: their flexibility. Parasites may become specialists on a single host thanks to their evolutionary arms race. But that doesn’t always mean that a parasite can’t use the same tricks to infect another species. If it should come across a new host with a similar physiology and a similar way of life, it may be able to eke out an existence in it. The parasite may simply never get a chance to try out that new host because of its ecology: if a species of tapeworm lives in a stingray in the Amazon, it probably won’t get a chance to try out stingrays in New Guinea. But sometimes parasites do get a chance—when, for example, continents slam together and animals on one of them colonize the other. That, in fact, seems to be how parasites survive through mass extinctions that claim so many of their hosts. They just jump from one host to a new one.

And so parasites carelessly introduced to new habitats can cause disasters, for all the reasons that make them so impressive when they work well. They have a sophisticated set of tactics they can use against their hosts, and they can fine-tune them through evolution to take on new hosts and new defenses. And once they get into a new habitat, there’s no way to get them back out. It is a one-way experiment.

The halt of the cassava mealybug may be a great success story, but there are stories of spectacular failure as well. The forests of Hawaii represent one. They’re filled with alien parasites brought there to destroy insect pests. Parasitic flies, for example, were brought in to keep down a species of stinkbug. But the fly could also live inside the koa bug, a big, showy native insect, and now the koa bug has almost disappeared. Parasitic wasps were brought in to control moths that attacked crops, and they also spread to many native species. Before the parasites came, the moths of Hawaii went through huge annual explosions; at their peak, their droppings falling from the trees sounded like a hailstorm. Birds would feast on their caterpillers and feed them to their young. But since the introduction of parasites, many native moths have managed to break out only once every decade or two. The forest birds of Hawaii are declining, and biologists suspect that the death of the moths may be partially to blame, because they can’t feed the birds. And without birds to pollinate the trees and disperse their seeds, the forests themselves may also be suffering.

Hawaii’s plight is the best documented of biological control’s failure because it’s a set of small, biologically distinct islands. But critics suspect that there are many other stories waiting to be told. In the United States, for example, over thirty different parasites were introduced during this century to kill gypsy moths. None of them worked well, and some of them have been destroying the exquisite giant silk moths, threatening them with extinction.

These disasters have made biologists like Lafferty and Kuris much more careful about using parasites. That was why they had set up such a long, tedious test of Sacculina in the first place. After the shore crabs started dying, they repeated their tests on Dungeness crabs. They got the same results: paralysis followed by death. “If I were to be responsible for the destruction of the Dungeness crab,” Kuris said, “my name would be mud. I would be like the guy who introduced the killer bees. The poor man has lived a life of public self-flagellation for forty years. Do I care about the native shore crabs? Sure I do. I yield to nobody on values on this.”

Lafferty broke the bad news to his colleagues in the fall of 1999. By then, the green crab had been spotted as far north as British Columbia, over a thousand miles from its landing point in San Francisco. Lafferty e-mailed me as well, and I immediately called him. I asked him if he was disappointed. “Well, as a scientist, you’re never supposed to be disappointed,” he said. “The truth exists, and you don’t have any control over what’s reality.”

But it was frustrating to watch the green crab keep spreading. “My gut feeling is that if you released these things on the West Coast, chances are they wouldn’t affect native crabs very much. All we found was that they have the potential to.” Putting Sacculina larvae in a cup with a Dungeness crab isn’t the same thing as putting them in the ocean. “It’s got to ask these questions, like where is it likely to find its host crab.”

Sacculina and its relatives use cues such as sunlight and chemicals given off by their hosts to position themselves where they’re likely to bump into a green crab. Those cues might not let them bump into any other species. Lafferty told me about another experiment he had run that supported this idea. He got his hands on another species of parasitic barnacle that is related to Sacculina and lives in the Pacific sheep crab. He then gathered California shore crabs that live in the same range as the sheep crab, but which have never been found carrying a parasitic barnacle of their own. When he exposed the shore crab to the parasite, he had no trouble infecting it. Something must be preventing the parasite from infecting the crab in the wild.

But if you’re trying to use parasites in the ocean as a biological control for the first time in history, you want to be utterly sure of yourself. I asked Lafferty if he had any other ideas for stopping the green crabs. “I don’t think we should sit back and watch the massacre,” he said. He started telling me about another parasite of green crabs called Portunion conformis. It’s an isopod, a relative of pill bugs, and it has independently evolved a Sacculina-like existence of its own in green crabs. It invades a crab as a microscopic larva and then destroys its host’s gonads, taking their place. Eventually it fills up a fair part of the crab’s body, making up a fifth of its weight. By destroying the crab’s gonads, it castrates its host, and like Sacculina, it feminizes male crabs. No one has ever cultured Portunion in a lab, but Lafferty wants to try. And then he wants to run the same tests on these parasites that Sacculina failed.

“They’re absolutely beautiful parasites,” Lafferty said. He had me picture a big, opaque pouch with a mouth at one end, carrying a collection of golden eggs inside. “It’s hard to describe them. They look like—God, they don’t look like anything you could ever imagine.” Parasites may be frustrating to work with sometimes, but for a parasitologist, there’s always a consolation in their beauty.

Herren and Lafferty work on the tattered edge of nature, the cassava fields and oyster banks where humans have transformed wilderness into a new sort of patchwork, where alien species can move thousands of miles in a matter of weeks, where the best-suited species is often the one that can thrive on perpetual chaos. Parasites may be able to soften the blow that we inflict in places like these if we respect their evolutionary power. But I also wondered about those parts of the world still left relatively untouched, and whether parasites might help keep them intact.

That was how I ended up in a Costa Rican jungle hunting frogs with Daniel Brooks. We were wandering around inside the Area de Conservación Guanacaste, a 220,000-acre reserve of dry forests, rain forests, and cloud forests, stretching from Pacific beaches to the tops of volcanoes. Twenty years ago, the forests of Guanacaste were disappearing as ranchers were cutting down trees to clear fields for their cattle, despite the fact that ranching was becoming less and less profitable. A biologist working in the area, a grizzled man named Daniel Janzen, decided to take advantage of the times. He set up a foundation that began buying up the ranches, and he hired the out-of-work cowboys to serve as “parataxonomists”—doing the work of documenting the diversity of Guanacaste by collecting species, dissecting them, and describing them. So the forest has not only been saved but expanded, and the people who live around it have a stake in protecting it. There are no fences around Guanacaste.

By the end of the 1990s, when I visited Guanacaste, Janzen was pretty much done with his reserve building. He was spending more of his time on his true love, the butterflies of Costa Rica. When you enter his little house at the reserve headquarters, three rooms under a corrugated tin roof, you have to stoop below the dozens of plastic bags hanging from the beams, each with a caterpillar feeding on a leaf. “My goal is to find all the caterpillars before I’m buried in the mud here,” Janzen said to me. Not only does Guanacaste contain a fair amount of pristine forest, but more important, in the future its forests will grow and turn into a self-sustaining ecosystem. “A thousand years from now, you come back and it’ll still be there,” he said.

One night Brooks and I burst into Janzen’s house. That day we had done a lot of dissections and looked at a lot of parasites, and we had decided to take a drive to a bar half an hour away for a drink. Along the way, the headlights of Brooks’s four-by-four lit up a furry body on the road. We stopped and backed up. It was a dead fox freshly killed, its tail still a delicate cloud of gray. It went into the back of the truck, and we headed back to Guanacaste. When we got to Janzen’s house, Brooks pulled the fox out and carried it to the front door. He laid it on the concrete floor of Janzen’s front room. The animal looked intact, but it had been hit so hard that its eyes bulged like domes out of its head. Janzen said, “Well, what do we have here?”

Janzen’s wife, Winnie, wandered out from the back room to see what was going on. She had their pet porcupine, Espinita, on her shoulder, and it raised its quills in fear. “You’ve been learning too much from your cats,” Winnie said to Brooks, “bringing gifts to people’s doors.”

It takes a strong friendship to flop a bloody fox on someone’s floor, and Janzen and Brooks have shared exactly that kind of friendship since 1994. (Janzen even named a species of parasitic wasp that he discovered after Brooks.) They met as Janzen was looking for help to count every species in the reserve. No one had ever done something on this vast scale—Janzen estimates that there are 235,000 species in Guanacaste. But he dreamed of having a full inventory of species, which scientists could use as a sort of yellow pages to let them pick out species to study and to help them discover how biodiversity is created and maintained in tropical forests. As soon as Brooks heard of the project, he wanted in.

Brooks has been a parasitologist since the mid-1970s. It was he who figured out how to use the relationships of parasites to reconstruct the wanderings of their hosts millions of years ago. He began working with frogs in Kansas but spent most of his career working in Latin America, looking at the parasites of stingrays, alligators, and other animals. It is slow work, and usually a parasitologist can hope to discover only a sliver of parasite diversity. And that’s why Brooks jumped at Janzen’s idea. “As soon as I heard about what was going on here,” says Brooks, “I turned over all my stingray stuff to my Ph.D. students. I realized this was the place I wanted to make the focus of my work.” For once, in one place, parasitologists might be able to know all the parasites. Guanacaste would become, as Brooks says, “a known parasite universe.”

Janzen was a little puzzled by Brooks when they first met, and I could see some of that bafflement in his face when Brooks laid the fox on his floor. How can someone get so thrilled by a corpse? But Brooks evangelized Janzen until he began to see the parasitic light. “This guy shows up, and my vision of a mouse is changed forever,” Janzen told me. “Now I see it as a bag of tapeworms and nematodes. You have this happy mouse and you open him up and he’s full of them.”

After showing off our find, Brooks and I took the fox to his shed. Brooks switched on the fluorescent light, and moths swarmed in through the chicken wire. He laid the fox down in the freezer, alongside an ocelot and a tapir—other lucky finds that he was going to open up eventually.

We got our drink—Cuba Libre in a can—and when we were done, at about eleven, we drove back to the reserve. Brooks pulled up by the shed and switched the light back on. The best way to see parasites is to open up a fresh body. As a corpse decomposes, the parasites lose their bearings and drift away from their natural homes. Soon they start to die themselves, their bodies disintegrating. So Brooks pulled the fox out of the freezer and got out a pair of scissors.

The fox’s inner ecology turned out to be pretty simple: it was loaded with hookworms, which had been gouging blood out of its bowels. “This guy had a screaming hookworm infection,” Brooks said, pulling apart the fox’s intestine under a microscope. What struck me most about the dissection was Brooks himself. He kept apologizing to the fox as he cut it open—“Sorry, sorry”—kept cursing its stupid death, kept complaining about how the collision had smashed its lungs. The other scientists who worked at Guanacaste looked on Brooks as something of a vampire, a scientist interested in the beautiful animals of the forest only if he could slit them open. But I had never seen someone mourn a dead animal so deeply.

Janzen’s dream of a full inventory fell apart in 1996 during negotiations with the Costa Rican government. Janzen didn’t like how the money for the project was going to be diverted from the central mission of counting species, so he decided he had to abandon it. “We shot the horse in the head,” was how he put it to me. But Brooks was able to get enough money from the Canadian government to keep going with the parasites. He estimates that the nine hundred forty vertebrates of the reserve harbor eleven thousand parasite species (including only the parasitic animals and protozoa), most of which will be new to science. “It’s going to take the rest of my career to do this inventory,” Brooks said. I wondered why he was planning to put himself through so much pain.

Over the course of the next day, I put the question to him a few times and got a new answer each time. Biodiversity is a staggering thing in a tropical forest such as Guanacaste, but you can’t see most of it without the aid of a scalpel. “There are undoubtedly more species of parasites than free-living organisms,” says Brooks. “When you preserve a species of deer, you’re preserving twenty species of parasites from four kingdoms.”

If that’s not enough, you can justify the project out of enlightened selfishness. Most medicines trace their genealogy to some natural compound in some organism, be it penicillin from a fungus or digitalis from foxglove. Only in the past few years have scientists begun to work their way through the parasite’s pharmocopeia. Cordyceps, a fungus that invades insects and sprouts flowerlike stalks out of its body, is the source of cyclosporin, an important antibiotic. Hookworms produce molecules that clasp perfectly with clotting factors in human blood, and biotechnology companies are putting them through trials as blood thinners for surgery. Ticks can also tamper with our blood to make their drinking easier, using chemicals that not only dissolve clots but reduce inflammation and kill bacteria that try to enter a wound. There are other parasitic tricks that still await an explanation. Blood flukes can steal substances out of our own blood to camouflage themselves from the immune system, but no one has figured out how they do it. If scientists did, they might be able to apply their discovery to transplanted organs. A doctor might be able to pump a patient’s blood through a donor lung and essentially turn it into a gigantic protected fluke. That could spare patients from the dangers of immune-suppressing drugs. And these are only a few parasites; who knows what sorts of chemicals the millions of others have evolved?

Another reason for a parasite inventory came up when Brooks and I took a day off from dissections. We drove up the side of Volcan Cacao, thrashing in the back of a Land Cruiser on a road made from boulders. Much of the forest up the sides of the mountain had been cut down by ranchers, but conservationists had bought the land back and were waiting for the forests to grow back down the slopes. We stopped driving at the border of the forest and hiked in, instantly dunked in an ocean of trees, blue morpho butterflies bounding through the shade like fish swimming overhead. A thin rain worked its way down through the thick canopy as we walked over a creek. Brooks stopped to look upstream and down. “This place should be packed with frogs,” he said. And there was nothing.

Beginning in the late 1980s, frogs began to disappear from the high elevations of Central America. On Cacao, not a single species of frog can be found. At first biologists had no idea what was causing the deaths; all they knew was that the corpses of frogs were piling up, untouched by birds. Only in 1999 did a biologist isolate what is probably the cause: a fungus brought down from the United States. Its spores travel through water until they meet the skin of a frog. Thereupon they dig into the animal and devour the keratin in its skin, releasing a toxin that quickly kills it. The only thing that keeps the fungus from killing every frog in Central America is the fact that it’s adapted for cool climates, and it’s too hot for the fungus to survive below a thousand meters.

By the time scientists had recognized the fungus, it was far too late to do anything. They could only watch the parasite bound southward from mountain to mountain. “We should have known about that fungus,” says Brooks. “If we’d had an inventory of parasites of frogs, we might still have frogs on the mountaintops of Central America. We didn’t know it was there.” Humans have no special protection from parasites either, and they can come bounding out of disturbed rain forests. It won’t be doctors who figure out where the Ebola virus comes from, but zoologists who can find the animal in the African rain forest that normally harbors it.

But Brooks doesn’t look at his inventory simply as a catalog of death and destruction. It may be able to help scientists measure the ecological health of Guanacaste and other forests like it. An ecosystem is a bit like a person. In a healthy person, all the parts interact the way they should: the lungs take in oxygen and the stomach takes in food, the blood carries it all to the tissues, the kidneys flush out waste, and the brain ponders the world or what it wants for dinner. In a sick person, a few of the parts stop working, and their shutting down disrupts the person’s whole body, sometimes forcing the rest of the parts to shut down as well. An ecosystem lasts for thousands or millions of years because it has parts that work together well: the worms aerate the soil, the fungus mingled with tree roots supplies them with nutrients and extracts carbohydrates in exchange, and so on. Water, minerals, carbon, and energy all circulate through the ecosystem like blood. And ecosystems, it turns out, can sicken. Introduce a parasite that kills koa bugs, and the damage can ripple out all the way to the trees in a forest.

Doctors don’t wait until their patients are dead to declare that something’s wrong with them. They look for early, easy-to-detect clues to trouble, even if they don’t know yet what the trouble is. If a potentially fatal colony of bacteria have established themselves somewhere in a person’s body, you don’t have to actually track the microbes down—you can just check for a fever. Ecologists want something that can tell them that an ecosystem is sick before the damage has rippled out to all the strands of its web. They have been auditioning the species that make up ecosystems in the hopes of finding one that could act as a sort of body-temperature index. Some have been looking at ants and other insects, others at the songbirds that nest on forest floors. Many candidates fall short in one way or another. It’s relatively easy to tell whether top predators such as wolves are declining, since they’re relatively few and big. But by the time the effects of some environmental stress have surged all the way up the food chain to the wolf, the ecosystem is probably already too far gone to help.

Some scientists, such as Brooks, think that parasites are a sign of ecological health, but not in the way most people would think. Until recently, most ecologists looked at parasites as nothing but a sign of environmental decline. If some pollutant wears down the immune systems of the members of an ecosystem, they become more susceptible to diseases. That does indeed seem to be true some of the time, but it’s easy—and wrong—to make it a generalization. The idea echoes all the way back to Lankester: the rise of parasites as a sign of degenerate times. The frogs Brooks and I had collected in the lower forests were healthy and so abundant that they threw themselves across our path, and they were riddled with parasites. Parasites are actually a sign of an intact, unstressed ecosystem, and the opposite, as strange as it may sound, is true: if the parasites disappear from a habitat, it’s probably in trouble.

As parasites travel through their life cycle they are often vulnerable to poisoning by pollution. A fluke, for example, hatches into a delicate form covered with hairlike cilia that swim in search of a snail; a couple of generations later, a cercaria emerges from the snail to find its mammal host. At both stages, the parasite depends on clean water to survive. That’s the theory, at any rate, and there’s some concrete evidence to show that it’s correct. The rivers of Nova Scotia have become acidified as a result of air pollution from coal plants upwind. Canadian ecologists added lime to the headwaters of one badly hit river, neutralizing the acid there, and then came back in the following years to collect the eels. They then compared them with eels from an untreated river that eventually joined the limed one. The eels from the limed river carried inside them a much richer diversity of tapeworms, flukes, and other parasites. The ecologists then expanded their survey to the rivers along much of the coast of Nova Scotia, and found that the most badly affected waters had eels that were the most free of parasites.

Parasites work well as ecological sentinels for another reason: they sit at the top of many ecological webs. If you dump nickel into a river, the little animals take up a little of it and don’t suffer too badly, but as the nickel rises up the food web—as copepods are eaten by small fish, which are in turn eaten by big fish, which are in turn eaten by birds—the pollution focuses to higher and higher concentrations. But parasites, which prey on even the top predators, concentrate even more pollution in their bodies. Tapeworms may carry hundreds of times more lead or cadmium than the fish they travel inside, and thousands more than the surrounding water.

Unlike free-living organisms, a parasite wanders through the many levels of its ecosystem, and it can report on the damage it comes across in its travels. Throughout its life cycle, a parasite may need to move through many hosts, each of which occupies its own niche in the habitat. Flukes in the Carpinteria salt marsh have to live in snails, which depend on the algae on the mud banks; from there they find a fish, which must eat zooplankton to survive; and finally the parasite must find the gut of a healthy bird in which it can mature. If any of those hosts should disappear, the parasite will suffer. In 1997, Kevin Lafferty found that in the degraded part of the Carpinteria salt marsh, there are only half the species of parasites as in the healthy part, and only half the number of individual parasites. Parts of the marsh are now getting restored, and by 1999, the snails there had regained the levels of parasites found in the pristine marsh.

This is why Brooks is cutting open frogs in Costa Rica. “You’ve got this guy walking around with nine or ten parasites, healthy and happy. Once you know all the parasites in the frogs, suddenly if something’s not there, something’s wrong with the frogs or with an intermediate host. If you’ve lost a parasite, you have lost something in the fabric of the ecosystem.” And once Brooks is done with his inventory, it may be possible to identify parasites by their eggs or larvae—and it won’t be necessary to sacrifice any more hosts.

Parasites may not only mark good ecological health; they may actually be vital for it. When ranchers overgraze their cattle and sheep on fragile grasslands, they can tip the ecology of the region over into a desert. As far as ecologists can tell, this move is pretty much irreversible, because the desert shrubs reorganize the soil in such a way that grasses can’t penetrate back in. It is a difficult and politically volatile matter to decide just how much grazing should be allowed on a given patch of land. Ranchers usually dope up their livestock with medicine to kill as many intestinal worms as they can, but the parasites might be able to keep the livestock in a careful balance with the grass they depend on. The larvae of some species of parasitic worms get into livestock by attaching to the grass they eat. When a worm gets into the gut of a sheep, it matures and starts siphoning off some of the sheep’s meals. Struggling with the effects of the worm, the sheep tends to live a shorter life and produce fewer lambs. In the end, the parasite shrinks the size of the herd.

Such ups and downs can alter an entire ecosystem. If a rancher is overgrazing his sheep on a semiarid grassland, the sheep may multiply and the plants will dwindle. At the same time the grazing changes the parasites: with more sheep available, they can breed in huge numbers, and they crowd on the dwindling blades of grass, making the probability that a sheep will become infected even higher. In other words, overgrazing automatically triggers an outbreak and scales back the herd, allowing the grass to recover. Soon the sheep population bounces back as well, but thanks to the management of the parasites, it never gets large enough to turn the grassland into desert. Rather than loading up their livestock with antiparasite drugs, and thereby ruining their grazing lands, ranchers may benefit by letting parasites keep the herd in check.

For now, though, the theory of parasitic stability remains mostly theory because scientists know so little about parasites in nature—which is another reason why Daniel Brooks is in Costa Rica. “People will be able to test their ideas on parasite stability because this won’t be a parking lot in thirty years. Parasites may dampen oscillations, and if they are having an influence, you don’t want to eradicate parasites.”

To manage Guanacaste, in other words, you need to understand its parasites. “If we want to preserve a place like this,” Brooks said, “we have to know what’s going on microscopically. We need to figure out how to work with parasites. We need to figure out what organisms need and want, so we can use them in ways that don’t terminate their existence.”

The way Brooks was talking about us humans reminded me of the way parasites use their hosts—evolving a sense of what their hosts need and want, what they can and can’t live without—so that they don’t destroy themselves. In my travels for this book I often thought about the natural world as the sum of its parts. I would look down out of planes at the mud lakes of Sudan, the circuit-board housing tracts around Los Angeles, the disintegrating ranches and scraps of forest of Costa Rica and think about a concept, called Gaia, which some scientists embrace. They think of the biosphere—the rind of ocean, land, and air that’s home to life—as a kind of superorganism. It has a metabolism of its own, which shuttles carbon and nitrogen and other elements around the world. The phosphorus that helps power the flash of a firefly ends up in the soil when the firefly dies, perhaps to be taken up by a tree and added to one of its leaves, dropping into a river and flowing to the sea, where photosynthesizing plankton take it up, only to be eaten by some grazing krill, which releases it into the ocean depths in its feces, only to be taken up by some bacterial scrounger, and cycled back up to the ocean’s surface, before finally, many years later, ending up entombed in the sea floor. Like our own bodies, Gaia is held together and kept stable by its metabolism.

We humans exist within Gaia, and we depend on it for our survival. These days we live by using it up. We strip topsoil away with our farms without replacing it; we fish out the seas; we clear out forests. I thought about what Brooks had just said, about learning how to use nature without terminating it.

“You talk as if we were a parasite,” I said.

Brooks shrugged his shoulders. The idea was fine with him. “A parasite that has no self-regulation is going to put itself out of existence and may take its host with it,” he said. “And the fact that most species on Earth are parasites tells us that hasn’t happened a lot.”

I chewed that over for a while. Here was a new meaning parasites could have for us—one that could take the place of Lankester’s degenerates, Jewish tapeworms, and all the old myths of failed evolution. One that could be biologically faithful without turning life into a horror movie, without having parasites come bursting out of our ribs. It is we who are the parasites, and Earth the host. The metaphor may not be perfect, but it chimes well. We reroute the physiology of life to our own ends, mining fertilizer and blanketing farm fields with it, much as the wasp reroutes the physiology of its caterpillar to make the kind of foods it needs. We use up those resources and leave behind our waste, like Plasmodium turning a red blood cell into a garbage dump. If Gaia had an immune system, it might be disease and famine, which can keep an exploding species from taking over the world. But we have dodged these safeguards with medicines and clean toilets and other inventions, and they’ve allowed us to put billions of people on the planet.

There’s no shame in being a parasite. We join a venerable guild that has been on this planet since its infancy and has become the most successful form of life on the planet. But we are clumsy in the parasitic way of life. Parasites can alter their hosts with great precision and change them for particular purposes: to take them back to their ancestral home in a stream, to move on to their adulthood inside a tern. But they are expert at causing only the harm that’s necessary, because evolution has taught them that pointless harm will ultimately harm themselves. If we want to succeed as parasites, we need to learn from the masters.