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.”