4

Escape Is Temporary

We may infer from these facts, what havoc the introduction of any new beast of prey must cause in a country, before the instincts of the indigenous inhabitants have become adapted to the stranger’s craft or power.

—Charles Darwin, The Voyage of the Beagle

We worry about pathogens that attack humans, such as Ebola, MERS (Middle East respiratory syndrome), and Zika. But if the potato famine teaches us anything, it is that we should also worry about the dangers to our crops. They are inevitable.

In 1970, a malady affecting cassava arrived in the People’s Republic of the Congo (later Zaire) and the Democratic Republic of the Congo. We don’t know for sure when it was first noticed. A farmer likely went to the cassava growing in the field behind her house, the cassava that was to feed her family, and saw that the plants were sick. The stalks and leaves lay black and twisted on the ground. When she got back to her village she found that other women, too, had gone to their fields to collect cassava. They also saw the sickness. The stems of the cassava plants were stunted and weak, their leaves distorted, their roots—the foodstuff—small. The women pulled the diseased cassava out of the ground and planted more of the same. The new plants also suffered.

What was causing this destruction? As farmers talked about the problems, they found a culprit, a small insect on the plants with the face of a monkey, complete with large “nostrils.” It was an emissary of some lesser and wicked god. This monkey-faced beast was to blame!1 Moreover, it seemed to be a kind of a messenger of misfortune, a bad omen. As a result, no one would touch the animal. Meanwhile, the malady spread.²

Technicians at an experimental farm in Mantsoumba, set up by the government twenty kilometers (twelve or so miles) west of Brazzaville,³ had seen the damage. They went out to their own fields and looked carefully at their plants. They noted the monkey-faced insect. It would prove to be the chrysalis of a butterfly (Spalgis lemolea), a chrysalis that is said to mimic a tiny monkey head so as not to get eaten by birds. It was not the cause of the malady. Instead the technicians observed sucking insects of diverse sorts on the cassava plants, including a species they had not seen before.⁴ The new species appeared to be the worst among them. It was small and white, an animal so modest and featureless as to scarcely deserve being called an animal. It looked to be a kind of mealybug.⁵

The details of what happened next are muddied by time and conflicting accounts. An agricultural engineer at the ministry of agriculture in the People’s Republic of the Congo and a professor of zoology at the University of Brazzaville became involved. In 1973, they or someone who worked with them sent a sample of the mealybugs plaguing the cassava to the National Museum of Natural History in Paris, where it landed on the desk of Danièle Matile-Ferrero,⁶ one of relatively few world experts in mealybugs. Matile-Ferrero prepared the specimen carefully. She then looked through a microscope at its features. Some attributes of insects can easily be seen through a microscope. Others must be inferred based on intuition developed over years of experience. Matile-Ferrero formed an image in her head of the specimen before her. She made a drawing. She compared the image and the drawing to other species in her collection and in every other key collection around the world. Slowly, meticulously, she was coming to the conclusion that the animal before her was totally new to science and that she would need to give it a name.

Cassava is an unglamorous food. Like the potato and other roots and tubers of the tropics, its story is one of food as energy rather than luxury.7 Like the potato, its taste is humble. Like the potato, it offers nearly complete nutrition. You can live off it with little else as a supplement if you eat both its leaves and its tubers. Whatever it lacks in subtlety and flavor it makes up for in fecundity. Plant the sticks in poor soil, and the plant will grow and make large storage roots, some weighing as much as ten pounds. Harvest one and another will grow. Come back several weeks later and there will be more.

Cassava is not native to Africa; it comes from the Americas. The conquistadors saw it, ate it, and left it where they found it. Tropical food plants were not useful to colonial powers unless they could be farmed in the relatively cool, dry conditions of Europe. The exceptions were luxury crops such as cacao, tobacco, sugarcane, and coffee, which could be farmed in the tropics and then exported to Europe. Cassava was no luxury. Yet four hundred years ago Portuguese traders collected cassava stems from Brazil and carried them first to the islands off West Africa, including São Tomé, then to the Congo River delta of Central Africa,⁸ where they were planted, then to Asia, where they were planted again. Once on these new continents, cassava spread from village to village, playing a bigger role each time there was a bad season, a bad year. The more frequent hardships were, the more quickly cassava spread, until many became dependent upon it. Just as with potatoes in Ireland. Just as with sweet potatoes in China. Cassava is the primary source of calories in sub-Saharan Africa, the primary source of calories for five hundred million people. This is particularly true in Central African countries such as the Democratic Republic of the Congo, where as much as 80 percent of all daily energy comes from it—a single type of plant. In the DRC, one brings cassava leaves when visiting a house;⁹ the plant is a gift synonymous with life.

If a pest or pathogen were ever to destroy cassava plants across Africa the way that late blight destroyed potatoes across Ireland, it would be a desperate tragedy. It would be a tragedy for the hundreds of millions of children and adults whose skin sweats with its scent. In sub-Saharan Africa alone, nearly ten million hectares are planted in cassava. Cassava is too important to collapse, especially as soils get worse and climates more hostile, conditions in which few other crops will grow. Cassava is the secret strength and weakness of Africa, tropical Asia, and the world. But whereas hundreds of millions of dollars are spent studying coffee and wine, the funds available to study and monitor cassava have always been modest. They always will be. Cassava lives in the empires of dirt, where the poor raise civilizations out of red mud.

As a result, one thing was clear even before Danièle Matile-Ferrero identified the mealybug that had been sent to her: the Congo basin, and potentially much of Africa and Asia, had a big problem—and since the mealybug appeared to be new to science, it was a problem about which essentially nothing was known. Mealybugs do not look ferocious. They are delicate and seemingly without skeleton, a smudge of life. But what research would soon make clear is that this particular mealybug has a special fondness for the food Africa depends on and an ability to reproduce with sufficient speed to decimate all the cassava in Africa and, with it, millions of Africans. Tiny, soft, harmless-seeming mealybugs have the power to destabilize a whole continent. In the villages in which the mealybug had arrived, it was an affliction tough to distinguish from fate. Little could be done. Then, as now, few could afford pesticide.

In France, Danièle Matile-Ferrero continued to study the mealybug she had been sent. Then in 1977, no fewer than five years after it was first detected, she named it Phenacoccus manihoti—the cassava mealybug. It took five years for the insect threatening the main foodstuff of much of Africa to be named. The question was, what next? What could be done now that the insect had been identified? Matile-Ferrero was invited to the Congo basin to study the insect in more detail. Once there, she found that it had spread widely and was destroying whole fields of crops. She recommended controlling the spread of the mealybug through three methods: the targeted use of pesticides to treat any cassava that was being moved (which was cheaper than spraying whole fields), the breeding of resistant varieties, and biological control—the use of the natural enemies of a pest—its predators, pathogens, and parasites—to control its populations. Ironically, the monkey-beast that was initially accused of killing the plants was, as a caterpillar, a predator of mealybugs and hence one potential agent of biological control, if it could be bred in a lab and released in large numbers. But the real hope, it seemed, lay in finding another insect that could more effectively eat the mealybugs, a superagent of biological control. She listed some possibilities, insects she had collected that eat mealybugs in the field, but she noted that they seemed relatively ineffective and that they would need to be sent to other experts for more study.10

One person who would follow up on Matile-Ferrero’s suggestions, Hans Herren, was, at the time, still thousands of miles away. He was a young Swiss hippie working at the University of California at Berkeley, steeped in the ethos of a town where even the marijuana is organic. Herren grew up in Switzerland on a farm in the lower Rhone valley, where his father grew tobacco, wheat, and potatoes using traditional farming techniques. Under the wheat, his father grew clover. The clover shaded out weeds and could be sold to farmers. Other farmers, in turn, would provide Herren’s father with manure for the fields. Herren’s father bought little in the way of pesticides and fertilizer. Then one day, Herren remembers, a big black car pulled up from Basel. Men from the city had come to try to persuade his father to use fertilizer and pesticides, to modernize.¹¹ After that, as Herren remembers it, the yields on the farm were higher, but so were the costs—the costs of fertilizer, pesticide, and gas for the tractor and the costs to the environment. It was the distinction between farming before the men in the black car pulled up and after that led Herren to think about the benefits of working with nature rather than against it.

Once Herren finished his PhD, he considered taking a pest-control job in Switzerland, but he suspected that such a job would focus on chemistry—on better ways to kill insects using pesticides. Herren wanted something else. He applied for and got a postdoctoral position at the University of California to work with Robert van den Bosch, who was himself the son of a Swiss mother who grew up on a farm. Van den Bosch (Van, to those who knew him) was in the process of writing a book critical of pesticide companies.12 These companies were, in turn, threatening him with lawsuits. Then, not long after his book was published, van den Bosch died of a heart attack. Inspired by Van’s work and saddened by his loss, Herren was determined to do meaningful work in the world. Then he saw an advertisement for a job in Nigeria with the International Institute of Tropical Agriculture (IITA): they were looking for a plant breeder to work with corn (or maize, as it is known outside the United States). It wasn’t what he was trained for, but it seemed like an adventure, and he had the necessary skills.

What Herren could not have known was that his application had landed on the desk of Bill Gamble, at the time the director general of the IITA. Seeing Herren’s résumé, Gamble decided he needed Herren not for the job for which he had applied but for another. He needed him, with his background in crops and insects, to help try to figure out how to control the cassava mealybug. The two men met, and Gamble offered Herren the job on the spot. Herren soon left for Nigeria with big aspirations. He would stop the cassava mealybug and, in doing so, save a continent.

By the time Hans Herren arrived in Nigeria, where the IITA office was located, the spread of the cassava mealybug was extensive. Since 1973, the mealybug had moved from field to field in the Congo basin and then began to arrive in countries as far away as Senegal. It was spreading from port city to port city, perhaps as folks traveled to see their friends, bringing with them bundles of cassava leaves as gifts. How long ago this spread had occurred was hard to say, as was just how common it was farther out in the bush. When a new pathogen or pest is found on a crop, it has typically been around for a while. Farmers rarely report new encounters until the invader has become a problem. In this case, farmers may have been slow to report, but what took even more time was for people to pay attention to the farmers’ pleas for help. The mealybug had spread without challenge through the unprotected fields; it could spread in the gift of leaves. It could even spread, it would later be learned, by floating up into the wind.

As Hans Herren considered the mealybug and how to control it, he read Matile-Ferrero’s papers. He paid special attention to her notes about the likely origin of the mealybug. The insect, she noted, was most closely related to other species from the Americas. On the basis of this information, she hypothesized that it was from the native range of cassava and was succeeding in West Africa in part because it had found its food and escaped its own threats. Relatively unhindered by predators and parasites (the monkey-beast notwithstanding), the mealybugs ate—or, rather, sucked—with impunity. They sucked and mated and sucked some more. But just where the cassava mealybug came from—that was anyone’s guess. “The Americas” was all Matile-Ferrero could determine with any certainty.

Cassava is a domesticated plant. Relative to their ancient, wild ancestors, the domesticated plants on which we depend are often defenseless.13 Author Annie Dillard has written at length about Galápagos Islands animals that do not seem to know predators. They came right up to her, she says—the tortoises with their shells full of meat, the salt-spitting iguanas, the wingless cormorants, the penguins and the sea lions. Even the flies, she says, hover as if unafraid. They are naive, innocent, living in a bloodless land as if before the fall. Our crops are similar. Their innocence is less conspicuous than that of a flightless bird walking toward a large human yet far more consequential.

The relative innocence of our crops has two origins. We breed crops to produce as much food as possible, to grow without any concern other than getting as close to the sun as possible as fast as possible. This growth comes at the expense of the plants’ ability to defend themselves, and the more industrialized agriculture becomes, the more this is true. In addition, in some cases the defenses we use to protect plants against herbivores are also toxic to us, so by making plants more productive and less well defended, we also make them more palatable. But there is also something else, what scientists call enemy release. We move those crops to places where they will be safest, where they can most easily be released from the threats of their enemies and put all their energy into luxurious growth. Life is easier if you can escape the things that eat you, and one of the ways to escape them is to move or, if you are lucky enough, be moved. This geographic release from danger, from enemies, is central to understanding the threat to cassava as well as many other modern agricultural plants,14 and once plants achieve it, they are even more likely to invest in growth over defense.

If one were to draw a map of where the major crop plants are farmed today and where they were first farmed and domesticated, an unusual geography emerges. Crops are usually domesticated in one place, often in mountainous regions. They are then shifted to a new place, often a river valley associated with the rise of one or another early civilization. Finally, with the globalization of the world, in the era of ships, they began their longest journeys, to continents where they are not native. The biological stories of our ancestors and their crops repeat themselves. Life, while diverse beyond our ability to measure, obeys a set of laws. We have not escaped these laws.

Potatoes, for example, were domesticated in Chile and the Andes but are now grown mostly in North America and Europe. Vanilla was domesticated in tropical Mexico and is now farmed in Madagascar, Indonesia, and China. Squash, pumpkins, and some gourds were domesticated in the Americas but are now primarily grown in China. Sweet potatoes were domesticated in Central and South America but are now farmed primarily in China. Bananas were domesticated in Papua New Guinea, but the bananas that are exported are grown mostly in Central and South America. Rubber (Hevea brasiliensis) is native to the Amazon, but nearly all commercial rubber is now grown in tropical Asia. The cacao tree (Theobroma cacao), whose beans are used to produce chocolate, is native to the Amazon, was domesticated in Mesoamerica, and is now mostly grown in West Africa. Similar patterns exist for most of our crops, especially those from the tropics. These geographies are all the result of enemy release, our temporarily successful creation of worlds for our plants in which they are free of enemies.¹⁵ Yet just as predictable as this release is the inevitability that eventually the pests and pathogens will catch up. The job of border controls and quarantine offices is to slow down this catching up, to stall the herbivorous monsters, fungi, bacteria, and viruses in the absence of which our crops have thrived. Today, given the number of airline flights and ship crossings people undertake, this task is harder than it has ever been.16

Just how many pests and pathogens have yet to move? How charmed is our moment in history? In 2014, Sarah Gurr and her colleagues at the University of Exeter tallied all the pests and pathogens affecting the world’s main crop species in the regions in which they are native and in each of the places where they have been introduced. They focused on 1,901 enemies for which data were available in the Plantwise database, developed by the Centre for Agriculture and Biosciences International. The data in Plantwise are incomplete. Some of the identifications of pests are likely wrong (many pests and pathogens do not yet have names, after all). And the identity of the host on which each enemy was found is often uncertain. An entry might indicate that a pathogen was found on wheat but not distinguish among the many thousands of different varieties of the plant. Yet these data represent the best understanding we have so far of the enemies of agriculture and their distribution.¹⁷ Sarah’s assumption was that each of these crops, upon initial introduction into a new realm, whenever that might have occurred, was devoid or nearly devoid of pests and pathogens. Then slowly, following accidental introductions on ships, on trains, and in cars, the beasts began to catch up. But how fast? And where do crops still have the greatest reprieve and hence the most to lose with further introductions?

The enemies, Gurr and her colleagues found, have all caught up to some crops. Other crops, though, a few, are still experiencing something akin to complete escape. They live in a world without dangers. Patterns emerged when Gurr considered which enemies have caught up and which have not. Small things—oomycetes, fungi, bacteria, and viruses—catch up first. Very few of these pathogens are still found only in the regions in which their host crops were domesticated. If a fungus is not yet in a particular place, the odds that it will get there quickly are high. But Gurr also found something else, a geographic pattern to where the pests had and had not caught up. The crops being farmed in affluent countries, including the United States, France, Italy, the United Kingdom, and Australia, and in some rapidly developing countries such as China and India, have already, for the most part, lost the advantage earned through escape. Most of the enemies of crops in these regions have caught up. At current rates of accidental introduction of enemies in these regions, they will have all arrived by 2050. For crops in these regions, the only new reprieve would come from moving them to another planet or moving them indoors. (In practice, we create fields free of enemies through pesticides, though this, too, has its limits, to which we will return.¹⁸) Conversely, in the tropics one finds many regions in which most pests have not yet caught up with their crops. On average, tropical countries still host just one-fifth of the crop enemies that could live there, were all crop parasites and pathogens to arrive everywhere their hosts live. One-fifth. The other way to put this, of course, is that four-fifths of the enemies that could live in any given tropical country have yet to arrive. We can’t say when this will happen, but we can predict which crops it is likely to happen to first.¹⁹

Figure 4. In tropical countries such as the Democratic Republic of the Congo, most crop pests and pathogens that could live in the country—an assessment based on the climatic conditions and the crops being grown—have yet to arrive. For now, these species are held back by good luck and the human control and inspection of products being shipped to these countries. Data are drawn from Daniel P. Bebber, Timothy Holmes, and Sarah J. Gurr, “The Global Spread of Crop Pests and Pathogens,” Global Ecology and Biogeography 23, no. 12 (December 2014): 1398–1407.

Don Strong, now with the University of California at Davis, spent years considering which factors determine the rate of colonization of animal species in various habitats, including crop fields. He studied, for example, the pests affecting sugarcane and coffee plantations.20 He was interested in general theories underlying the workings of life on earth. But sometimes the search for generality also offers us particulars. When Strong compared the number of kinds of pests in sugarcane fields around the world, that number was predicted by one variable: the size of the field. Bigger fields had more kinds of pests;²¹ this pattern was a specific instance of a general phenomenon, the species-area rule. In 1977, when Strong carried out the study, the tiny island of Dominica, less than a third the size of Rhode Island, had around two hundred hectares of sugarcane and just six kinds of sugarcane pest. Much larger Puerto Rico had 151,000 hectares of sugarcane and more than a hundred kinds of sugarcane pests. A similar pattern emerges when one studies cacao plantations.²² Big areas provide more food than small areas; they are also bigger targets for pests arriving from afar. Where crops are non-native, pests arrive both through adaptation to the new crops (evolution favors any organism that can feed on an abundant food) and through dispersal from the crop’s motherlands. Subsequent research has shown that time also matters. The longer a crop has been planted in a particular area, the more likely the species that can colonize it will have done so.²³ For the theory of island biogeography, this is a beautiful illustration of the general rule that habitats with the largest amounts of area will contain the largest number of species. But for agriculture it has a sinister significance. It means that the more dependent we become on a crop, the more likely it is to be destroyed; this is what I am calling Strong’s paradox. Our most precious foods are the most susceptible targets.

Combined with Gurr’s results, Strong’s paradox leads us to the ominous prediction that our enemies will all eventually catch us and that they will attack us first where it most hurts, in our biggest, most sustaining fields—our fields of cassava, for example. Such targets are humanity’s weakness. If the mealybug hadn’t shown up, something else would have. In fact, since the mealybug first appeared, nearly a dozen other pests that affect cassava have arrived in Africa. Given the area over which cassava is planted, it was only a matter of time before some pest arrived. That the pest would be the mealybug was up to chance or luck. Very bad luck, as it would turn out.

For the cassava mealybug the most likely scenario was that nothing would stop its spread across Africa and maybe even across Asia. And while the losses these mealybugs caused were variable—depending on climate, the rainfall in a particular year, and the variety of cassava planted—they were often high, sometimes complete. Whole farms of cassava were lost. Wherever they went, the mealybugs seemed likely to leave hunger in their wake as certainly as if they had attacked the humans themselves.