Promoting Pests

As terrifying as environmental catastrophes may be, they have a redeeming virtue: they are self-limiting. After an earthquake has released its energy, a fault may move little for tens, hundreds, or even thousands of years. A storm dissipates at sea. Even without human suppression, a forest fire runs its course. A power plant spews sulfur and nitrogen compounds and particulates only so long as it is in operation. A tanker or pipeline has only so much fuel to leak.

The pest, usually a less visible hazard than the physical or chemical one, is a more persistent, open-ended one. A pest is any plant or animal that flourishes by taking advantage of human-made environment change--in a way that injures human interests. The alteration can be the result of either a modified or a disrupted habitat. It can also be a means for migration to new territory. Whatever is changed, the pest competes successfully for resources with existing organisms, either domesticated or preferred wild ones, and may soon displace them entirely. Pests may also be directly or indirectly harmful to human health. Natural hazards are feared, but pests more often than not are hated as well.

Although infrequent but dramatic oil spills are more visibly destructive, the daily routine of world shipping has killed far more wildlife and endangered more species by spreading pests than by fouling seas and shores. John Balzar, writing in the Los Angeles Times, acknowledges that the Exxon Valdez spill killed hundreds of thousands of birds, but notes that by spreading rats to over 80 percent of the world's islands, shipping is condemning millions more. In the last four hundred years, rat predation has eradicated more species of land and freshwater birds--mostly on oceanic islands--than all other causes combined. Once established, rats are so difficult to eliminate that when they were suppressed on a New Zealand island with a surface of less than a square mile, a documentary film commemorated the feat. Environmentalists now recognize that the greatest threat to the abundant wildlife of the Aleutians and the Pribilof Islands is not from supertankers but from tiny creatures stowing away on smaller ships and barges. Pamela Brodie of the Alaska chapter of the Sierra Club told the Times: ``It bothers me personally how we set our priorities. We tend to ignore the chronic problems--which can be much more serious--in favor of the occasional accidents.''

Pests are a problem in environmental ethics, including rats--our fellow omnivores, camp followers, and laboratory surrogates. We despise them. Few people would begin to weigh a rat's right to eat birds against the preservation of native bird colonies--even though similar birds were routinely exterminated in a designated wildlife sanctuary when they were found to endanger planes at New York City's Kennedy Airport. Where should we draw the line in correcting the effects of our own disruptions? Thanks to our garbage, rats now mature more rapidly in cities, and grow larger, than they did in their original grassland habitat; house cats, once their ancient enemies, now are as likely to feed alongside them as to devour them. The cowbird is a native bird, but now it threatens songbirds as a nest parasite because our patterns of agriculture, settlement, and road building have broken up woodland habitats.

It is one thing to build nesting boxes for Eastern bluebirds that our agriculture has displaced and endangered. It is quite another to trap and drown the house sparrows that occupy these boxes, wring their necks, or put them in a sack tied to an automobile exhaust pipe--all techniques recommended in a pamphlet distributed by the North American Bluebird Society. Is it the house sparrows' fault that they too fit the holes designed for the bluebirds? More to the point, are they to blame if development has dangerously reduced the number of the bluebirds' preferred nesting places, decaying trees?

To many environmentalists, any intervention, even in favor of ``struggling'' species against ``aggressive'' ones to correct the results of human intervention, can amount to biological fascism. One distinguished environmental historian wonders whether a campaign to eradicate invasive plants in the Everglades might not be Nazi in spirit. The garden writer Michael Pollan and others have noted that Heinrich Himmler supported a movement to promote native German plants and garden designs to the exclusion of foreign organisms and landscape ideas. Other gardeners and nurserymen deplore a prejudice against new and useful plants, regarding as futile the search for an authentically native landscape. Even the smallpox virus has its advocates. Many microbiologists, ecologists, and philosophers question the ethics of destroying the last remaining laboratory stocks of it fifteen years after the last known naturally transmitted case of the disease. Some believe pragmatically that we can never be sure we will not need the organisms in the future. Others object that we have no right to extinguish any other life-form.

Pests reflect humanity's stunning success in modifying its environment by intensifying production. Farming land means replacing a natural plant community and, usually, growing a single crop. When the original cover is removed, long-dormant seeds of pioneer succession plants germinate, and others invade the disturbed land. Even a low-yield, preindustrial field of crops is an artificially uniform habitat. Its homogeneity rewards organisms, notably insects, that specialize in eating it. Natural selection assures that these organisms appear and flourish. Recently this process has accelerated. As farmers mechanized in the late nineteenth century, they also bred and selected their crops for easier machine processing. These uniform cereals, vegetables, and fruit also invited pest specialists. Moving a crop to a new habitat may turn a previously unimportant species into a serious pest, as happened when potatoes were introduced to the American Southwest from South America. A local insect that had limited its diet to the wild sandbur took on a new menace, and identity, as the Colorado potato beetle. And the twentieth century's Green Revolution of higher-yielding cereals has produced not only more food for people but greater losses to insects, weeds, and microorganisms. Farmers traditionally had selected for adaptation to natural conditions -- including characteristics that resisted local pests. By the 1970s, pathogens, weeds, and pests were shrinking the world's food production by about half, if losses both before and after harvest are included.

The Hazards of Improvement

It's a truism that human life is impossible without some pollution. Because, as we have seen, a prosperous industrial society seems to be good for people's health, a decline of economic output threatens it. Morbidity and mortality rates from a number of causes jump during depressions. This doesn't mean we can't make large gains. Most consumer goods could still be made to be more energy-efficient. Americans could shift (back) to the denser land-use patterns typical of Europeans and Japanese if they had the political will. Products could be redesigned for easier recycling. New control devices are already reducing industrial emissions. Many or most of these matters involve not revenge effects but trade-offs: price-competitive consumption goods against environmental values.

Consider polluted harbors again. Despite the continuing problems of oil seepage and chemical releases, most of America's and Europe's rivers and harbors are cleaner than they have been in years. Fish species have returned that were absent for decades.

Too bad cleanliness is part of the problem. We have already seen that soot from smokestacks helped neutralize acid rain before it was precipitated out, and that dust that farmers and builders pour into the atmosphere can do the same. The clouds of mud and toxic substances in America's harbors, we are learning, had surprising benefits. While killing game fish or making them inedible, they also poisoned the animals that attacked wooden harbor structures. Harbors became dirtier than ever but, for human purposes, more stable.

Reducing pollution changed all this. It helped not only sea bass but the marine animals that feed on the wood in piers and bulkheads. These crustaceans and mollusks, like many other pests, have been associated with humanity for centuries, plaguing Christopher Columbus himself. The tar and oil of the mid-nineteenth century killed off the creatures. These substances were concentrated enough in parts of the harbor that merely sailing into port could treat a ship's hull free. Later generations of petrochemicals also suppressed the populations of boring animals and even deposited a protective film on steel structures.

By the 1980s, wooden piers in the cleaned-up harbors were showing signs of renewed attack. Siphon-equipped shipworms (Teredo navalis), mollusks growing to nearly an inch in diameter and two feet long, were burrowing random channels in the wood with their spiny shells. Tiny gribbles (Limnoria lignorum), finishing the job, chewed the hollowed-out pilings to their core. The efficient intestinal bacteria of these organisms helped them to reduce timber at rates that amazed New York Harbor officials by the early 1990s--the diameters of pilings were reduced from up to a foot to only a few inches in just two years. For unexplained reasons, whether harbor structures contained different wood or the creatures have improved their performance, 150 years earlier, in the 1840s, it took not two but up to seventeen years to destroy a piling. Although coating structures with creosote and other protective chemicals can slow the attack, in another resurging effect, creosote-resistant strains of boring animals soon appear, while the chemicals eventually pollute the harbor themselves. Plastic coatings work but need laborious wrapping.

Marine borers do have surprising positive uses. The shipworm's method of lining the tube it digs with a chemical from its body reportedly inspired Marc Isambard Brunel's movable shield for constructing tunnels--a method still used after many refinements over a century later. And the harbor crisis is helping create a new market for millions of recycled plastic milk cartons in the form of borer-proof piers. Improving the environment may improve the environment after all. But until present structures are phased out, the borers are yet another chronic problem, and one estimated to cost hundreds of millions of dollars in New York Harbor alone.

What shipworms and gribbles do to wood and harbors, the shipworms' cousins the zebra mussels (Dreissina polymorpha) are doing to waterworks, rivers, and lakes--but on a scale of billions rather than hundreds of millions of lost dollars. Over more than two hundred years, these two-inch-long, gray-striped mollusks extended from their habitat in the Aral, Black, and Caspian seas and the rivers feeding them to the waters of western Europe. In German-speaking Europe their rapid spread earned them the romantic popular name Wandermuschel. By the early nineteenth century they had turned up in England. Not until the 1980s were they able to join the tide of immigration and trade to North America. Already in the late nineteenth century, the practice of taking on harbor water as ballast (as opposed to dry bulk materials like bricks, cobblestones, sand, and lead) accelerated the worldwide dispersal of marine organisms. The discharge of ballast water in harbors exchanges plankton and hordes of small organisms. A ship's tanks may carry dozens of species, and a harbor in Oregon has become home to fully 367 nonnative taxa. But the zebra mussels have been unique in their visibility, range of dispersion, and immediate destructiveness.

Within Europe, zebra mussel eggs and free-swimming larvae (veligers) traveled locally in ballast water; but transatlantic voyages were long enough to kill any young mussels that had managed to survive the filth of the harbors. Nor could adult zebra mussels endure the salinity of the open ocean. Europe built its public waterworks just as the mussel was spreading in its rivers and lakes. European systems were engineered to minimize the impact of obstructions by the mussels. Today, some have dual intake systems that are shut down alternately for cleaning. European engineers also specify short pipes and shun the right-angled sections where mussels flourish.

In the late twentieth century, mussels crossed the Atlantic barrier as they had traversed the English Channel 150 years earlier. Marine scientists are not sure why the mussels spread so much more rapidly in North America than in nineteenth-century Europe. There, they were present in Germany by the 1830s but did not reach Swiss lakes until the 1960s. On this continent, they were first found in Lake St. Clair only in 1986, yet were established throughout the Great Lakes and major Midwest river systems by the mid-1990s. Part of the reason may be the disturbance of the Great Lakes ecosystem by logging, but a series of otherwise positive innovations prepared their way. After the Second World War, European harbors (like American ones) became cleaner, and ships became far faster, taking as little as a week to cross the Atlantic and thus retaining more oxygen in their ballast water. With the opening of the St. Lawrence Seaway in 1959, ships from Europe with their bivalve stowaways moved directly into the Great Lakes, and the mussels could invade the rivers feeding them. Unaware of the potential disaster, U.S. and Canadian governments initially allowed unlimited release of water ballast in the Great Lakes. The size of the ships and the volume of water released were massive by nineteenth-century standards, possibly accelerating the spread of the mussels. Despite restrictions in the 1990s, zebra mussels now circulate throughout the fresh waters of North America. The 1993 Mississippi floods heightened their circulation in the Midwest. Now they use not only ballast water of commercial ships on the Great Lakes, but the bilgewater of pleasure boats--not to mention the hulls, hull openings, outboard and inboard motors, pumps, anchors, propellers, shafts, and other ship parts. Worse yet, fishery employees may be unwittingly helping to destroy their future catch by moving the destructive mussels among natural waters and hatcheries as they stock rivers and lakes with game fish.

Zebra mussels retain in adulthood a mass of threads called a byssus that lets them attach themselves to many surfaces. They use it. In North America they have alarmed manufacturers and public works authorities by proliferating in the intake pipes of factories, power plants, and water systems, blocking the flow of water. They also clog the water channels of locks and dams along the Mississippi, accumulating on the interior walls of pump valves and threatening to block the flow of water needed to cool air compressors operating the locks. One female can produce from forty thousand to one million larvae a year. Tens of millions of adults have been found in two-foot-diameter pipes. In the intake pipes of a large Midwestern electric plant, mussels have reached a density of 700,000 per square yard.

The mussels threaten not only public works but other aquatic organisms. They attach themselves to native mussels, smothering them and raising concerns about extinction. (Illinois has suspended the harvesting of native freshwater mussels until more is known about the prospects for their survival.) Their numbers and efficient filtering reduce the supply of nutrients to fish and to other shellfish. Although zebra mussels, like other exotic organisms, often are cited as ``natural pollution,'' they ironically clarify lakes and streams even while they impoverish them. Where excessive algae are a problem, the mussels can work apparent wonders. Russians use them to purify canals; Netherlands biologists are studying their value in managing lakes. (In some Dutch lakes, mussels filter the entire body of water in a month or less.) Wisconsin scientists have found that they eliminate 95 percent of the parasitic cryptosporidium protozoa that infected hundreds of thousands of Milwaukee residents in 1993. But the mussels' positive side has a negative side of its own; in absorbing contaminants, the mussels transfer them to lake beds and shorelines, poisoning some species of ducks.9 Conversely, the most popular and effective treatment, chlorine, is toxic to other organisms. Some biologists hope for deliverance by natural predators. Divers in Lake Michigan have found that native freshwater sponges of the Spongilla family share the zebra mussels' attraction to waters moving around breakwaters and piers. These sponges grow around the mussels, which are immobilized by their own filaments.

Parasitic worms and microorganisms can be as opportunistic as mussels and crustaceans in taking advantage of environmentalist measures and healthier eating habits. In the late 1980s, physicians on the West Coast and in Hawaii began to notice dozens of cases of painful infection by a nematode larva, Anisakidae, one of the revenge effects of environmental protection. Marine mammals have been flourishing under legal protection for over twenty years; sea lions alone have multiplied sixfold, reaching an estimated population of 177,000 by 1992 and decimating the steelhead population of the West Coast. Even more serious for people is what happens to the resurging mammals' parasites. Tiny crustaceans ingest from their feces and spread to fish--some of which are eventually eaten uncooked as sushi and sashimi, creating a small epidemic.10 Residents and vacationers on Italy's Adriatic coast have a related problem: the chemicals that have replaced phosphates in ``environmentally friendly'' European detergents have eliminated algal blooms, but in interacting with other minerals and chemicals found naturally in waters, they have also formed floating mats nourishing vast colonies of bacteria. These microorganisms in turn secrete a slimy, malodorous foam to attach themselves to the mats. Boaters are nostalgic for the algae, and phosphate lobbyists are campaigning to make their material legal for detergents again.

Of course, it was not foolish to clean up harbors. And water ballast costs much less and is far easier to control than other ballast materials. There are effective ways to slow the inadvertent spread of marine organisms; exchanging ballast water on the open seas will kill the stowaways. Redesigned marine structures and a variety of chemical and physical weapons are reducing some of the expected damage. But zebra mussels and other shipboard migrants will never be eradicated, only controlled. Species like the Asian clam Potamocorbula amurensis in San Francisco Bay are transforming their surroundings radically. They are another case of a chronic problem for which all solutions demand an added degree of vigilance.

Hazards of Improvement: The Household

The quest for comfort can be as hazardous as the pursuit of environmental purity. Living standards in North America and Europe have increased substantially over the last fifty years. As Ruth Schwartz Cowan has shown, twentieth-century domestic technology did not mean the end of the household as a productive unit: ``Households are the locales in which our society produces healthy people, and housewives are the workers who are responsible for almost all of the stages of the production process.'' Many household technologies, including those that lead to repeating effects (like more frequent washing), really do make people healthier. Central heating, for example, has caused chilblains--skin inflammations following prolonged exposure to damp cold--to become relatively rare.

But even home comforts can be hazardous to our health. Some have nothing to do with pests or bacteria. Vacuum cleaners and shampooing machines helped promote the wall-to-wall carpet as an emblem of middle-class comfort and health for generations. After the Second World War, manufacturers introduced a new method of gluing tufts of synthetic fiber to backing, which was originally jute and then polypropylene. As the near-universal choice for houses and offices alike from the 1950s, the new technology seemed to promise cleanliness; nobody can sweep dirt under a nailed-down rug. But environmentally it was (and is) an unknown quantity, especially in the ``tighter'' houses and offices of the last twenty-five years, in which insulation and weatherproofing have reduced the exchange of indoor and outdoor air. The fibers, latex glue, and backing are fused in an oven in a complex process that can leave two hundred known substances in the carpet. Many people believe chemical emissions from their carpets are making them sick, and a few have filed consumer complaints and lawsuits. Carpet emissions, in some experiments, appear to kill mice, although scientists at the Environmental Protection Agency have not been able to replicate the best-known study, by Anderson Laboratories. Still, years before the Anderson tests, when nearly a hundred EPA employees became ill in its redecorated office in 1987 and 1988, the carpeting was blamed. The agency subsequently removed nearly 27,000 square yards of carpet and introduced a carpet- and chemical-free zone for sensitive staff.

In fact there are proved health hazards of wall-to-wall carpeting. One is occupational: installers risk arthritis and other joint problems when they kick material into place with knee-mounted stretchers. But the problem affecting the largest number of people is that wall-to-wall carpets seem to have helped spread a pest--dust mites--that has in turn favored one of the most chronic diseases of childhood and adulthood: asthma.

We have already seen how hygiene can foster sickness: how young upper-middle-class adults were at greater risk of polio than age-mates with a dirtier upbringing. Hay fever is another disease associated with higher living standards. When the English physician Michael Bostock wrote the first description in 1819, he was one of the few known sufferers. In fact, as hay fever and other allergies multiplied in the nineteenth century, it was not working-class children growing up amid industrial haze but instead the scions of the best households that were affected. Epidemiologists are beginning to believe that large families, messy play, and early infections could have helped condition children's immune systems not to gear up against a common substance like pollen when they first encountered it. The protein that mediates hay fever, IgE, appears designed to defend the body against worm infestation. The allergist and historian Michael Emanuel has speculated that hay fever results from IgE deprived of its original target, noting that ``man evolved with his parasites and there may be a price to pay for their removal.'' (Other medical historians believe nineteenth-century industrial emissions and the rise in smoking were largely responsible. Bostock himself grew up in the industrializing North and worked with harsh laboratory chemicals.)

Comfortable middle-class households turned out to be technological and social systems that produced not only healthy people but chronically sick ones as well. The warm, humidified, well-insulated Western home is as comfortable for pests as it is for human beings. The medical entomologist John W. Maunder has evidence of ``a vast flea epidemic'' throughout Western Europe and large parts of the United States; taken together, the world's fleas probably weigh more than its people. From 1991 to 1992 alone, the number of requests for flea extermination in England increased by over 70 percent. Fleas are starting to appear even in middle-class households, and pet owners have yet to admit the need for disinfecting the whole house--not just the dogs and cats. The cat flea, Ctenocephalides felis, spends nearly all its time in carpeting or on draperies waiting for a cat or other warm-blooded host; ten thousand may be lingering, with only two dozen on a host at any time. People in flea-infested quarters, shunned by visitors, are said to cope with their loneliness by acquiring more cats.

With the virtual end of bubonic plague, fleas are more of an annoyance than a menace. But other arthropods are a more serious matter. The carpet, larger than ever, is a country club for dust mites as well as for fleas, just as the mats produced by those ``environmentally friendly'' detergents in Italy are floating resorts for bacteria. Cousins of the spiders, and less than a fiftieth of an inch long, the mites live on tiny flakes of dead skin in common dust. They thrive in the warmth and humidity of well-insulated, centrally heated housing. Cleaning may not work, since cold-water detergents apparently help save dust mites as well as energy. The fecal pellets of dust mites contain a powerful airborne allergen, Der p 1, which stimulates the immune system to inflame the airways. A study of children growing up in England showed that those from houses with high levels of dust-mite allergens were up to five times as likely as others to become asthmatic by their teens. Researchers estimated that children were being exposed to as many as 500,000 fecal particles per gram of house dust. Some of the worst risk factors are luxury goods like down comforters and pillows, and finely woven oriental rugs. But even simpler improvements in living standards may inadvertently spread mites and promote disease; the adult asthma rate increased 5,000 percent in part of Papua New Guinea after people began wrapping their heads in their recently introduced blankets at night.

Vacuum cleaners, long promoted for healthy living, actually make this problem worse, according to another English study. They do suck up dirt, but they also bounce mite pellets into the air, where they may stay suspended for days on end as they sink back slowly into the carpet--a classic rearranging effect. Vacuuming can triple the density of suspended droppings. Only a few expensive vacuum cleaner models use high-efficiency particulate air (HEPA) filters that trap such micro-debris.

Of course, carpets, drapes, and vacuum cleaners aren't the only promoters of asthma. Good construction and airtight insulation also contribute to the problem. Studies by U.S. Department of Agriculture entomologists suggest that cockroaches thrive in tightly built housing, and cockroach allergens are more prevalent. Materials shed by dogs, cats, fleas, mites, and cockroaches, plus secondhand smoke and industrial air pollution, might play a part, though asthma rates have sometimes risen even where air quality has appeared to improve. Poverty also increases the likelihood and severity of asthma. But since none of these risk factors is new, there is reason to think that the rise in severe cases of asthma comes at least in part from more home comforts and better insulation.

Acute episodes of asthma can be fatal; about 4,600 Americans died of attacks in 1990, double the number ten years earlier. But for most of the more than ten million Americans who suffer, the disease is a chronic one. Inhalants can control symptoms and open airways. Corticosteroids produce no significant side effects when inhaled, but another common family of asthma control drugs, the beta agonists, may increase the risk of a fatal attack. If one theory is correct, pocket inhalers with metered doses of beta agonists may suppress the symptoms of an asthma attack while leaving the patient continually exposed to dangerous antigens--another medical revenge effect. The search for comfort, then, helps further a situation of prolonged discomfort in a significant minority of the population. And while the discomfort can be managed and controlled, it is only with constant vigilance, monitoring, and adjustment--the hallmarks of a chronic problem.

Like the dust mites and more visible insects of the household, the most annoying organisms of agriculture are themselves revenge effects--animals that have flourished by seizing on resources we have assembled for them, or by accompanying us into territories where their natural predators are absent.

Transportation and air conditioning helped open the American Southwest for year-round living after the Second World War. A rising number of allergies sent hundreds of thousands of people from the Northeast and Midwest to seek a healthier working and retired life in the deserts of Arizona. ``Send your sinuses to Arizona'' became a legendary television pitch for antihistamines. And for a number of years, the arid Southwest delivered on its promise. It was never perfectly pollen-free; recent research has shown that pollen from at least one favorite native plant, the paloverde tree, may become windborne and irritate allergy sufferers. At first there probably were not enough landscaped specimens of these trees to be a noticeable problem. In any case, most native Arizona desert plants are pollinated by birds, bees, butterflies, and other insects, not by the wind.

If the new residents of the Southwest had followed traditional housing patterns, building adobe houses right up to their property lines, Arizona might have stayed largely allergy-free. But the migrants did not really want to go completely native. Like settlers everywhere, they grew nostalgic for the plants they had left behind. In the 1950s and 1960s, Arizonans started to build ``ranch'' houses with lawns. They brought Bermuda grass with them and showered it with the water that federal projects were diverting from Western rivers for their benefit, in the best traditions of self-reliant American individualism. Golf courses, country clubs, and resorts followed. The grass became a major seasonal producer of pollen. Lawns also provide food and moisture for allergenic molds, which increased by nearly tenfold in Tucson after the migration began.

Grass and spores are a relatively small problem compared with the wind-pollinated trees and plants that newcomers brought: olive, mulberry, cottonwood, sycamore, pecan, ash, and elm. Most of these produce large quantities of pollen over intervals of several months each spring. Warm temperatures sometimes extend the hay fever season to as many as ten months. Now Arizona has a relatively high concentration of people with pollen and mold allergies, about 230,000 in the Phoenix area alone, according to one estimate. Since 1985, Tucson has banned new olive and mulberry trees, with a corresponding reduction of 35 to 70 percent in tree pollen. Still, the city has to send out hundreds of letters each year reminding people to cut their grass before it grows to seed.

Fortunately for Arizona's allergy sufferers, an almost pollen-free variety of olive tree was discovered not long ago in Swan Hill, Australia. Its blossom drops without opening, and the little oily pollen that it does produce is too heavy to become windborne. It is now widely available. So too are sprays to stop older trees from pollinating. Plant biologists are beginning to talk about transplanting an engineered bacterial olive gene originally developed for pollenless corn. It could, they say, produce a nearly pollenless landscape within a decade. It is always possible that the new varieties may have revenge effects of their own, but these will not be known until the new trees are widely planted. So far, none has appeared. In the U.S. Southwest, at least, technology may indeed have the last laugh. The Frustrations of Extermination

Revenge effects come not only from the quest to make our own surroundings more comfortable. They also arise from the attempt to extirpate the pests that surround us. The impulse to slaughter whatever appears to threaten livestock and crops is almost certainly as old as agriculture. In the nineteenth century, cultivators and ranchers nearly wiped out many of the larger predators of North America and Europe with the not terribly high technology of firearms, poison, traps, and habitat destruction--with regrettably little knowledge of what might truly support agriculture. Not until the turn of the century did some states eliminate the bounties they offered for dead hawks and owls. Later-twentieth-century opinion, reflecting presumably enlightened urban and suburban environmentalism, substituted protection for persecution. There is new respect if not affection for predators as capstones of ecosystems, mirrored in books and films like Farley Mowat's Never Cry Wolf. It has generally benefited not only the animals themselves but their surroundings. When respect shades into sentimentality, however, revenge effects are sure to follow. Wolf hybrids, bred with sled dogs and German shepherds, can forget their lowly place in the chain of being and turn into New Age pit bulls without provocation. Rangers must warn visitors to U.S. national parks to keep their distance from bears--perhaps another unintended consequence of the Smokey campaign--and protected alligators are again a menace in Florida. Paradoxically, the one group of carnivores that has retained its ancient dread, the shark family, may be the one most threatened by humanity.

Internationally, rare and protected or otherwise popular animals can become pests with the disruption of their habitats. New Zealand parrots--scavengers and omnivores before the introduction of sheep to the islands--somehow acquired a taste for the fat around the animals' kidneys, and began attacking live sheep. Pandas in China, under intense stress from human intrusion in their habitats, have been known to raid penned-up sheep. (Of course, they are too slow to threaten free-range flocks.) Gray squirrels, harmless to forests in their native North America, have become woodland pests in the United Kingdom. The thicker phloem layer of beech and sycamore trees grown widely spaced in British tree farms (``plantations'') is filled with delicious sap, which appears to encourage squirrels to strip the bark for access. In growing numbers, the white-tailed deer and Canada geese of the United States have adapted all too well to a combination of resurging forests, encroaching suburbs, clipped lawns, and corporate ponds.

The most notable revenge effects come not from our efforts to control the larger pests--some of which, like the geese and deer, retain human admirers--but from our attempts to crush the omnipresent smaller ones.

Nineteenth-century farming, as its penchant for slaughtering resident wildlife suggests, was no Arcadia of stone-ground wholesomeness. The historian of science James Whorton, in Before Silent Spring, reminds us that American farmers used a fearsome array of toxic copper- and arsenic-based chemicals against molds, fungi, and insects. Preparations with deceptively colorful names like Paris green and London purple joined arsenate of lead and other substances as persistent poisons that endangered not only workers applying them but livestock, consumers, and, of course, the plants themselves. The burgeoning profession of economic entomology boosted the use of arsenical pesticides, dismissing the reservations of old-fashioned farmers about the chemicals' health effects and high cost. Consumers would have to eat hundreds of pounds of fruit to get sick, the entomologists countered. The head of the Department of Agriculture's Bureau of Entomology declared fears of injury from spraying to be ``utterly groundless,'' and his professional colleagues deplored the warnings of ``a few ignorant alarmists.'' Doggerel exhorted:

Spray, farmers, spray with care,
Spray the apple, peach and pear;
Spray for scab, and spray for blight,
Spray, O spray, and do it right.

In fairness to the entomologists, manufacturers were also using arsenic liberally, even in children's toys and coloring paper. It was only after the First World War that a few entomologists and physiologists began to study how cumulative doses of lead and arsenic compounds could cause neuritis, stomach disorders, skin disease, and cancer--all chronic side effects of a more intensive style of agriculture.

The paradox of pesticides--the chemicals that were supposed to replace discredited metallic compounds--became an even greater public issue. On the eve of the Second World War, a Swiss chemist named Paul Muller found what he and an entire generation thought was a miracle insecticide, DDT. Marketed in Switzerland, it was applied to over a million residents of Naples in 1944, saving them and Allied troops from an incipient typhus epidemic. It crushed insect-borne epidemics on the islands of the Pacific theater. Remarkably, it seemed perfectly safe. As Rachel Carson later noted, the body absorbs little DDT when the chemical is externally applied as a powder, though it can accumulate small amounts of DDT in its liquid form. Thus DDT appeared to offer almost miraculous protection from acute insect-borne illnesses without raising any of the risks to human health that the older generation of inorganic insecticides had posed. Even workers with heavy, prolonged exposure seemed to suffer no ill effects. Only one case of a directly lethal exposure is known: some DDT powder, confused with flour, was cooked in pancakes. (It was actually much less toxic than the potentially lethal organophosphate chemicals like parathion deployed after it was banned. Hundreds died working with them.) Postwar entomologists, armed with DDT, prepared to crush what one popular book of the period--ironically by a future satirist of science, Anthony Standen--assailed as the Insect Invaders. The magazine Popular Science foresaw ``total victory on the insect front.'' DDT came to menace the future because it seemed so safe in the present. Wartime entomologists discovered that aircraft could spray it economically diluted in an oil-based solution requiring a quarter pound or less of DDT per acre. After 1945, former military pilots continued the air war on bugs with surplus planes, including big transports retrofitted for spraying and dusting. Direct contact did not seem to hurt people, whereas the inorganic pesticides that preceded DDT had chronic as well as acute effects.

Slow arsenic poisoning has long been a cliche of mystery writing. Similarly, the most alarming damage done by DDT was not the immediate deaths of birds and fish from massive spraying but the effects of its invisible accumulation in their tissues and especially in their reproductive systems, including its thinning of eggshells. Eventually, the public began to worry more about the slow buildup of the compound in body fat and the risk of cancer, not to mention the hazards to those who actually worked with insecticides. Banned in 1972, DDT remains a suspected human carcinogen, but only on the basis of limited animal tests. Like other revenge effect technologies, DDT defused one problem by diffusing another. Yet abandoning DDT has had revenge effects as well, especially the contamination of aquifers by new generations of water-soluble pesticides.

DDT also showed for the first time the power of the resurging effect. In the teens of this century, entomologists began to report that some orchard pests were starting to resist the inorganic chemicals used against them. Cases remained relatively uncommon, though, because these chemicals attacked multiple sites in the target animals; they acted in a way that offered little scope for metabolic defenses. DDT and other synthetic organic substances changed all that. They opened the door to a new class of natural defenses: metabolic enzymes that could dexotify the new poisons.

There is a perverse logic to the spread of resistance genes. We saw it in antibiotic-resistant bacteria. The more effective a pesticide, and the more widely and intensively farmers apply it, the greater the potential reward for genes that confer immunity to it. In Sweden and elsewhere in Europe and North America, DDT-resistant flies appeared as early as 1947. By the mid-fifties, only ten or fifteen years after the Naples campaign, body lice in many parts of the world were already unaffected by DDT treatment. So were many farm, orchard, and forest insects in the United States.

The resurging effect was not limited to the unnatural selection of chemically self-protected strains. DDT actually fostered the reproduction of some insects by killing their natural predators. In the rubber and oil palm plantations of Malaysia, where there had never been a serious insect problem despite the hot and rainy climate, the application of DDT for a relatively small infestation of cockchafers led to a new plague of caterpillars, followed by still heavier insecticide doses and widespread defoliation. An entomologist called to investigate the case discovered that the poisons had wiped out the wasp parasites that had kept the most troublesome caterpillars in check. Unlike the wasps, the caterpillars could shield themselves from the poison by curling up while it was applied. Even after the poisonings were stopped and the parasites returned, the caterpillars were still on the rampage. The poisoning apparently had synchronized their life cycles so that vast numbers of caterpillars appeared at once, overwhelming the wasps. Stomach poisons harmless to the wasps finally kept caterpillars in check. In North America and Britain, use of DDT (beginning in 1949) against another insect, the codling moth, promoted the red spider mite to a major pest of fruit trees, again by killing its natural insect predators. In fact, in small doses DDT even appeared to make the mites breed faster, by mechanisms that still are not understood. Fortunately gardeners of the 1990s now can order other, harmless, mites that prey on red spider mites and die off when their hosts are gone.

Deployment of DDT revealed yet another revenge effect. Like antibiotic-resistant bacteria, strains of insects surviving DDT (or any other widely used chemical) tend to resist other compounds as well. Insecticides select not only for defenses against themselves, but for genes that make the animal better adapted to other aspects of its environment. Once this selection has taken place, it seems to make a population of insects more resilient in general. Even after DDT or another insecticide is withdrawn and immunity is gradually lost, it is reacquired more quickly. Intensification of the battle against insects seems to harden the enemy's defenses through natural selection. Megadoses build superbugs. While DDT has been banned in the United States for over twenty years, worldwide resistance to pesticides of all kinds continues to grow steadily. There were over five hundred resistant insect and mite species alone in 1990, and overcoming resistance requires higher doses and less cost-effective alternatives. Despite the vast improvement of agricultural yields since the Middle Ages, the proportion lost to insects, diseases, and weeds combined has not changed in the last fifty or five hundred years: it remains about a third. Internationally, the return of malaria has been due less to local bans on DDT than to increasing resistance of both mosquitoes and malarial parasites to pesticides and drugs of all kinds.

Insects learn to resist not only poisons but environmentally friendly chemicals like those that disrupt their development by mimicking juvenile hormones. In the 1960s, insect physiologists considered hormone mimics resistance-proof, but an experiment using a mutagen showed that a single genetic change could increase resistance by a factor of one hundred. Used intensively enough, even natural control agents like hormones, parasites, and predators can select for resistant pests. After heavy applications of the bacterium Bacillus thuringensis (Bt)--hailed as an effective, nontoxic agent--some pests have become resistant to it as well. The quantities present in nature, where Bt outbreaks are rare, had been too small to exert selective pressure for resistance genes. Bt's popularity changed that. Meanwhile another bacterial weapon, milky spore, has lost credibility against the Japanese beetle.

Revenge effects don't make either chemical pesticides or ``natural'' agents useless. Resistance has changed the strategy and tactics of combat from roundhouse slugging to biological judo. Subtle, time-winning gambits mean more than the search for a devastating first strike. Growers have been limiting doses, delaying them until a threshold of economic damage is reached, applying pesticides less often to smaller areas at more carefully calculated times. They have been alternating and coordinating them with introduction of natural enemies. (Unfortunately, some pet owners still combine insecticides, a practice that helped make multiple-resistant fleas among the most difficult insects to control in the early 1990s.) And they have even been reintroducing nonresistant strains of pests to mate with small populations that are beginning to develop resistance. These ploys don't eliminate the underlying resurging effects, but they do buy time in delaying them.

Regrettably, agribusiness is still killing weeds in the good old way: dousing them with more chemicals. In ten years, the number of herbicide-resistant weed species has grown from a dozen to more than a hundred worldwide. To permit use of longer-lasting herbicides in the U.S. Midwest, seed producers now offer corn that tolerates one of the most popular but persistent chemical groups--chemicals that, after having been sprayed in soybean fields, would otherwise damage the young shoots of the next corn crop. (Farmers plant corn and soybeans in rotation.) But by using the same herbicides year after year instead of in rotation, farmers growing herbicide-resistant crops will inadvertently promote herbicide-resistant weeds. Still worse, there is growing evidence that genes from crops can find their way into weeds, which may not only be their next-door neighbors but their unruly cousins. If this happens, the resistance genes can spread so rapidly that superweeds could join the ranks of multiple-resistant problem organisms.

Fire Ant Follies: The Vietnam of Entomology

Disastrous as it has been in the long run, DDT at least had the virtue of working for a few years--and even now, in limited, carefully defined circumstances, it can still work, according to its defenders. But the pesticide campaign mounted by the Department of Agriculture's crusade against fire ants, a struggle that continues, was not only damaging to wildlife but counterproductive. Natives of the upper Paraguay River floodplain where Brazil, Argentina, and Paraguay meet, red fire ants (Solenopsis invicta) first landed in Mobile in the 1930s. They soon proved to be among the insect world's fastest and fiercest colonizers. Not only did they overwhelm the native species of fire ant by building over forty mounds per acre against the natives' four or five, but they came to dominate the related and previously introduced black fire ant (S. richteri) as well. Like other pest species, fire ants specialize in changing and disrupted landscapes, including flooded riverbanks in South America, nurseries and sod farms here. They take to rich suburban lawns just as fleas and dust mites do to household carpeting. In fact, their colonies are five to ten times as dense in the United States as they are in their native territory, probably because South American parasitic flies (phorids) disrupt their foraging and reduce their ability to compete for food with other insect species. The phorids are still not present in the United States.

S. invicta ants still produce more bloodcurdling anecdotes than provable economic damage. Because they kill just about everything that comes too near their mounds, they fight boll weevils as well as attack birds, reptiles, and small mammals. (Ironically, the boll weevil is one of the few major pests that are losing the war against pesticides; it is vanishing from the Cotton Belt.) A mound can be up to a yard high and five feet in diameter, linked to a wider tunnel system and housing hundreds of thousands of ants. Although a number of children have died from multiple bites, fire ants are not a life-threatening menace to people but a chronic nuisance. They hold on with their jaws as they inject venom, and while the results are not as intense as a bee sting, the itch is worse than a mosquito bite. In infested areas, between 30 and 60 percent of the population are stung each year, and tens of thousands of Southerners seek medical treatment for fire ant bites. Children playing in the grass are at highest risk.

S. invicta meets any definition of a pest. It devours seedling trees. It kills young calves and fawns. Its mounds obstruct and damage farm equipment. With its boundless appetite for insulation, it even disables traffic signals and other electrical equipment. But until recently it was not a fearsome economic threat. It does eat germinating crop seeds, but its favorite dish is the grubs and larvae of other insects, and it generally doesn't attack mature crops. It is not in the same league as the corn borer, the Colorado potato beetle, and other crop and livestock pests. What probably helped raise its status in the world was the resurging effect that an eradication campaign made possible.

The agricultural historian Pete Daniel has revealed the background of the chemical fire ant campaign in the power struggles of the farm bureaucracy. A network of agencies, including the land grant universities and the Agricultural Research Service (ARS) of the U.S. Department of Agriculture, emerged from the Second World War believing that biological control was outmoded. In the Cold War climate, the future appeared to belong to new chlorinated hydrocarbon insecticides like DDT, which could defeat the fire ant as the atomic bomb had beaten Japan and was containing the Soviet Union.

The first pesticide sprayed massively by the ARS with congressional authorization, dieldrin, was twenty times more toxic than DDT and drew unsuccessful protests from the Fish and Wildlife Service of the Interior Department--all the more after studies showed that the dosage of active ingredient per acre was 60 percent higher than necessary. Heptachlor was substituted, then discontinued in the early 1960s in favor of another pesticide, mirex, which in turn was found to harm wildlife and marine life, and possibly to cause cancer in human beings. Meanwhile biologists were discovering that the ants were killing sicker and weaker animals, as good predators should, while pesticides were wiping out quail and other wildlife indiscriminately. Not until 1978 did the spraying program stop. By then, the USDA had sprayed millions of acres, spent $200 million, and left more fire ants than ever.

As some of the early DDT sprayers discovered, heptachlor, mirex, and the rest killed not only fire ants but their natural insect enemies--for example, species that eat fire ant queens. Armed with a genetic heritage of explosive reproduction and colonization, invicta not only recovered swiftly but moved into the niches its insect enemies and competitors left behind. One study at the University of Florida has shown that a broad-spectrum insecticide helped fire ants increase their share of the resident ant population from 1 percent to 99 percent in only four years. By 1990 they occupied 400 million acres in the South and Southwest. And the campaign may have promoted an even more ominous trend that already dominates Florida: densely spaced supercolonies, as many as five hundred per acre, each with a hundred queens or more, resulting in average densities of over 175 ants per square foot and peak densities of over 500 per square foot.

Single-queen colonies compete with one another and attack individual ants that stray into them from neighboring colonies. But multiple-queen colonies, linked by tunnels, seem to form an extended fighting organization capable of wiping out almost all other forms of insect, reptile, bird, and rodent life in its path. This is all the more enigmatic because such behavior was unknown in the ants' original South American habitat. Since multiple queens were first observed in 1972, after the heptachlor and mirex campaigns had been under way for fifteen years, pesticide spraying may have promoted the change by inadvertently selecting for genes previously expressed only rarely.

Whatever their origin, the new colonies now appear to have a foothold in Southern California and are said to be poised to move up the West Coast and to extend their domain in the Southeast at least as far as the Mason-Dixon line. They are so resilient that they migrate not just in potted plants but as part of insecticide shipments. Workers can defend and relocate queens, thereby reestablishing colonies so fast that fire, boiling water, and most poisons are ineffective in combating them. While there are baits that will slowly kill the ants, scientists and politicians alike have abandoned eradication for control. Control in this case means exercising constant vigilance to live warily with a chronic nuisance. Excursus: An Electrical Alternative?

The problems of suppressing insects chemically make it natural to wonder whether there might be revenge effects in attacking them electrically. In fact, insect electrocution is a venerable theme in America's technological history. We have seen in the first chapter that ``bug'' was telegraphers' jargon for a hidden fault in the circuits, but it had a related and literal meaning for the operators. Western Union's city offices were notoriously dirty and insect-infested. Thomas Edison himself, as a young telegrapher desperate to ``debug'' his desktop in 1868, invented a pioneer electrical roach trap rigged to the office battery.

While Edison chose not to pursue this line of work, insect-electrocuting devices were big business a century later. U.S. sales of products with trademarked names like Zappers, Bugwackers, and Bug Blasters reached a peak of nearly $100 million in 1984. Sales have declined since then, with Asian imports battering domestic producers, but the industry is still large enough to have its own trade association. The products work similarly: intensified ultraviolet light lures insects to an electrically charged grid of alternating high-voltage current. Fluid in their bodies closes the circuit--the current arcs even if they don't touch the grid--killing the insects by dehydration and action on the nervous system. Suburbanites supposedly enjoy hearing the zapping sounds.

The relief may be only illusory. In the early 1980s a team of scientists at the University of Notre Dame did a field test using a range of backyards adjoining a drainage ditch or other desirable insect habitats. Notre Dame graduate students (acting as bait) sat in yards with and without the bug zappers; they collected the mosquitoes as they began to bite. When the investigators later studied the insects killed, they found that the overwhelming majority were gnats. Female mosquitoes, the only ones that draw blood, accounted for a paltry 3.3 percent of insect remains. More important, the presence of a zapper did not reduce the number of attempted bites. Mosquitoes prefer warmth and carbon dioxide to ultraviolet light. And the insects breed so rapidly that even if zappers were more efficient, they would be unlikely to reduce their numbers significantly.

Even if successful, insect electrocution devices might have the revenge effect of selecting for mosquitoes and other insects that avoid ultraviolet light, just as electric fly killers in some Mississippi barns are said to have selected for a tendency not to land on walls. But the real revenge effect of zappers may be yet another chronic one: promoting allergies. If used indoors or close to food preparation areas, electrocuting traps are known to produce allergenic debris, especially since some moths arc as long as thirty seconds. High voltage tends to fragment insect bodies and disperse particles. Over four hundred papers have been published on rhinitis, conjunctivitis, and asthma among people working with insects or inhaling insect parts. Researchers point to the physical characteristics of moth and butterfly scales that keep them suspended in air. To make things worse, pathogens carried by flies and other insects may be aerosolized and spread, negating the most important reason for killing them. When intensification of the battle goes too far, insects can bite back even postmortem.

It may in the end seem that all intensification of agriculture, and of the struggle against household dirt and insects, has been some terrible mistake, that all we need is to return to nature. And indeed we are doing some things overintensively. Even after abandoning mirex and DDT, we are nevertheless multiplying some pests by killing their natural enemies or by disorienting the behavior of insect predators with sublethal doses. David Pimentel and his colleagues list over a dozen pests helped by pesticides that destroy predators. In cotton fields, these include cotton bollworms, tobacco budworms, cotton aphids, spider mites, and cotton loopers; in apple orchards, three species of mites, three of aphids, and two of scales persist. Pimentel has placed the price of chemical interference with natural enemies in the United States at over $520 million, about half due to extra control costs and half to lost production.

Many farmers are discovering that ``low-impact'' agriculture can actually be more profitable than intensive chemical applications. Farmers and gardeners have cut back insecticide use significantly since the 1970s. They can get many of the benefits of chemical treatment and avoid many of the costs and revenge effects by using much smaller quantities, timed and applied more accurately. Some are making more money without chemicals than they did with them, partly because their prices jumped sharply in the early 1960s. (The market has been at least as powerful a damper on pesticide use as any environmentalist group.) Like antibiotic producers, agricultural chemical manufacturers continue to find ways to keep ahead of the insects; but they are finding that even in Asian countries where there is an overwhelming dependence on heavily treated high-yield rice crops, cutting back on spraying is actually increasing harvests.

On balance, pesticides can increase crop production safely, if used in the right places, at the right times, and in the right quantities. Even most of their scientific critics acknowledge their value, at least for the near future. Taking into account all their adverse environmental and health consequences, and their cost, our nutrition and health would suffer if we suddenly tried to do without them. Just as California earthquakes taught lessons about collapsing buildings and bridges that were written into each new generation of building codes, biological revenge effects are not futile. The question is what lessons we should draw from failures like mirex and equivocal successes like DDT. If we insist upon the search for a new wonder substance that will effectively eliminate a given problem, the result will probably be the same: the adaptability of more resilient creatures will inevitably win. If we learn from revenge effects we will not be led to renounce technology, but we will instead refine it: watching for unforeseen problems, managing what we know are limited strengths, applying no less but also no more than is really needed.