The Forest Unseen: A Year's Watch in Nature (15 page)

The dependence of leafhoppers on their bacterial helpers is of particular interest to entomologists in the pest control business. Leafhoppers and aphids exact a heavy toll on crops and often transmit disease to the plants that they puncture. If the relationship between the insect and its bacteria could be poisoned or otherwise disrupted, the entomologists might be able to wash fields clean of these troublemakers. This idea has yet to be put into practice, but I hope that if it ever is, we will not let the bright light of our ingenuity blind us to the possible costs of our actions. Chemicals that sever the tie between beneficial bacteria and their hosts may have effects well beyond ridding crops of leafhoppers. The soil’s vitality depends on the action of such bacteria, as does the health of our own gut. At a deeper level, all animals, plants,
fungi, and protists have ancient bacteria living inside their cells. Leafhoppers are the tip of the iceberg. Hammering at this tip risks sending fractures throughout.

The mandala contains insects designed to steal every part of a plant. Flowers, pollen, leaves, roots, sap are all preyed upon by a diverse toolbox of insect mouthparts. Yet the mandala is green. Leaves are a little tattered, but they still dominate the forest. Above, leaves are stacked in layers, blocking my view of the sky; around me, shrubs stretch out across the hillside, again hemming in my sight; below, my feet rest in a carpet of saplings and forest herbs. The forest seems to be an herbivore’s heavenly banquet. Why is the mandala not stripped bare? This is a simple question, but it is much fought over, and it stirs up controversy among ecologists for good reason. The relationship between herbivores and plants sets the stage for the rest of the forest ecosystem. If we don’t get the answer right, or if we cannot produce an answer, our understanding of forest ecology is shipwrecked and we are swimming in ignorance.

Birds, spiders, and other predators may give part of the answer. Their hunger might hack back the ravenous hordes of insects, protecting plants by preventing herbivore populations from getting large enough to fulfill their destructive potential. A corollary of this idea is that herbivorous insects seldom compete among themselves; they are suppressed by their predators, not by their peers. This is important because competition is the force that drives evolution. If herbivore populations were limited solely by predation, we would expect natural selection to have lavished more effort on helping herbivores to avoid predators than on giving them an edge in competition for food.

The idea that insect populations are suppressed by their predators has been tested by building cages around plants. If predation rules the insects’ world, insect numbers should explode inside the cages, and caged plants should be eaten down to nubs and stumps. The results of
caging experiments are mixed. Insect populations do sometimes increase when their predators are kept away, but the jump in numbers is seldom dramatic, and in some seasons and locations the cage has no effect at all. Even when cages do boost insect populations, the caged plants remain leafy green, albeit more chewed than their uncaged kin. Predation cannot, therefore, be the only explanation for the seeming paucity of herbivores.

We too are plant eaters, and our feeding behavior suggests another approach to the puzzle of the greenness of the woods. I live surrounded by maples, hickories, and oaks but have never sat down to a tree-leaf salad. Forest herbs grow in profusion at my feet but, again, I have not dined on them. My medicinal botany books tell me that small doses of the herbs in the mandala may alleviate ailments, but anything more than a nibble will cause (depending on the species of herb) heart stoppage, glaucoma, stomach upset, tunnel vision, or irritation of the mucous membranes. Our domesticated crops have had the toxins bred out of them, giving us a distorted view of the realities of herbivory. Granted, we have not evolved to be leaf eaters and lack the detoxifying biochemistry of most true herbivores, but our inability to eat most of the plants that surround us reveals an important point: the world is not as green as it seems. This point is reinforced by the very fact that other herbivores have specialized biochemical methods to neutralize the toxins in their food. The mandala is not a banquet waiting for guests to arrive but a devil’s buffet of poisoned plates from which herbivores snatch the least deadly morsels.

Organic chemists confirm the experience of our taste buds. The world is a bitter place, full of deterrents, digestive disrupters, and poisons. Hawks know this also, using fresh greenery to line their nests and drive out fleas and lice. Consider also the
New York Times
. Insects grown in containers lined with old copies of the paper fail to reach maturity. The quality of the insects’ reading material is not the culprit, although insects raised on the London
Times
mature into adults. The
New York Times
is printed on paper containing the pulped wood of
balsam fir. The fir tree produces a chemical that mimics the hormones of its insect herbivores, thus protecting itself by stunting and neutering its enemies. The London
Times
is made from trees that lack hormonal defenses, making the pulped flattened remnants of their bodies safe to use as bedding for laboratory insects.

We can now turn our question around and ask not how plants manage to survive assaults by herbivores, but how herbivores cope with noxious plants. The puzzle is no longer that the world is green but that the greenery has holes made by creatures who do not die after dining. Detoxifying countermeasures are the foundation of the herbivores’ ability to eat poisoned plants, but the insects also try to dodge around the defenses by feeding on those parts of plants they are most able to digest. It is no coincidence that the green caterpillar in the mandala is feeding on
young
maple leaves. Maples, like many tree species, defend their leaves with bitter tannins. Tannins are effective deterrents only in high concentrations, so young leaves have not yet accumulated enough of these chemicals to make them noxious. If the same caterpillar were to hatch in August it would face a forest steeped in tannin. The springtime emergence of many herbivores allows them to sidestep the plants’ defenses.

The biochemical swordplay between plants and their herbivores has created a tense stalemate in the mandala. Neither side has yet routed the other. The holes and incisions of the mandala’s leaves are the marks of this year’s round of cut and parry, the venerable duel from which emerges the mandala’s fundamental character.

H
ungry ladies dance in the air, swoop at my arms and face, then land and probe. They have flown upwind, excited by my smelly mammalian promise. No doubt the nakedness of my skin further stimulates them; no dense mat of hair to obscure their dinner table. What an easy meal!

One of the mosquitoes lands on the back of my hand, and I let her probe my skin. She is mousy brown, just a little furry, with scallop patterns along her abdomen. Slender curved legs hold her body parallel to my skin. A needle juts from under her head. She slowly moves this lance across my skin, seeming to test for a suitable spot. She stops, holds steady, then I feel a burn as her head drops between her forelegs and the needle slides in. The sting continues as she penetrates deeper, sliding in several millimeters. The sheath that held the needle has bent back between her legs, leaving a tiny length of thin tube exposed between her head and my skin. The needle looks like a single shaft, but it is a bundle of several tools. Two sharp stylets help cut into the skin, making way for salivary tubes and a strawlike food canal. The salivary tubes ooze chemicals that prevent blood from clotting. These same chemicals cause the allergic reaction that we call a mosquito bite.

The needle is flexible, so it bends after it enters the skin and, like a worm seeking a soft patch of soil, it probes around inside my skin, sniffing out a blood vessel. Capillaries are too small, so the mosquito
searches for a larger vessel, a venule or arteriole, the state highways of our blood system. Veins and arteries, the interstates, are too tough-coated to be of interest. When the needle finds the object of its search, the sharp end punctures the vessel’s wall. The flow of blood over the needle stimulates nerve endings that signal pumps in the insect’s head to start pulling blood. If the mosquito fails to locate a suitable vessel she will either pull out and try again, or feed on the small pool of blood that the needle created as it tore through capillaries in the skin. This pool-feeding method is much slower, so most mosquitoes that fail to hit a good-sized vessel prefer to pull out and try again, seeking a rich seam elsewhere under the skin.

The mosquito on my hand has evidently pierced a productive vessel. In just a few seconds her light brown underbelly balloons into a shining ruby. The brown scallops on her back that mark each segment of the abdomen move apart, seeming to dislocate her tidy body. She rotates as she feeds, perhaps pushing the needle around a curve in the blood vessel. When her belly is stretched into a half globe, she abruptly lifts her head and, in a blink, flies off. I am left with a slight burn on my hand and two milligrams less blood.

These milligrams are a trifle for me, but they have doubled the mosquito’s body weight, making her flight ponderous. The first thing she’ll do after finishing her meal is rest on a tree trunk and offload by urination some of the water she has swallowed. Human blood is much saltier than a mosquito body, so she’ll also pump salts into the urine, preventing my blood from disrupting her physiological equilibrium. Within an hour she will have dumped about half the water and salt from her meal. What remains, the blood cells, will be digested, and my proteins will find themselves turned into yolk in a batch of mosquito eggs. The mosquito will also take some of my nutrients for herself, but the vast majority will be used for egg production. The millions of mosquito bites inflicted on us every year are therefore preliminaries to mosquito motherhood. Our blood is their ticket to fecundity. Male mosquitoes and females who are not breeding feed like bees or butterflies,
sipping nectar from flowers or drinking sugars from rotting fruit. Blood is a proteinaceous boost for mothers only.

The mosquito’s colors and fuzz mark her as a member of the
Culex
genus. This means that she will lay a small raft of eggs on the surface of a pond, ditch, or stagnant pool.
Culex
often breed in the fetid water that surrounds human habitations, giving them their common name, “house mosquito.” Females fly a mile or more from these breeding areas, searching for a suitable blood donor. My blood may end up in eggs laid in the pond half a mile behind me or in a blocked gutter or sewer in the town a mile away. Here the eggs will hatch into aquatic larvae that live suspended just under the water’s surface. Their rear end is an air tube that adheres to the water’s surface film, providing both an anchor and a breathing hole. Their heads hang down into the water and filter bacteria and dead plant matter from the cloudy water. During their life cycle, mosquitoes therefore exploit three of the richest food sources available to animals: the bounty of wetlands, the concentrated sugars in nectar, and the viscous feast of vertebrate blood. Each meal propels them into their next life stage, creating a nearly unstoppable momentum.

If I had not visited the mandala the
Culex
would likely have found another blood donor for her meal. Despite their fondness for human habitations,
Culex
usually feed on bird blood. This is to the birds’ detriment because
Culex
transmits disease, most notably avian malaria and, of late, West Nile virus. Avian malaria lives in the blood of about one-third of the birds flying above the mandala. Most of the infected birds appear to be able to live out their lives without being substantially weakened by the parasite. Birds infected with the invading West Nile virus have higher mortality, perhaps because America’s birds have no natural resistance to this virus from Africa.

When
Culex
mosquitoes cannot find a crow or chickadee, they feed on humans. This flexibility of dining arrangements brings bird parasites into contact with human blood. Some, like avian malaria, die in the alien surroundings. But others, including West Nile virus, will
sometimes take hold and infect the human. This jump from bird to human blood requires that a mosquito first feed on an infected bird and pick up the virus, which then multiplies in the mosquito’s salivary glands. If the mosquito then feeds on a human, the drop of saliva may now carry an unwelcome guest, and the West Nile virus may jump from crow to human.

Perhaps I should not have been so sanguine about my exsanguination; my curiosity may have allowed another life-form to take over my body, perhaps even kill me. I am, however, hardly dancing on the precipice. In the whole of North America only four thousand people were infected with West Nile virus last year, fifty-six of them in Tennessee. About fifteen percent of these cases are fatal, making the virus fearsome if you get it but, compared to the other risks we face each day, a very minor threat. The newsworthiness of the virus comes not from the magnitude of the threat that it actually poses to us but from its novelty, its indiscriminate choice of targets, and our inability to predict whether it will bloom into a larger threat. The virus is also a gift to pesticide manufacturers, scientists feeding from the government’s largesse, and news editors desperate for sensational copy. Fear and profitability have launched the virus to stardom.

Until recently a much more deadly threat to humans hung over the mandala. Another species of malaria lurked in mosquito salivary glands, waiting not for a bird but for a human. In the first years of the twentieth century the average rate of mortality from malaria for people living in the southern United States was about one percent per year. In the swamplands of Mississippi the rate was three percent; in these Tennessee hills it was lower but still significant. Malaria’s terrible weight used to oppress people all across the eastern United States, but eradication programs eliminated it from the Northeast in the nineteenth century, decades before clearing the South. Malaria’s end in the South came about in the early twentieth century after a campaign that targeted many stages of the malarial life cycle. Huge quantities of quinine were distributed to treat infected people and prevent reinfection of
mosquitoes. Screens on windows and doors were encouraged or required, breaking the link between mosquito saliva and human blood. Wetlands and ponds were drained to remove breeding sites for mosquitoes, or oiled to smother their larvae, or poisoned with insecticides. Although malaria’s hosts, mosquitoes and humans, still lived across the South, the distance between them was stretched sufficiently that the parasite dropped into extinction.