kinglets, and downy woodpeckers wander through the woods foraging for insects. The birds, so to speak, gang up on the insects. Field studies summarized by Kimberly Sullivan “showed that individuals can benefit from membership in a flock by decreasing their risk of predation and increasing their foraging efficiency.” Flock members constantly sound contact, or social, calls—such as the chickadee's chick-a-dee-dee—that announce their presence and help to maintain flock cohesion. They also have calls, such as the chickadee's high-pitched zeee, that warn of an approaching hawk or other predator. Most small birds, according to Susan Smith, freeze or dive for cover in response to the warning calls of their own and other species. Sullivan, as we will see in chapter 9, found that downy woodpeckers spend much more time eating and much less time cocking their heads from side to side to watch for predators when they are with a flock than when they are alone, because the constant contact calls of their companions assure them that others are also keeping an eye out for predators.
Many of the 4,500 species of mammals—from tiny shrews, bats, and mice to huge bears—feed on insects to varying extents. Some, such as the African aardvark and the giant anteater of South America, eat nothing but ants and termites. Many omnivores, among them bears, raccoons, opossums, chipmunks, foxes, squirrels, mice, and skunks, include insects in their diets.
Primates such as lemurs, tarsiers, monkeys, baboons, chimpanzees, and humans are omnivores that, to varying degrees, feed on insects. In the early 1960s, Jane Goodall made the famous discovery that chimpanzees create tools from twigs and use them to “fish” for one of their favorite snacks, termites—the tropical species that build large cementlike mounds. Early in the rainy season, swarms of thousands of male and female termites of the reproductive caste leave the mounds through tunnels dug by workers, who keep the exit holes thinly sealed until conditions are favorable for the reproductives to fly off and found new colonies. When a hungry chimpanzee spots one of these lightly sealed holes, Goodall observed, it removes the seal with its index finger and pokes a tool into the hole. A moment later the chimpanzee withdraws the tool and then eats the termites clinging to it. Children in Africa use the same technique to get a few termites for a snack, but adults on the continent make ingenious traps to catch swarming termites—a much-favored food, delicious when roasted—by the thousands. As I explained in Fireflies, Honey, and Silk, people of almost all non-Western cultures eat insects, usually as a special treat.
Bats, the masters of the night sky and the only mammals capable of true flight, are not blind, but they find their way in the dark by means of echolocation (sonar). In flight they emit sounds too high-pitched for our hearing and sense obstacles and their prey, usually insects, by listening for the echoes that bounce back from them. As the Nobel laureate Niko Tinbergen observed, on what is to us a quiet summer evening, to the bats flying about and the moths that can hear them “the evening is anything but calm. It is a madhouse of constant shrieking. Each bat sends out a series of screams in short pulses, each lasting less than a hundredth of a second.” In chapter 9 we will consider the bat's echolocation in more detail and the question of how moths benefit from an ability to hear bats.
Shrews, which may weigh as little as a tenth of an ounce, are the smallest mammals on earth, and because of their tremendous metabolic rates—their hearts may beat 1,200 times a minute—they are the most voracious of the insectivorous mammals, and probably the most voracious of all mammals. Every twenty-four hours, a shrew eats the equivalent of its own body weight or more in insects, other arthropods, and occasionally a mouse or other small mammal. Shrews live and hunt in extensive runways at or just above ground level.
The mouse-size short-tailed shrew (Blarina brevicauda), common in the eastern half of southern Canada and the United States, is active both day and night throughout the year and is one of the world's few venomous mammals. Delivered in the saliva as the shrew bites, the venom is toxic to both small mammals, which this shrew seldom attacks, and insects, which are the most important part of its diet. The experiments of Irwin Martin showed that crickets and cockroaches are immobilized by the venom but do not die until three to five days after being bitten. “Venom,” Martin reasoned, “was therefore acting as a slow poison as well as an immobilizing agent. Immobilization for 3 to 5 days may extend the availability of fresh non-decomposing food, and thus enable Blarina to optimally exploit a sudden abundance of insects by caching some. If all hoarded insects were dead, many might [decay and] lose substantial nutritive value before the shrew could eat them.”
Insect eaters do, of course, help to prevent insect populations from soaring to ecologically disruptive levels—always a possibility because an insect, depending upon the species, will lay anywhere from a few to thousands of eggs. If, on average, two of a female's eggs survive to become reproducing adults, she will have replaced herself and her mate, and the population of her species will not increase. But if only an additional two survive, the population will increase by a factor of two in each generation and will soon become an ecologically disruptive force. Clearly, dozens or even thousands of a female's offspring must perish—and predators eat many of them.
The many insect-eating animals, from the little crab spiders to birds and even huge bears, consume enormous numbers of insects. In so doing, they exert the powerful selection pressure that results in the evolution of the many ways in which insects can survive by avoiding or defending themselves against predators. A few examples from agriculture show how great the selection pressure from predators can be.
In 1887, sap-sucking cottony cushion scales, insect invaders from Australia, infested California orange groves, threatening to destroy them all. Knowing that these scales were uncommon in Australia, where they were never destructive, Charles V. Riley, a great pioneering entomologist, reasoned that they were controlled in Australia by an enemy absent from California. He postulated that the scale population would crash if this enemy were introduced into California. Therefore a few hundred vedalias, ladybird beetles that eat these scales, were imported from Australia, and in less than two years only a small and inconsequential population of cottony cushion scales survived, coexisting with a few vedalias that kept them in check. In 1945, DDT, which kills vedalias but not the scales, was sprayed in the orchards to control another insect. As was to be expected, seriously destructive outbreaks of cottony cushion scales followed, but the benign balance of vedalias and scales was restored when the use of DDT was discontinued. Robert L. Metcalf and Robert A. Metcalf underscored the importance of predators in controlling pest insects with an example involving two native American insects. In 1899 in Maryland, in just a few days, sieves used in packaging fresh peas separated out twenty-five bushels of hoverfly larvae, which feed on aphids. “They were so abundant that they almost completely destroyed the pea aphids in the fields.”
In 1979, Richard Holmes and his coworkers showed that birds alone can significantly decrease populations of some plant-feeding insects. They covered plots of striped maple shrubs in a New Hampshire hardwood forest with nets that excluded birds but not insects. Nearby uncovered areas of similar size and with comparable growths of striped maple shrubs served as controls. The exclusion of birds, especially ovenbirds, black-throated blue warblers, veeries, and Swainson's thrushes, caused a significant increase in the numbers of leaf-eating caterpillars.
Similar experiments by Robert Marquis and Christopher Whelan in Missouri showed that insectivorous birds decreased the number of plant-feeding insects on white oak saplings by half, which in turn allowed the saplings to increase their aboveground growth by one-third. Like Holmes and his coworkers, they covered some saplings with nets that excluded birds and left other saplings uncovered.
Many of you have seen grasshoppers leap into the air and use their wings to make a speedy retreat when you come threateningly close to them. When I turn on the lights in my laboratory at night, panicked cockroaches swiftly run off to find a hiding place. (Entomologists can't use insecticides in their laboratories; insecticides kill not only cockroaches but also the insects that are the subjects of our experiments.) Many insects do not respond to most disturbances by fleeing, because they
