How Not to Be Eaten. Dr. Gilbert Waldbauer
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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 are camouflaged or hidden, perhaps on the underside of a leaf, under debris on the ground, or in some other nook. Some, however, will leave their hiding place to flee if an intruder comes too close—within a critical distance whose length will vary with the species of the prey insect. The next chapter considers running away and hiding as ways to escape from predators, to avoid becoming a meal for a bird, a mantis, a mouse, or some other insect eater.
THREE
Fleeing and Staying under Cover
A well-hidden insect will be safe from many, if not most, insect-eating predators. But since natural selection is inexorable, predators will inevitably evolve with the anatomical and behavioral specializations needed to find and capture even the most thoroughly concealed insects. For example, if you hear what sounds like the blows of an ax in a winter woodland, it may well be a pileated woodpecker, the largest of our surviving North American woodpeckers, using its powerful, chisel-like bill to chop out chips of wood the size of a child's hand as it works to get at the larva of a long-horned beetle hidden deep in the trunk of a tree. An insect burrowing in the soil, such as a wireworm or a white grub, may be found by a mole or the probing bill of a grackle or some other bird. Nevertheless, hiding—although not always successful—can be advantageous, and insects of all sorts, and other animals too, have adopted this strategy for survival.
Natural selection favors—often very much so—an insect's normal lifestyle, especially its feeding behavior, if it keeps the insect out of sight and thereby protected from at least some potential predators. Usually only the larvae, insects in the immature stage, bore into plant tissues, burrow in the soil, or are otherwise hidden, and the usually immobile pupae generally remain hidden in the larval tunnel or burrow. The much more active adults are exposed to many more predators—spiders, insects, birds, mice, shrews, bats—as they fly and run about searching for nectar or other food, for a mate, and for appropriate places to lay their eggs. Most female insects lay hundreds of eggs, and many are exposed to predators as they fly long distances to distribute their eggs one by one or in small clutches on widely dispersed plants, often of only one or a few closely related species.
Figure 3. Disturbed by a predator, a grasshopper leaps into the air and flies off to make its escape.
In July and August we hear, high in the trees, the loud, shrill drone, the “love call,” of male dog-day cicadas—even in cities and towns. The females are frequently on the move as they disperse their eggs in small clutches laid in small cavities slashed into woody twigs by their sharp ovipositors, their egg-laying appendages. After hatching, the tiny nymphs drop to the ground and burrow deep into the soil, where they suck sap from roots until they emerge from the soil as adults about 2 inches long two or more years later. (Because the generations overlap, some cicadas emerge every year.)
Although the nymphs are relatively safe in the soil, the adults are eaten by birds of many kinds. Large, scary-looking but harmless solitary (nonsocial) wasps called cicada killers also