Prairie

by Candace Savage

  And so a pest was born. The wheat-stem sawfly, as this insect somewhat misleadingly came to be known, went on to cause massive crop losses on the northern plains in the 1950s and 1960s. Although its importance diminished after solid-stemmed varieties of wheat were developed, it has recently staged a remarkable comeback. In the past, sawflies infested only spring-sown wheat, a crop that grows through the summer and is harvested in the fall, giving the insects time to emerge, lay eggs, hatch, and initiate larval development during the growing season. But they could not survive in fall-sown, or winter, wheat, which matures more rapidly and dries down before the sawflies are ready. Lately, however, evolution appears to have pressed fast-forward and produced a new strain (or perhaps species) of sawfly that emerges a month earlier than before, allowing it to colonize winter wheat as well. And so the process of adaptation continues.

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  > WHERE HAS ALL THE GRASSLAND GONE?

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  > THERE’S MORE TO A SKUNK THAN ITS STINK

  Much of the current research on farming and wildlife focuses either on insects, especially those that are troublesome to farmers, or on birds, especially those that are troubled by agriculture. But there are many other creatures that live in farming country and that are equally interesting.

  Take striped skunks, as an example. These pretty, placid little carnivores are found everywhere across the Great Plains, especially in farming country. Once restricted to wooded streambeds or shrubby meadows, they are now entirely at home in a landscape of grain fields, fencerows, and shelterbelts. Active primarily at night, they pad down farm lanes and windrows, stalking grasshoppers, digging up wasp nests, or sniffing at lumps of excrement to nose out beetles. Insects are their staple foods, but the skunks are opportunists and will readily consume fruits and vegetables, as well as mice, frogs, lizards, carrion, and the eggs and young of ducks and other ground-nesting birds.

  Although often described as solitary, striped skunks are actually quite companionable. Breeding females spend the spring and summer tending families of twittering young, usually four to six in a litter. Their nursery is a sheltered den under a building, inside a hollow log, or dug into the ground, sometimes the abandoned burrow of a badger, coyote, or fox. Completely helpless at birth, the kittens cannot walk until they are five weeks old, but they grow up fast and follow their mother on hunting trips by the age of two months, one plumy tail after another. A few weeks later, the youngsters are ready to fend for themselves, and the family disperses.

  As winter closes in, however, the skunks den up again, this time in groups that include one male for every roughly half dozen females. Sometimes as many as twenty animals curl up together to wait out the cold, not quite hibernating but very drowsy and slow. This peaceful atmosphere is disturbed briefly in late winter, when first the older females and then the yearlings come into estrus, but calm is quickly restored and the animals slumber on until the weather improves. In due course, the mated females choose their birthing dens and the cycle is repeated.

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  Hopper Heaven

  Wheat-stem sawfly

  Of all the insects that have benefited from farming, none have been more successful—or more devastating to human interests—than certain species of grasshoppers. During severe infestations, they may consume crops, gardens, shrubbery, and everything else in their path until, as the book of Exodus puts it, “not any green thing” is left. People who lived through the Dirty Thirties may still remember walking through a green slime of swarming insects. No wonder we tend to think of them as intruders. Yet grasshoppers are not intruders; they are native herbivorous insects that, over the last 50 million years or so, have evolved in concert with the natural vegetation of the grasslands.

  Although plants may look like passive victims, they are really small green warriors armed to fight off attack. Grasses, for instance, are girded with strands of abrasive silica that make them hard to chew and digest; leaves coated with hairs have much the same effect. Many forbs, including sagebrush and other aromatics, are laced with substances that block digestion or prevent nutrients from being absorbed. To survive, grasshoppers (like other herbivores) have had to come up with ways of overcoming these defenses, whether by developing more powerful mandibles and larger gizzards for grinding up tough plants or by producing antidotes to specific plant toxins. No one species can rise to every challenge.

  The result has been the evolution of hundreds of species of grasshoppers— more than four hundred in western Canada and the United States alone—each of which has overcome one or more sets of obstacles and gained the ability to feed on one or more types of vegetation. Some grasshoppers are so specialized that they can eat only a single type of native grass, while others are restricted to one family of plants. The rattling Carlinian snapper, for example, is thought to feed mainly on wheatgrasses, and Turnbull’s grasshopper (the farmer’s friend) eats only Russian thistle and other related plants. These highly specialized hoppers seldom, if ever, become pests, mainly because they are such fussy eaters.

  Turnbull’s grasshopper

  By contrast, the small number of grasshopper species that do infest croplands tend to be generalists, with wide habitat tolerances and broad food preferences. For example, the clear-winged grasshopper—a major pest of cereal crops across the north-central plains—is naturally adapted to eat many types of wild prairie grasses, including fescues, bromes, and bluegrasses. And what are wheat and barley but highly nutritious kinds of grass? The introduction of agriculture has provided clear-wings with mile upon endless miles of top-quality sustenance, far better than anything available to them under natural conditions. The network of mowed road margins has also benefited them by providing ideal sites for egg laying. (When laying, a female grasshopper bores a hole into the soil with her abdomen, releases a number of cream-colored-to-orange eggs, and then covers them with a protective secretion. This pod is the overwintering stage for all the pest species.)

  Another notorious pest species, the two-striped grasshopper, has had an even better run. A real go-anywhere, eat-anything herbivore, it is equipped to feed not only on grasses but also on a wide range of broad-leafed plants, including everything from mustard to sowthistle and from ragweed to leadplant. It can even eat timber milkvetch, a wild legume (found in the mountains and the Black Hills) so poisonous that a few mouthfuls can kill a cow. This hopper is one tough hombre, a kind of six-legged supervillain. Not surprisingly, it consumes all types of crops, including grains, alfalfa, corn, and garden produce, and lays its eggs in a variety of agricultural environments: roadside ditches, weedy summerfallow, stubble fields, and tame pastures. For species like the two-striped grasshopper, farmland is heaven.

  Having provisioned these insects so abundantly, farmers find it no easy task to prevent them from taking over. Grasshopper damage can be reduced by tactics such as early seeding, crop rotation, tillage, and the use of trap strips (green strips planted adjacent to a germinating crop, where grasshoppers can be lured and poisoned). When worst comes to worst, however, producers often end up relying on lethal force, by spraying infested regions with insect poisons. The current generation of insecticides is effective, but it is also crude, killing not only pests but any other insects or spiders that happen to be in the way. And while modern insecticides do not persist in the ecosystem as older control agents (like ddt) did, they can still leave behind a wreckage of collateral damage. One popular preparation, carbofuran, proved to be so toxic to birds—including several dozen California gulls that died after feeding on poisoned hoppers in Saskatchewan—that it was taken off the market in Canada and the United States as a grasshopper-control agent, though it is still used in other countries and for other purposes. And even when these poisons do what they are supposed to do and kill only insects, they still weaken the web of life by wiping out the food supply for insect eaters. Thus, the battle against grasshoppers may unintentionally take a toll on snakes, lizards, mice, skunks, bats, quail, small hawks, and songbirds—all of
which need insects to feed their young— among many others.

  Ultimately, the course of a grasshopper outbreak is beyond human control. Like other insects, grasshoppers are creatures of the weather. As cold-blooded organisms, they regulate their body temperature by seeking sun or shade (for example, by basking at the top of a stem or hiding under a leaf) to keep their tissues in the optimal functioning range. Warm, dry summers give grasshoppers an advantage, because they don’t have to spend so much time soaking up the sun and can put their energy into feeding and growing. If sunny weather persists into late summer and autumn, when most species lay their eggs, the scene is set for an explosive population increase. Under optimal conditions, a single female can produce as many as 250 eggs. If the following spring brings more fine weather, the new generation of nymphs, or hatchlings, will prosper and reproduce in their turn, until the whole place is hopping with hoppers.

  About the only thing that can rein in this runaway population is a good spell of rain. By preventing the insects from maintaining their body temperature, damp conditions hinder their development and growth. At the same time, humidity encourages the spread of deadly grasshopper diseases. Of special note is a fungal pathogen called Entomophaga grylli, which, when conditions are right, can decimate a population of grasshoppers almost overnight. A strange malady if ever there was one, E. grylli causes its victims to clamber into the top of plants just before they die. (Perhaps the illness makes them feel chilly and so they climb up to bask in the sun.) There they cling, so many small, faded cadavers grasping onto stems in what can only be described as a death grip. Forced into the service of their nemesis, the grasshoppers apparently serve as aerial capsules in which the fungus can mature and from which it can broadcast its spores. Although this disease can bring a quick end to a grasshopper outbreak, it requires ten days of warm, wet weather to develop full epidemic force and therefore is not reliable enough to be deployed as a biocontrol agent.

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  > “BARE . . . AS NEWLY PLOUGHED GROUND”

  This report, about a severe grasshopper outbreak in Cass County, west-central Missouri, appeared in the Globe-Democrat Correspondence on July 16, 1875:

  I do not exaggerate, but state the simple truth when I say that I have traveled time and again over the most of this township, and I do not believe there is one sprig of timothy, clover, wheat or corn left standing an inch above the ground in the township; that not a bundle of oats will be cut; not a pound of hay or grass of any kind will be saved this season; vegetables of every kind have been totally destroyed, and all the fields, without a single exception, so far as I have been able to learn, are as bare of vegetation, even weeds, as newly ploughed ground—notwithstanding the fact that some farms have been planted as often as twice and three times this season, and the wild grass and weeds on the outlands in both prairie and timber, have either been entirely devoured or cut down so close to the ground that cattle have been and still are starving to death by the hundreds. The owners having paid out all their money, sold everything they could get along without, and mortgaged their farms to get money to carry their stock through the winter and plant their crops, now are left with nothing to eat, their stock have starved to death, and they have no money, and no means of raising any by loan or mortgage, to buy food or to get away from here to more favored sections of the country.

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  > EDIBLE INSECTS

  Most grasshoppers breed in late summer and die before winter sets in, leaving the next generation to overwinter as eggs in the soil. There are, however, a number of notable exceptions, among them a smallish gray-to-green insect with bold geometric markings that is somewhat confusingly known as the brown-spotted range grasshopper. This species lays its eggs in midsummer, whereupon they either (a) sit there for an entire year and hatch out the following July—the usual pattern on the northern plains, though no one knows why—or (b) get busy and hatch out immediately, as one might expect and as commonly happens in the central and southern states. In either case, winter soon closes in on the young hatchlings and they are forced to take shelter under leaf litter or are buried alive in the earth.

  Come early spring—usually sometime in April— the half-grown hoppers emerge from their winter hideaways, resume feeding (on a diet of sedges and grass), and proceed to grow and develop. But no sooner do they become active, than they are set upon by a rush of breeding birds that spill across the plains for the nesting season. So many mouths to feed, so many bite-sized grasshoppers to stuff into them. Because these early-season grasshoppers are available in May and June, when other insects are scarce, they serve as a critical resource for many species of grassland birds. By the time the summer is over, a single pair of prairie songbirds and their nestlings can consume up to 11 pounds (5 kilograms) of insects, including as many as 149,000 grass-hoppers.

  Brown-spotted range grasshopper

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  Benefiting the “Beneficials”

  Because they are so responsive to the weather, grasshoppers and many other insects have a built-in tendency to experience cycles of boom and bust. Yet in a state of nature, it must have been relatively uncommon for even the most voracious among them to cut loose and reach plague proportions. The resources they required for survival—including their preferred food plants and suitable egg-laying sites—were often limited and dispersed. They had to struggle to meet their needs or, frequently, die trying. At the same time, they were faced with perils that included not only disease but a large and varied assault force of spiders and predatory and parasitic insects. Although these natural control agents have been pushed to the sidelines by high-tech agriculture, they are still out there in the ecosystem.

  Beefly

  Grasshoppers, for example, are beset by insect foes, including both bee-flies and blister beetles. Although beeflies really do look like bees, all hairy and striped, they only have one set of wings, a characteristic that identifies them as members of the order Diptera, the flies. (True bees, by contrast, always have two sets, fore and hind.) Blister beetles are typical beetles, though with long, narrow bodies that are often clad in wonderfully iridescent or striped wing casings, or elytra. (And, yes, they do contain chemicals that can cause blistering.) Although totally unrelated to each other, beeflies and blister beetles have adopted similar reproductive strategies. They lay their eggs in the soil close to grasshopper egg-pods, a resource that the gravid females likely locate by smell. When the larvae hatch, they wriggle around in the dirt and attack the egg-pods, consuming grasshopper embryos by the dozen. Although not numerous enough to control a severe outbreak on their own, they do their part in limiting grasshopper populations.

  The same is true of certain species of tachinid flies, which look like houseflies festooned with bristles or hairs. All members of this group are parasitic, which is to say that they typically lay their eggs on—or sometimes in—the larvae of other insects. There the eggs develop and pass through various larval stages before bursting out of their unwitting host as adult tachinids. Some of the species that infest grasshoppers are live bearers, hatching their eggs in their own bodies and then laying their larvae on, or in, the early nymphal stages of their victims. But however the attack is launched, these parasitic invasions are fatal.

  The banded argiope spider lives in open grasslands. Its wispy-looking webs are sturdy enough to capture and hold grasshoppers, which are thought to be the principal food of this species.

  The trick to maintaining populations of these natural pest-control agents is to provide food for them. Even predators and parasitoids have to eat. After they emerge as free-flying adults, beeflies, tachinids, and blister beetles all feed on pollen or nectar from their own preferred flowering plants. In other words, a weed-free, wall-to-wall monoculture cannot support them. Instead, they require, at a minimum, strips and patches of land that are left to grow weedy and wild, whether along roadsides, grassed watercourses, or fence lines. A diversity of plants supports a diversity of “beneficial” insects. For instance, t
he introduction of wild sunflowers has been known to reduce the population of greenfly on a nearby sorghum field, probably by providing nectar for a particular species of greenfly-killing parasitic wasp. And sometimes the linkages are even more circuitous. There is a certain tachinid fly, for example, that parasitizes the European corn borer, a pest that (as its name suggests) tunnels into cornstalks and destroys them. The tachinid is most effective in the presence of giant ragweed, a plant that is often infested by yet another type of stalk-boring insect. Since the tachinid parasitizes this insect as well, it can survive in its alternate host when European corn borers are rare, and so it is ready and waiting to mount an attack when the pest reappears.

  Tachinid fly

  It’s easy to see how providing food and alternate hosts for beneficial insects would help control pests. But there may well be more to the story than that. When plants are attacked by herbivorous insects, the leaves release volatile compounds into the air, which waft out from their tissues like distress signals. Predatory and parasitic insects can pick up these chemical messages (or smells) and interpret them, perhaps even to the extent of identifying the species of plant-eating insect that is present. If the predator or parasitoid catches the scent of an edible insect or a prospective host, it will home in on the signal and attack, thereby protecting the plant from extensive damage. The more species of plants that are present, the more varied and stimulating their chemical output will be, as if the sos call were beaming out on every available wavelength. Certain parasitoids have proven to be more effective in controlling pest infestations in mixed plots of corn, soybeans, and weeds, with their intermixed signals of distress, than they are in pure stands of corn, which all send out the same message. Diversity multiplies into complexity in natural systems.