Innumerable Insects
Page 12
The precise use of these creatures in the economy of the universe is not very easy to define, and although I cannot go so far as with [Linnaeus] to give the louse full credit of preserving full-fed boys from coughs, epilepsy, &c. yet I do think it probable they may be conducive to health, in a certain degree, by promoting cleanliness; for were it not for the great increase which soon takes place, if a colony are allowed undisturbed possession, there are individuals, probably, who are so lost to all sense of decency, that they would never clean themselves at all. But by means of this peculiar stimulus, it becomes absolutely necessary to have a bath now and then.
Had Denny understood their role in transferring the bacteria responsible for diseases as devastating as trench fever and typhus, or similar agents that infect livestock and fowl, then he might have looked upon them with a bit more trepidation and concern. It is fair to say that the smaller bacterial foes are far greater enticements toward cleanliness than the mere itch of a louse.
Denny had intended to expand his work into the exotic species of lice, amassing specimens from colleagues, including material from Darwin taken during his voyage on the HMS Beagle. He spent years preparing the fine lithographs necessary to accompany his grand vision for this supplement. Unfortunately, the scope of the work envisioned was massive and left incomplete at the time of his death. John O. Westwood (see pages 50-53), the Hope professor and curator of entomology at Oxford, purchased Denny’s collection and plates from his widow, intending to see the work finished, but even the indomitable Westwood was overwhelmed by its scope and projected cost. Today, the material remains in Oxford’s museum.
In the end, Denny published only two works, his monograph on British lice, and an earlier (1825) and equally lovely treatment of the British pselaphine and scydmaenine beetles. Nonetheless, his contribution to the study of parasites was tremendous, and it would be nearly half a century before any real improvement would arrive in the form of the work Les Pédiculines: Essai monographique (Lice: Monographic Essay) (1880), by Swiss entomologist Édouard Piaget (1817–1910). Although Piaget’s coverage of species surpassed that of his predecessor, Denny’s sublime portraits remain superior, and through his artistic hand we lose some of our revulsion toward these animals, perhaps even discovering in them a subtle beauty.
The lice of humans: clockwise from top left, the body louse (Pediculus humanus humanus), the head louse (P. h. capitis), and the crab louse (Pthirus pubis). The body louse and head louse are subspecies of the same species, presumably having diverged from each other only about one hundred thousand years ago. From Denny, Monographia.
Of course, pests do not only feed on us—they also attack our livestock and crops. For every species of mammal or bird that we raise, including those we cherish as pets, there are a number of parasitic insects that are more than delighted to make a meal of them. At their worst, the swaths of destruction caused by some infestations can lead to famine and all of the evils that may bring.
Horses and cattle can be greatly affected by numerous insects, including blood-feeding horse flies, black flies, bot flies, screwworms, lice, fleas, and many others. These insects can directly cause injury and death or act as vectors for debilitating or fatal infectious diseases. Myiasis is the presence of a fly maggot feeding within the tissue of a mammalian host. Bot flies and screwworms (the maggots of certain species of blow flies) are some of the more notorious culprits in cases of myiasis. For instance, the screwworm Cochliomyia hominivorax is particularly devastating to cattle; the maggots cause grotesque lesions in the skin as they burrow into healthy tissue and feed. Once the maggots—which resemble small screws—burrow in, the only portion that remains visible are their breathing tubes at the tail end of the body. A related species, C. macellaria, prefers to eat dead tissue and has become an important tool for medical examiners and detectives as the progression of the maggots’ development within a body can reveal the time and location of death. This practice helped spawn the field of forensic entomology.
LOCUSTS AND OTHER HERBIVORES
Less sickening to behold than maggots in festering wounds are the varieties of caterpillars, aphids, bugs, and beetles that feed on fruits, vegetables, and grains. Perhaps the most famous of agricultural pests, however, are locusts, a species whose plagues have been written about since cuneiform and hieroglyphics were our primary scripts. Locusts are grasshoppers that may undergo swarming under specific conditions; numerous grasshopper species are subject to such influence. Usually solitary insects, overcrowding spurred by food shortages or drought can trigger a gregarious phase initiated by an increase in serotonin. This leads to a cascade of physiological and behavioral shifts, including a higher metabolism, which causes individuals to eat more voraciously, breed more readily, become more attracted to one another, and ultimately act as a cohesive entity. In several species, individuals in the gregarious phase also take on different body colors relative to those in the solitary phase.
As they continue to compete for our crops, locusts and other grasshoppers—such as the large and colorful Tropidacris cristata shown here—have greatly influenced human societies, culture, and mythology over the millennia. From Rösel von Rosenhof, De natuurlyke historie der insecten.
The great father of forest entomology, Julius T. C. Ratzeburg, gorgeously illustrated his monographs, depicting here the appearance and biology of grasshoppers found in Germany’s forests. From volume 3 of his Die Forst-Insecten (1844).
The largest swarms consist of billions of individual locusts that can cover vast areas. When a swarm takes flight it can easily be redirected by wind currents, so some swarms can get pushed out to sea to die or even deposited onto glaciers. Such swarms have been found in thick layers of ice thousands of years old, giving us a glimpse into ancient outbreaks. As one might imagine, billions of locusts require a great deal of food—each insect eats the equivalent of its own body weight in vegetation per day—and cultivated farmland is ripe for the reaping. It is therefore not surprising that Egyptians and other people of antiquity were so scared when winds unexpectedly brought down from the skies thick clouds of locusts that devoured their food and so invited famine and death. Locusts still plague us today, and damage to crops can run well into the billions of dollars. The most devastating species are the desert locust, Schistocerca gregaria, and the migratory locust, Locusta migratoria.
Leaf beetles (family Chrysomelidae) are varied in biology and form, with some appearing like miniature colorful tortoises, owing to the expansion of their hardened wing coverings. From Jules-Sébastien-César Dumont d’Urville, Voyage au pôle Sud et dans l’Océanie sur les corvettes l’Astrolabe et la Zélée (1842–1854).
Herbivorous insects of all varieties feed on our fields and food stores or ravage our ornamentals and hardwoods, skeletonizing and blighting leaves, boring through twigs and stems, and producing unsightly galls. The list of such herbivore pests is extensive, but the majority is found among the larvae of beetles and moths, and the nymphs and adults of aphids, thrips, true bugs, grasshoppers, and katydids.
One of the world’s major pests is the Colorado potato beetle, Leptinotarsa decemlineata. Although named for their destruction of potato crops, the beetles are equally fond of other vegetable crops of the plant genus Solanum, which includes tomatoes. The larvae are the ones that do the damage, and populations can quickly reach levels in which nearly every plant is being nibbled away by plump, red larvae. Originally discovered in the mountains of Colorado in the early nineteenth century, this beetle is native to Mexico and the United States but it is now a pest throughout the world wherever potatoes, tomatoes, and eggplants are grown. During the Cold War, the CIA was falsely accused of using these beetles as a form of biological weapon meant to undermine Soviet agriculture. Little did the Central Committee of the Soviet Union realize that the beetles required no help from any government in spreading and making a plague of themselves. Today, this species is becoming even more troublesome owing to its increased resistance to pesticides, and so sust
ainable practices have the greatest hope of achieving “détente” between the beetles and ourselves.
Leaf beetles (family Chrysomelidae) are a diverse group with over thirty-seven thousand species, and their hungry larvae can defoliate plants. From Ratzeburg, Die Forst-Insecten volume 1.
INSECTS OF THE FORESTS
In any field of endeavor there is always progress, with new and important information steadily accumulating every year. Eventually, there can come a tipping point in which a dramatic synthesis and reshaping of the field takes place—a paradigm shift that places all past efforts in a new light and casts aside many misperceptions. The catalyst for just such an intellectual revolution in one branch of entomology was Julius T. C. Ratzeburg (1801–1871), the patron saint of forest entomologists, who may rightly be credited with having founded the field as a discrete science. Many insects live in forests, but forest entomology concerns itself with advancing forest resources as they relate to those species that promote or hinder forest management. Prior to Ratzeburg, works that one might loosely classify as covering forest entomology principally documented little more than major insect outbreaks without any attempts to really investigate the underlying biology. Johann M. Bechstein (1757–1822) and Georg L. Scharfenberg (1746–1810) had together produced the definitive work on forest insects at the time. Their three-volume Vollständige Naturgeschichte der schädlichen Forstinsekten (Complete Natural History of Destructive Forest Insects) (1804) was, however, greatly lacking as neither author wrote from much personal experience in dealing with the species covered. They instead summarized much from other, often erroneous, sources.
Ratzeburg’s representations of forest insects on their host plants were so accurate that volumes of Die Forst-Insecten were distributed throughout Germany as a standard reference for identification and reference. This plate, from volume 2, depicts five different species of geometrid moths (family Geometridae), many of which can be notorious pests going by various common names, such as the peppered moth (Biston betularia) depicted at top center.
Ratzeburg began his career teaching in Eberswalde, not far from Berlin, and witnessed firsthand extensive damage to the surrounding forests. He immediately went for Bechstein and Scharfenberg’s volumes and promptly found them to be superficial. Recognizing that something substantive and rigorous needed to be done, in 1835 he launched into a major effort to provide the much-needed synthesis. Today it is taken for granted that any scientific investigation begins with a thorough review of the available literature, yet at the time such a course of action was rare. Still, Ratzeburg pulled together and thoroughly digested all of the available literature pertinent to forest insects. He also wrote to every forester and natural history scientist and assimilated the responses to his letters. He inquired with them as to their own observations, the veracity of past findings, and what experiences they might have had in dealing with particular pests. In time, the German government instructed all of their forest officials to send information directly to Ratzeburg, and his office became the hub for all things relating to insects and forests. All of this was fine and valuable, but Ratzeburg felt that the only true means of developing a real understanding of these species was through personal observation and experience. He therefore spent large parts of nearly every day out in the forest, making his own detailed notes on the lives of the many pertinent species, including corroborating or refuting the numerous observations sent to him by correspondents. Rearing many species back in the lab, Ratzeburg was able to experiment with conditions to see how these might impact the survival or development of each insect. In this way he was able to correct misguided notions that had persisted for decades, if not centuries.
The life cycle of various species of plant-feeding wasps, colloquially known as “sawflies” (family Tenthredinidae, order Hymenoptera), as depicted in a plate from volume 3. The sawfly larvae superficially resemble caterpillars and feed on leaves, denuding entire trees if left unchecked.
In 1837, Ratzeburg published the first of his three volumes on forest entomology, introducing the subject and discussing beetles and weevils. This was followed in 1840 by a volume on moths, and in 1844 by a book concerning other groups such as locusts, aphids, sawflies, lacewings, and true flies. Each volume was accompanied by detailed color plates depicting particular species, and, quite importantly, Ratzeburg included illustrations of the immature stages of many species with examples of the plant injuries they inflicted. These illustrations are exceptionally accurate, and most of them were drafted by Ratzeburg himself. As a testament to the scope and value of the work, the frugal ministry paid to have individual copies sent to every forest official in Prussia. Ratzeburg’s Die Forst-Insecten (The Forest Insects) would stand as the most extensive and detailed work of its kind for a century, and even today, his beautiful etchings of insects are remarkable for their fine accuracy.
The three volumes Ratzeburg produced on forest insects were filled with detailed observations on injurious insects, but made all the more useful by the detailed lithographs depicting the insects, their life stages, and the sometimes characteristic damage they produced, such as with the depictions of three butterflies shown here from volume 2: the pine hawk moth (Sphinx pinastri, top), and the black-veined white (Aporia crataegi, lower left), and blackleg tortoiseshell (Nymphalis polychloros, lower right).
WEEVILS
One of the most successful of all insect groups is the weevil (superfamily Curculionoidea), a lineage of specialized beetles famed for their elongate snouts but also infamous for the many injuries they wreak upon plants. There are already around sixty thousand species of weevils that have been documented from around the world, and new species turn up on even the most cursory of entomological expeditions. Weevils of one kind or another have evolved to feed on virtually every part of a given plant, specializing to feed as adults and larvae on roots, stems, leaves, flowers, seeds, and everything in between. This wide breadth of feeding is largely due to their snouts. Despite misconceptions, the long, slender snouts of weevils are not for sucking fluids; they actually have the usual complement of jaws at their apex, merely in reduced form. The snout permits the weevil to not only feed on otherwise inaccessible plant parts, but also to chew deep into specific plant tissues or into the soil, making holes in which to place their eggs. Weevils, like most beetles, have less-than-impressive ovipositors, and the snout permits them to access spaces for their eggs that would otherwise be out of reach. The hatching larvae are then, ideally, placed to feed within the roots, stems, or other parts of the plant, leading to considerable damage, particularly when populations are large.
Numbering over sixty thousand species worldwide, weevils (family Curculionoidea) are among the most injurious of beetles, and their characteristic anatomy permits different species to feed on virtually every part of a plant. From Ratzeburg, Die Forst-Insecten, volume 1.
The galleries made by bark beetles (family Scolytidae), although seriously deleterious to trees, can nonetheless be stunning to behold. Bark beetles are nothing more than specialized weevils that have lost their characteristic snouts during the course of evolution. From Ratzeburg, Die Forst-Insecten, volume 1.
The cotton boll weevil (Anthonomus grandis) was originally native to central Mexico but was introduced into the United States in the 1890s, and by the roaring 1920s had ravaged the entirety of the South. Today, it remains the primary pest of cotton. Species of the weevil genus Sitophilus are devastating to many of our most critical crops, feeding on and developing within the grains of rice, wheat, and corn. Not all weevils compete for our foods, and some, like the bright red Merhynchites bicolor, or rose weevil, drill through the buds and petals of flower gardens, rendering them horticultural nightmares. Others, like Ips calligraphus, are difficult to recognize as weevils owing to the secondary loss (the reversion to an ancestral state) of their snouts, but they remain equally destructive. Commonly referred to as bark beetles—obscuring their actual status as modified weevils—these insects infest many of the hardwo
ods we are so fond of using for timber. Ips calligraphus is so named for the fine “calligraphy” these beetles etch through the wood as their larvae burrow, creating remarkable patterns but rendering the wood useless. Bark beetles in all of their variety represent some of the greatest threats to the forest industry, particularly the invasive species that lack the natural checks and balances native species have evolved as a control against their populations running amuck.
THERE ARE TENS OF THOUSANDS more species of parasitic and herbivorous insects that do not attack us or feed on our crops. Accordingly, they are not pestiferous to us. These insects are pests to those species they impact, but their pestilence usually goes on without our taking any notice. Most parasitic insects victimize other insects, and although they are technically parasites, we consider several of them as beneficial since we can use their powers to our advantage—usually as a natural control over the populations of those species that do impact us. For example, parasitic wasps are fearsome to those insects that they attack, but they can be used to our advantage in biological control programs. By way of illustration, the minute wasps of the genus Aphytis are parasitic upon mealybugs and scale insects that feed on citrus, olive, and other orchard trees. The wasp injects its eggs into the host’s body, and the developing larvae consume the victim from within. One species’ demon is another’s savior.
Some parasites are beneficial; we employ chalcid wasps (family Chalcidoidea) as biological control agents, using them to keep crop pests at manageable levels. From Amédée Louis Michel Lepeletier, comte de Saint Fargeau, Histoire naturelle des insects (1836–1846).