Insects and Human Welfare
Insects and
Human Welfare
Beneficial Insects
Although most of us think of insects primarily as pests, humanity would have great difficulty in surviving if all insects were suddenly to disappear. Some produce useful materials: honey and beeswax from bees, silk from silkworms, and shellac from a wax secreted by lac insects. More important, however, insects are necessary for the cross-fertilization of many crops. Bees pollinate almost $10 billion worth of food crops per year in the United States alone, and this figure does not include pollination of forage crops for livestock or pollination by other insects.
Very early in their evolution, insects and flowering plants formed a relationship of mutual adaptations that have been to each other’s advantage. Insects exploit flowers for food, and flowers exploit insects for pollination. Each floral development of petal and sepal arrangement is correlated with the sensory adjustment of certain pollinating insects. Among these mutual adaptations are amazing devices of allurements, traps, specialized structures, and precise timing.
Many predaceous insects, such as tiger beetles, aphid lions, ant lions, praying mantids, and lady bird beetles, destroy harmful insects. Parasitoid insects are very important in controlling populations of many harmful insects. Dead animals are quickly consumed by maggots hatched from eggs laid in carcasses (Figure 20-35). Insects serve as an important source of food for many birds, fishes and other animals.
Harmful Insects
Harmful insects include those that eat and destroy plants and fruits, such as grasshoppers, chinch bugs, corn borers, boll weevils, grain weevils, San Jose scale, and scores of others (Figure 20-36). Practically every cultivated crop has several insect pests. Humans expend enormous resources in all agricultural activities, in forestry, and in the food industry to counter insects and the damage they engender. Outbreaks of bark beetles or defoliators such as spruce budworms and gypsy moths have generated tremendous economic losses and have become a major element in determining the composition of forests in the United States. Gypsy moths, introduced into the United States in 1869 in an illadvised attempt to breed a better silkworm, have spread throughout the northeast as far south as Virginia. They defoliate oak forests in years when there are outbreaks. In 1981, they defoliated 13 million acres in 17 northeastern states.
Lice, bloodsucking flies, warble flies, bot flies, and many others attack humans or domestic animals or both. Malaria, carried by Anopheles mosquitos, is still one of the world’s major diseases; mosquitos also transmit yellow fever and lymphatic filariasis. Fleas carry plague, which at times in history has wiped out significant portions of human populations. House flies are vectors of typhoid, as are lice for typhus fever; tsetse flies carry African sleeping sickness; and bloodsucking bugs, Rhodnius and related genera, transmit Chagas’ disease.
There is tremendous destruction of food, clothing, and property by weevils, cockroaches, ants, clothes moths, termites, and carpet beetles. Not the least of insect pests are bed bugs, Cimex, bloodsucking hemipterous insects that humans probably contracted early in their evolution from bats that shared their caves.
Control of Insects
Because all insects are an integral part of the ecological communities to which they belong, their total destruction would probably do more harm than good. Food chains would be disturbed, some of our favorite birds would disappear, and the biological cycles by which dead animal and plant matter disintegrates and returns to enrich the soil would be seriously impeded. The beneficial role of insects in our environment is often overlooked, and in our zeal to control the pests we spray the landscape indiscriminately with extremely effective “broad-spectrum” insecticides that eradicate good, as well as harmful, insects. We have also found, to our dismay, that many chemical insecticides persist in the environment and accumulate as residues in the bodies of animals higher in the food chain. Furthermore, many insects have developed a resistance to insecticides in common use.
In recent years, methods of control other than chemical insecticides have been under intense investigation, experimentation, and development. Economics, concern for the environment, and consumer demand are causing thousands of farmers across the United States to use alternatives to strict dependence on chemicals.
Several types of biological controls have been developed and are under investigation. All of these areas present problems but show great possibilities. One is the use of bacterial, viral, and fungal pathogens. A bacterium, Bacillus thuringiensis, is quite effective in control of lepidopteran pests (cabbage looper, imported cabbage worm, tomato worm, gypsy moth). Other strains of B. thuringiensis attack insects in other orders, and the species diversity of target insects is being widened by techniques of genetic engineering. Genes coding for the toxin produced by B. thuringiensis also have been introduced into other bacteria and even into the plants themselves, which makes the plants resistant to insect attack.
A number of viruses and fungi that have potential as insecticides have been isolated. Difficulties and expense in rearing these agents are being overcome in certain cases, and some are nearing commercial production.
Introduction of natural predators or parasites of the insect pests has met with some success. In the United States vedalia beetles from Australia help control cottony-cushion scale on citrus plants, and numerous instances of control by use of insect parasites have been recorded.
Another approach to biological control is to interfere with reproduction or behavior of insect pests with sterile males or with naturally occurring organic compounds that act as hormones or pheromones. Such research, although very promising, is slow because of our limited understanding of insect behavior and the problems of isolating and identifying complex compounds that are produced in such minute amounts. Nevertheless, pheromones will probably play an important role in biological pest control in the future.
A systems approach referred to as integrated pest management is practiced with many crops. This approach involves integrated utilization of all possible, practical techniques to contain pest infestations at a tolerable level, for example, cultural techniques (resistant plant varieties, crop rotation, tillage techniques, timing of sowing, planting or harvesting, and others), use of biological controls, and sparing use of insecticides.
Classification of Class Insecta
Insects are divided into orders mainly on the basis of wing structure, mouthparts, and metamorphosis. Entomologists do not all agree on the names of the orders or on the limits of each order. Some choose to combine and others to divide the groups. However, the following synopsis of the orders is one that is rather widely accepted.
Beneficial Insects
Although most of us think of insects primarily as pests, humanity would have great difficulty in surviving if all insects were suddenly to disappear. Some produce useful materials: honey and beeswax from bees, silk from silkworms, and shellac from a wax secreted by lac insects. More important, however, insects are necessary for the cross-fertilization of many crops. Bees pollinate almost $10 billion worth of food crops per year in the United States alone, and this figure does not include pollination of forage crops for livestock or pollination by other insects.
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| Figure 20-35 Fly maggots (order Diptera) feeding on a deer carcass. |
Very early in their evolution, insects and flowering plants formed a relationship of mutual adaptations that have been to each other’s advantage. Insects exploit flowers for food, and flowers exploit insects for pollination. Each floral development of petal and sepal arrangement is correlated with the sensory adjustment of certain pollinating insects. Among these mutual adaptations are amazing devices of allurements, traps, specialized structures, and precise timing.
Many predaceous insects, such as tiger beetles, aphid lions, ant lions, praying mantids, and lady bird beetles, destroy harmful insects. Parasitoid insects are very important in controlling populations of many harmful insects. Dead animals are quickly consumed by maggots hatched from eggs laid in carcasses (Figure 20-35). Insects serve as an important source of food for many birds, fishes and other animals.
Harmful Insects
Harmful insects include those that eat and destroy plants and fruits, such as grasshoppers, chinch bugs, corn borers, boll weevils, grain weevils, San Jose scale, and scores of others (Figure 20-36). Practically every cultivated crop has several insect pests. Humans expend enormous resources in all agricultural activities, in forestry, and in the food industry to counter insects and the damage they engender. Outbreaks of bark beetles or defoliators such as spruce budworms and gypsy moths have generated tremendous economic losses and have become a major element in determining the composition of forests in the United States. Gypsy moths, introduced into the United States in 1869 in an illadvised attempt to breed a better silkworm, have spread throughout the northeast as far south as Virginia. They defoliate oak forests in years when there are outbreaks. In 1981, they defoliated 13 million acres in 17 northeastern states.
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| Figure 20-36 Some insect pests. A, Japanese beetles, Popillia japonica (order Coleoptera) are serious pests of fruit trees and ornamental shrubs. They were introduced into the United States from Japan in 1917. B, Longtailed mealybug, Pseudococcus longispinus (order Homoptera). Many mealybugs are pests of commercially valuable plants. C, Corn ear worms, Heliothis zea (order Lepidoptera). An even more serious pest of corn is the infamous corn borer, an import from Europe in 1908 or 1909. |
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Lice, bloodsucking flies, warble flies, bot flies, and many others attack humans or domestic animals or both. Malaria, carried by Anopheles mosquitos, is still one of the world’s major diseases; mosquitos also transmit yellow fever and lymphatic filariasis. Fleas carry plague, which at times in history has wiped out significant portions of human populations. House flies are vectors of typhoid, as are lice for typhus fever; tsetse flies carry African sleeping sickness; and bloodsucking bugs, Rhodnius and related genera, transmit Chagas’ disease.
There is tremendous destruction of food, clothing, and property by weevils, cockroaches, ants, clothes moths, termites, and carpet beetles. Not the least of insect pests are bed bugs, Cimex, bloodsucking hemipterous insects that humans probably contracted early in their evolution from bats that shared their caves.
Control of Insects
Because all insects are an integral part of the ecological communities to which they belong, their total destruction would probably do more harm than good. Food chains would be disturbed, some of our favorite birds would disappear, and the biological cycles by which dead animal and plant matter disintegrates and returns to enrich the soil would be seriously impeded. The beneficial role of insects in our environment is often overlooked, and in our zeal to control the pests we spray the landscape indiscriminately with extremely effective “broad-spectrum” insecticides that eradicate good, as well as harmful, insects. We have also found, to our dismay, that many chemical insecticides persist in the environment and accumulate as residues in the bodies of animals higher in the food chain. Furthermore, many insects have developed a resistance to insecticides in common use.
In recent years, methods of control other than chemical insecticides have been under intense investigation, experimentation, and development. Economics, concern for the environment, and consumer demand are causing thousands of farmers across the United States to use alternatives to strict dependence on chemicals.
Several types of biological controls have been developed and are under investigation. All of these areas present problems but show great possibilities. One is the use of bacterial, viral, and fungal pathogens. A bacterium, Bacillus thuringiensis, is quite effective in control of lepidopteran pests (cabbage looper, imported cabbage worm, tomato worm, gypsy moth). Other strains of B. thuringiensis attack insects in other orders, and the species diversity of target insects is being widened by techniques of genetic engineering. Genes coding for the toxin produced by B. thuringiensis also have been introduced into other bacteria and even into the plants themselves, which makes the plants resistant to insect attack.
A number of viruses and fungi that have potential as insecticides have been isolated. Difficulties and expense in rearing these agents are being overcome in certain cases, and some are nearing commercial production.
Introduction of natural predators or parasites of the insect pests has met with some success. In the United States vedalia beetles from Australia help control cottony-cushion scale on citrus plants, and numerous instances of control by use of insect parasites have been recorded.
Another approach to biological control is to interfere with reproduction or behavior of insect pests with sterile males or with naturally occurring organic compounds that act as hormones or pheromones. Such research, although very promising, is slow because of our limited understanding of insect behavior and the problems of isolating and identifying complex compounds that are produced in such minute amounts. Nevertheless, pheromones will probably play an important role in biological pest control in the future.
A systems approach referred to as integrated pest management is practiced with many crops. This approach involves integrated utilization of all possible, practical techniques to contain pest infestations at a tolerable level, for example, cultural techniques (resistant plant varieties, crop rotation, tillage techniques, timing of sowing, planting or harvesting, and others), use of biological controls, and sparing use of insecticides.
Classification of Class Insecta
Insects are divided into orders mainly on the basis of wing structure, mouthparts, and metamorphosis. Entomologists do not all agree on the names of the orders or on the limits of each order. Some choose to combine and others to divide the groups. However, the following synopsis of the orders is one that is rather widely accepted.
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Order Protura (pro-tu´ra) (Gr. protos, first, + oura, tail). Minute
(1 to 1.5 mm); no eyes or antennae;
appendages on abdomen as well as
thorax; live in soil and dark, humid
places; direct development
Order Diplura (dip-lu´ra) (Gr. diploos, double, + oura, tail): japygids. Usually less than 10 mm; pale, eyeless; a pair of long terminal filaments or pair of caudal forceps; live in damp humus or rotting logs; development direct.
Order Collembola (col-lem´bo-la) (Gr. kolla, glue, + embolon, peg, wedge): springtails and snow fleas. Small (5 mm or less); respiration by trachea or body surface; a springing organ folded under the abdomen for leaping; abundant in soil; sometimes swarm on pond surface film or on snowbanks in spring; development direct.
Order Thysanura (thy-sa-nu´ra) (Gr. thysanos, tassel, + oura, tail): silverfish and bristletails. Small to medium size; large eyes; long antennae; three long terminal cerci; live under stones and leaves and around human habitations; development direct.
Order Ephemeroptera (e-fem-erop ter-a) (Gr. ephemeros, lasting but a day, + pteron, wing): mayflies. Wings membranous; forewings larger than hindwings; adult mouthparts vestigial; nymphs aquatic, with lateral tracheal gills.
Order Odonata (o-do-na´ta) (Gr. odontos, tooth, + ata, characterized by): dragonflies, damselflies (Figure 20-22B, and 20-27B). Large; membranous wings are long, narrow, net veined, and similar in size; long and slender body; aquatic nymphs with gills and prehensile labium for capture of prey.
Order Orthoptera (or-thop´ter-a) (Gr. orthos, straight, + pteron, wing): grasshoppers (Figure 20-4), locusts, crickets, cockroaches, walking (Figure 20-9B), praying mantids (Figure 20-7). Wings, when present, with forewings thickened and hindwings folded like a fan under forewings; chewing mouthparts. Many entomologists divide Orthoptera as given here into additional orders, such as Orthoptera (limited to grasshoppers, crickets, and related forms), Blattaria (cockroaches), Mantodea (praying mantids), Phasmida (walking sticks), and Grylloblattaria (rockcrawlers).
Order Dermaptera (der-map´ter-a) (Gr. derma, skin, + pteron, wing): earwigs. Very short forewings; large and membranous hindwings folded under forewings when at rest; biting mouthparts; forcepslike cerci.
Order Plecoptera (ple-kop´ter-a) (Gr. plekein, to twist, + pteron, wing): stoneflies (Figure 20-27A). Membranous wings; larger and fanlike hindwings; aquatic nymph with tufts of tracheal gills.
Order Isoptera (i-sop´ter-a) (Gr. isos, equal, + pteron, wing): termites (Figure 20-33). Small; membranous, narrow wings similar in size with few veins; wings shed at maturity; erroneously called “white ants”; distinguishable from true ants by broad union of thorax and abdomen; complex social organization.
Order Embioptera (em-bi-op´ter-a) (Gr. embios, lively, + pteron, wing): webspinners. Small; male wings membranous, narrow, and similar in size; wingless females; chewing mouthparts; colonial; make silk-lined channels in tropical soil.
Order Psocoptera (so-cop´ter-a) (Gr. psoco, rub away, + pteron, wing) (Corrodentia): psocids, book lice, bark lice. Body usually small, may be as large as 10 mm; membranous, narrow wings with few veins, usually held rooflike over abdomen when at rest; some wingless species; found in books, bark, bird nests, on foliage.
Order Zoraptera (zo-rap´ter-a) (Gr. zoros, pure, + apterygos, wingless): zorapterans. As large as 2.5 mm; membranous, narrow wings usually shed at maturity; colonial and termite like.
Order Mallophaga (mal-lof´a-ga) (Gr. mallos, wool, + phagein, to eat): biting lice (Figure 20-15). As large as 6 mm; wingless; chewing mouthparts; legs adapted for clinging to host; live on birds and mammals.
Order Anoplura (an-o-plu´ra) (Gr. anoplos, unarmed, + oura, tail): sucking lice (Figure 20-16). Depressed body; as large as 6 mm; wingless; mouthparts for piercing and sucking; adapted for clinging to warm-blooded host; includes head lice, body lice, crab lice, others.
Order Thysanoptera (thy-sanop ´ter-a) (Gr. thysanos, tassel, + pteron, wing): thrips. Length 0.5 to 5 mm (a few longer); wings, if present, long, very narrow, with few veins, and fringed with long hairs; sucking mouthparts; destructive planteaters, but some feed on insects.
Order Hemiptera (he-mip´ter-a) (Gr. hemi, half, + pteron, wing) (Heteroptera): true bugs. Size 2 to 100 mm; wings present or absent; forewings with basal portion leathery, apical portion membranous; hindwings membranous; at rest, wings held flat over abdomen; piercingsucking mouthparts; many with odorous scent glands; includes water scorpions, water striders (Figure 20-11), bed bugs, squash bugs, assassin bugs, chinch bugs, stink bugs, plant bugs, lace bugs, others.
Order Homoptera (ho-mop´ter-a) (Gr. homos, same, + pteron, wing): cicadas, aphids, scale insects, leafhoppers, treehoppers (Figure 20-29). (Often included as suborder under Hemiptera.) If winged, either membranous or thickened front wings and membranous hindwings; wings held rooflike over body; piercingsucking mouthparts; all plant-eaters; some destructive; a few serving as source of shellac, dyes, and so on; some with complex life histories.
Order Neuroptera (neu-rop´ter-a) (Gr. neuron, nerve, + pteron, wing): dobsonflies, ant lions, lacewings. Medium to large size; similar, membranous wings with many cross veins; chewing mouthparts; dobsonflies with greatly enlarged mandibles in males, and with aquatic larvae; ant lion larvae (doodlebugs) make craters in sand to trap ants.
Order Coleoptera (ko-le-op´ter-a) (Gr. koleos, sheath, + pteron, wing): beetles (Figure 20-9A, 20-30, and 20-36A), fireflies (Figure 20-31), weevils (Figure 20-37). The largest order of animals in the world; front wings (elytra) thick, hard, opaque; membranous hindwings folded under front wings at rest; mouthparts for biting and chewing; includes ground beetles, carrion beetles, whirligig beetles, darkling beetles, stag beetles, dung beetles, diving beetles, boll weevils, others.
Order Strepsiptera (strep-sip´ter-a) (Gr. strepsis, a turning, + pteron, wing): stylops. Females with no wings, eyes, or antennae; males with vestigial forewings and fan-shaped hindwings; females and larvae parasitic in bees, wasps, and other insects.
Order Mecoptera (me-kop´ter-a) (Gr. mekos, length, + pteron, wing): scorpionflies. Small to medium size; wings long, slender, with many veins; at rest, wings held rooflike over back; scorpion-like male clasping organ at end of abdomen; carnivorous; live in most woodlands.
Order Lepidoptera (lep-i-dop´ter-a) (Gr. lepidos, scale, + pteron, wing): butterflies and moths. Membranous wings covered with overlapping scales, wings coupled at base; mouthparts a sucking tube, coiled when not in use; larvae (caterpillars) with chewing mandibles for plant eating, stubby prolegs on the abdomen, and silk glands for spinning cocoons; antennae knobbed in butterflies and usually plumed in moths (Figure 20-37).
Order Diptera (dip´ter-a) (Gr. dis, two, + pteron, wing): true flies. Single pair of wings, membranous and narrow; hindwings reduced to inconspicuous balancers (halteres); sucking mouthparts or adapted for sponging, lapping, or piercing; legless larvae (maggots); includes crane flies, mosquitos, moth flies, midges, fruit flies, flesh flies, house flies, horse flies, bot files, blow flies, and many others.
Order Trichoptera (tri-kop´ter-a) (Gr. trichos, hair, + pteron, wing): caddisflies. Small, soft bodies; wings well veined and hairy, folded rooflike over hairy body; chewing mouthparts; aquatic larvae of many species construct cases of leaves, sand, gravel, bits of shell, or plant matter, bound together with secreted silk or cement; some make silk feeding nets attached to rocks in stream.
Order Siphonaptera (si-fon-ap´ter-a) (Gr. siphon, a siphon, + apteros, wingless); fleas (Figure 20-14). Small; wingless; bodies laterally compressed; legs adapted for leaping; ectoparasitic on birds and mammals; larvae legless and scavengers.
Order Hymenoptera (hi-men-op´tera) (Gr. hymen, membrane, + pteron, wing): ants, bees, wasps (Figure 20-37). Very small to large; membranous, narrow wings coupled distally; subordinate hindwings; mouthparts for biting and lapping up liquids; ovipositor sometimes modified into stinger, piercer, or saw (Figure 20-10); both social and solitary species, most larvae legless, blind, and maggotlike.
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| Figure 20-37 A, Papilio krishna (order Lepidoptera) is a beautiful swallowtail butterfly from India. Members of the Papilionidae grace many areas of the world, both tropical and temperate, including North America. Compare the knobbed antennae with the plumed antennae in B, Rothschildia jacobaea, a saturniid moth from Brazil. Hyalophora cecropia is a common saturniid in North America. C, Paper wasp (order Hymenoptera) attending her pupae and larvae. D, Curculio proboscideus, the chestnut weevil, is a member of the largest family (Curculionidae) of the largest insect order (Coleoptera). This family includes many serious agricultural pests. |
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