animals that injure or kill cultivated plants.
The harm done by plant pests and diseases is great. According to the data of the United Nations Food and Agriculture Organization, annual worldwide losses amount to approximately 20-25 percent of the potential worldwide yield of food crops. Among vertebrate animals, many crop pests are mammals, especially in the order of rodents. Among invertebrates, certain species of gastropods and a large number of roundworms from the class of nematodes harm crops. The most varied and numerous species of crop pests are arthropods: insects, arachnids (mites), and some species of millipedes and crustaceans (wood lice).
The most injurious pests are insects, chiefly because of their biological characteristics, abundance of species, high fecundity, and rapid reproduction. Insect pests are classified according to the taxonomic principle (by orders) and by the nature of their feeding. Herbivorous insects and mites are divided into polyphages, which feed on plants of different families; oligophages, which feed on plants of different species from the same family; and monophages, which feed mainly on plants of one particular species.
Polyphagous insects do great harm to different crops. They include locusts, certain crickets (for example, the mole cricket), click beetles, darkling beetles, cutworms and related species of boring moths, and corn and gamma moths. Among the numerous oligophagous insects are the fruit fly, green-bottle fly, Hessian fly, the beetle Anisoplia austriaca, and others that feed solely on cereals. The sweet clover weevil, pea moth, pea aphid, and so on are injurious to leguminous plants. The insects that feed on the mustard family—cabbage butterfly, diamond-backed moth, turnip flea beetle, and cabbage root fly, for example—are highly diverse species. The greatly harmful monophagous insects include the grape phylloxera, pea weevil, and sweet clover weevil. Harmful insects and mites are also classified by the crops that they damage—for example, grain pests or fruit pests—which is convenient for practical purposes.
Two main types of plant injuries are distinguished. The first is characteristic of insects with gnawing mouthparts, the second of insects with piercing-sucking mouthparts. Gnawing insects eat around the plants or edges of the leaves; skeletalize the leaves; gnaw the parenchyma and other parts; gnaw through or nibble at the leaves, stems, and shoots; eat away ducts; mine leaves and stems; and gnaw the phloem, cambium, and xylem under the cortex. Piercing-sucking insects—that is, aphids and true bugs, among others—introduce the secretions of their salivary glands into plants before feeding; the enzymes induce a number of biological changes. Some crop pests often confine themselves to certain organs of the plants; thus, there are root, stem, leaf, bud, flower, and fruit pests. Another important species characteristic is the more or less pronounced selectivity with respect to the age and physiological condition of the plant organ to be attacked—for example, aphids prefer to feed on young tissues, and the cherry slug on adult tissues.
The distribution of crop pests and formation of a complex of species in agrobiocenoses are directly dependent on changing environmental conditions and the ecological plasticity of the species. Each species has its own territory. A distinction is made among the general range of a species, the zone of injury to crops, and the habitat. The range of a species is the territory on which the pest is found. Ranges, natural or primary, are created by the independent settlement of a species. Their boundaries are determined mainly by climatic conditions, location of large mountains and seas, presence of plants suitable for consumption, and some other factors. Insects enter artificial or secondary ranges with seeds, planting material, and so on. Secondary ranges are characteristic, for example, of the grape phylloxera, Corn-stock mealybug, and many other pests imported into the USSR. The zone of injury is that part of the overall range where a particular species is regularly found in the largest numbers and where it does the most damage. The habitat is a place where the ecological conditions are favorable for a particular species. However, habitats may be different for the same species in different natural zones. This depends on where a species finds its ecological optimum. For example, the June beetle lives mainly on fallow fields and virgin land in the steppe, but in Middle Asia it lives in shady and humid orchards. Several species (locusts, aphids, and others) have annual and seasonal changes of habitats.
Temperature conditions are a major factor in the development and reproduction of insects and mites. Each species has a characteristic temperature regime during which all the vital processes take place most intensively. Substantial deviations from the optimum frequently kill a pest. The ability of insects to tolerate prolonged cold differs not only from species to species but even within a single species, depending on its physiological condition. With knowledge of the total mean daily effective temperatures, the approximate time of the appearance of insects can be determined and the duration of the individual developmental phases and number of generations in a season can be forecast. The chemical composition, acidity, physical structure, aeration conditions, and moisture content of soil are important for insects, whose development is closely bound up with the soil. Agricultural methods (soil cultivation, addition of fertilizers, and so on) can significantly alter the conditions in a direction unfavorable for harmful insects. For example, liming acid soils worsens the conditions for reproduction of many species of click beetles.
The relations between crop pests and animals also significantly affect the reproduction of pests. Complex “food chains” that develop in a biocenosis greatly influence the relations between the components occupying a given biotope. For example, various aphid species feed on plant juice, and the sugars they release serve as food for ants, ichneumon flies, and some other flies. Many species of predatory insects feed on aphids, including ladybug beetles and larvae and the larvae of stink flies and hover flies. Aphids and their enemies, predatory insects, are eaten by various insectivorous birds who become, in turn, prey to predatory birds. The breaking of any link in an existing food chain sometimes results in significant and unforeseeable or undesirable change in the biocenosis as a whole.
Various combinations of environmental factors produce more or less sharp changes in the abundance of insects; the causes vary from species to species. The presence and composition of food, weather conditions, influence of predators, parasites, and diseases, and so on may play a decisive role. Fluctuations in abundance are the result of the relationship between the fecundity of a species and the survival rate of the offspring, which depends on the environmental conditions and on the ability to adapt to them. Valuable forecasts of insect reproduction are based on regular counts of their numbers and on indications of when they will appear.
The systematic changes brought about by man in biotope conditions as a result of his farming activity cause a corresponding reconstruction of the biocenological relations and structure of the biocenosis. So-called secondary biotopes and agrobiocenoses are formed. For example, plowing up virgin land in the USSR and substituting cultivated grasses with the required cultivation techniques for the various steppe plants brought about drastic changes in the species composition and abundance of insects. On one hand, some monophagous species that previously fed on plants specific to virgin land died, impoverishing the species composition of the entomofauna in the new cultivated biotope. On the other hand, certain species that previously dwelt in wild grasses shifted to wheat, where they found an abundance of more nutritious food. This also explains, in part, the rapid increase in numbers of the wheat thrips and gray grain moth. It is a major scientific and practical task to foresee and control such changes.
The measures employed to reduce the damage done by insects are (arbitrarily) divided into preventive measures—agrotechnical and biological—and exterminative measures—physicochemical, chemical, biophysical, and biochemical. The agrotechnical method (which is basically biological) includes the breeding of pestresistant plants, choosing and observing correct crop rotations, and applying management methods at the most effective times in order to create conditions that increase to the utmost the self-protective properties of plants and that reduce the numbers of crop pests and their capacity for doing damage. The biological method entails the use against pests of their parasites and predators bred in special laboratories (Trichogramma, Cryptolaemus, and Aphelinus, for example) and the use of microbiological preparations (entobacterin, boverin, and others) and virus diseases of insects; the protection and utilization of natural enemies of pests (predatory animals that attack rodents, insectivorous birds, parasitic and predatory insects, mites, and nematodes); and the establishment of artificial breeding sites for birds and supplemental feeding for them in winter.
The physicomechanical method includes the use of rodent traps, pits and trenches to collect insects such as beet pests, beetle catchers, caterpillar catchers, and light traps and bait; the collection and burning of winter nests of brown-tailed moth and pierid butterfly caterpillars; the destruction of gypsy moth and lackey moth eggs; and the use of sticky bands around the trunks of fruit trees to control the codling moth. The chemical method controls pests with toxic chemical agents—for example, acaricides, insecticides, zoocides, nematocides, and fumigants. The biophysical and biochemical methods use gamma radiation and chemical agents to sterilize insects and mites in conjunction with attractants and agents that impair the physiological functions of the insects (antimetabolites, for example).
Pest control consists in carrying out measures based on a rational and differentiated combination of various methods with primarily preventative aims.
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V. N. SHCHEGOLEV