Pesticides


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Pesticides

 

chemical agents used in combating plant pests and diseases, weeds, and pests attacking grain, grain products, wood, cotton, wool, and leather, as well as ectoparasites of domestic animals and carriers of diseases dangerous to humans and animals. Pesticides also include defoliants and desiccants, which facilitate the mechanized harvesting of certain crops; plant growth factors (auxins, gibberellins, and retardants); and paint additives to combat overgrowth on ships.

Pesticides belong to various classes of organic and inorganic compounds. Most pesticides are synthetic organic compounds. Important organic pesticide groups include organochlorine and organophosphorus compounds, derivatives of carbamic acid, plant-derived pesticides, triazines, and urea derivatives. Compounds of copper and sulfur are important inorganic pesticides. Pesticides are the basis of the chemical method of plant protection and are still the most effective means of combating pests, diseases, and weeds. They facilitate a significant reduction in losses in agriculture and lumbering, as well as in the wood-products industry. The costs of using pesticides are returned by a factor of 5-12.

Pesticides are divided into the following main classes, depending on the type of pest against which they are used: (1) acaricides, which are used against mites and ticks; (2) insect repellents, which repel insects from plants on which they feed; (3) insecticides, which exterminate harmful insects; (4) herbicides, which are used against unwanted vegetation; (5) zoocides, which exterminate harmful vertebrates (compounds used against rodents are called rodenticides, and those used specifically against rats are called raticides); (6) bactericides, viricides, and fungicides, which are used against agents of bacterial, viral, and fungal diseases of plants; (7) nematocides, which kill roundworms; and (8) mollus-cicides, which eliminate harmful mollusks (poisons used against slugs are called limacides).

Pesticides also include seed disinfectants; repellents of harmful insects, mites, ticks, mammals, and birds; attractants, which attract arthropods to permit their subsequent extermination or to disclose their concentrations or the beginning of their flight; and chemosterilants, which render insects, rodents, mites, and ticks sterile rather than killing them. There are also complex-action pesticides, such as seed disinfectants that contain fungicides, bactericides, and insecticides. The use of such pesticides permits a reduction in processing labor.

In some cases, pesticides are grouped according to the phase of development of the pest against which they are used. For example, ovicides are poisons that kill the eggs of insects, mites, and ticks, whereas larvicides exterminate the larva.

According to the means of penetration of pest organism, a distinction is made among stomach poisons, which enter through the mouth and intestines; contact poisons, which act on the surface of the pest’s body (that is, through the cuticle); fumigants, which reach the organism as a vapor or gas through the respiratory tract; and systemic poisons, which easily penetrate the tissues of animals or plants and destroy pests that feed on plant juices or animal fluids.

Pesticides are divided into six groups, depending on their period of decomposition in the soil: (1) more than 18 months— organochlorine and selenium compounds; (2) about 18 months —triazine herbicides, picloram, and diuron; (3) about 12 months —derivatives of halobenzoic acids and some carboxylic acid amides; (4) not less than six months—acid nitriles, derivatives of aryloxy acetic acids, trephlan and its analogues, and nitrophenols; (5) not less than three months—derivatives of arylcar-bamic and alkylcarbamic acids, some derivatives of urea and heterocyclic compounds; and (6) less than three months—or-ganophosphorus compounds. In agriculture, the use of compounds that decompose over a growing season is preferred, whereas long-acting pesticides are preferred for use on airfields and in combating the overgrowth of roads.

Pesticides are divided into four groups according to toxicity to humans and warm-blooded animals: acute, high-toxicity, medium-toxicity, and low-toxicity. The LD50 values (the minimum doses of pesticide that result in the death of 50 percent of the test animals) for pesticides of these groups are up to 50, 50-200, 200-1,000, and greater than 1,000 mg/kg, respectively. This division is arbitrary, since the toxicity of pesticides to humans and animals depends not only on the absolute value of the lethal doses of the preparations but also on their other properties. These other properties include the possibility of long-term consequences in cases of systematic action on the organism, tendency to accumulate in the organism and the environment, stability in the environment, blastogenic properties (capacity for causing tumors), mutagenic properties (effects on the heredity of the organism), embryotoxic properties (effects on the development of offspring), teratogenic properties (production of deformities), and allergenic properties (production of an unnatural, heightened sensitivity of the organism to the pesticide).

The mechanisms of action of the various classes of pesticides vary widely and have not been studied sufficiently. For example, organophosphorus compounds and esters of alkylcarbamic acids inhibit the cholinesterase enzyme of arthropods, whereas derivatives of thiourea block oxidation-reduction processes in insects.

The rates of application of pesticides range from 0.2 to 40 kg of active substance per hectare (ha), depending on the properties and intended use of the pesticides; the most common rates are 0.5-2.0 kg/ha. To achieve uniform distribution of such a small quantity of pesticide over the area to be treated, pesticides are used in a suitable prepared form, such as wetting powders, emulsion concentrates, dusts, solutions in water and organic solvents, aerosols, and granules, and are applied by various means, including spraying, dusting, fumigation, poisoned bait, and seed disinfection. In addition to the pesticide, auxiliary substances, diluents, and emulsifiers are present in the prepared form. The most promising pesticides are the sprays (wetting powders, emulsion concentrates, and solutions in water and organic solvents) and the granules for application to plants and the soil. Of particular interest are solutions in nonvolatile organic solvents used for ultralow-volume spraying, with a rate of 0.5-10.0 liters per ha. Crops are treated using ground machinery and airplanes.

Scalding of plants, decreased vitality of pollen, death of pistils, and a significant drop in crop yield can result from the use of doses of pesticide that are higher than the officially recommended doses or of concentrations that do not correspond to the methods and periods of application, and also from failure to take into account the weathering conditions of the pesticide. The plants may become contaminated by the pesticide or acquire an unpleasant odor and taste (for example, when hexachlorane is used), and also accumulate pesticides on their surface in the form of poisonous residues that are dangerous to humans and animals. Systematic use of pesticides often causes pests to become resistant. To avoid the development of strains of pests that are resistant to certain pesticides, a wide selection of preparations for a single purpose is necessary, with planned alternation of their use.

The effect of pesticides on biocenoses is complex and varied. Especially significant damage is noted in the case of systematic application of stable, highly toxic pesticides, mainly insecticides and acaricides. The elimination of parasitic and predatory arthropods by means of pesticides often results in massive increases in the population of other harmful types of insects, mites, and ticks. For example, the massive reproduction of the European red mite observed after treatment of fruit crops with DDT in several countries, including the USSR, is explained by the elimination of predatory mites of the genus Typhlodromus, and an increase in the numbers of the woolly apple aphid (also after the use of DDT) is due to the elimination of the aphelinid, which is a parasite of aphids.

The incorrect use of pesticides is known to have negative effects on humans, as well as on honeybees, bumblebees, and other insect pollinators and on fish (when the pesticides enter bodies of water), birds, wild animals, and domestic animals, and on nature as a whole. To prevent possible harmful effects of pesticides on humans, animals, plants, and water, the effect of the pesticides not only on a particular pest but also on biocenoses must be taken into account and the final results of the measures taken must be predicted. Important measures include rigorous monitoring of residual quantities of pesticides in food products and observance of the rules for storage, transport, and use of pesticides, which are mandatory for all institutions and individuals working with pesticides.

Great attention is devoted to the isolation, study, and synthesis of pesticides and to the development of means of using pesticides with new types of highly specific action, such as sexual attractants (pheromones), insect repellents, chemosterilants, and compounds with the action of a juvenile hormone secreted from bodies adjacent to the brain of insects. The introduction of a juvenile hormone or its analogues at the stage of development at which the hormone should be absent leads to failure of metamorphosis or causes the death of the insect. The high specificity of these groups of pesticides apparently will make possible the selective extermination of certain types of insects without affecting the biocenosis as a whole. Pesticides should change from a means of eliminating pests to a means of controlling their numbers. The least danger in the use of pesticides for beneficial insects (en-tomophages, pollinators, and honeybees) is achieved by pre-planting treatment of grain and planting materials and the use of selective pesticides, which are less toxic to entomophages than to phytophages.

The use of pesticides is regulated by law in all developed countries. The aim of this regulation is to permit use of only those preparations that are sufficiently effective and are acceptable according to labor and nutritional hygiene. Soviet and foreign pesticides approved by the State Commission on Chemical Agents Against Pests, Plant Diseases, and Weeds of the Ministry of Agriculture are used in the USSR. The List of Chemical and Biological Agents Against Pests, Plant Diseases, and Weeds for Use in Agriculture is published annually in coordination with the Ministry of Health of the USSR and with the approval of the Ministry of Agriculture of the USSR. Pesticides should be used strictly according to their specific action and only when chemical agents cannot be replaced by biological agents. Permissible concentrations of many pesticides in the air in manufacturing work areas have been established, as have maximum permissible residual amounts in food products.

In connection with the great importance of pesticides in agriculture, pesticide production is continually expanding. Production in the USSR was 103,200 tons in 1965, 163,800 tons in 1970, and 200,000 tons in 1973 (in terms of active substance). In 1972, production in the Federal Republic of Germany was 162,700 tons, and in the USA, more than 550,000 tons. World production of pesticides in 1973 was about 2 million tons. A decrease in the scale of use of pesticides, in view of the side effects of their use, is possible as pesticides are replaced by biological agents.

Most pesticides enter the human body through the respiratory system, the skin, and the gastrointestinal tract. Especially dangerous pesticide poisoning occurs during the treatment of closed areas and during preplanting treatment. Organochlorine pesticides have a general toxic action on the body and usually affect internal organs (the liver and kidneys) and the nervous system. The symptoms of poisoning, which are not very specific, include general fatigue, dizziness, nausea, and irritation of the mucous membranes of the eye and respiratory tract.

Most organophosphorus pesticides easily penetrate the body through the skin and have pronounced anticholinesterase activity. The symptoms of acute poisoning by these pesticides are specific: ptyalism, constriction of the pupils, muscular spasms, and convulsions. In cases of acute poisoning by organomercury pesticides, symptoms include ptyalism, a metallic taste in the mouth, nausea, and sometimes mucous diarrhea, headache, and loss of consciousness. (For a discussion of treatment of pesticide poisoning, see.)

The use of personal protective gear, such as special clothing and shoes and a respirator, gas mask, or safety goggles, is mandatory for all types of work with pesticides. Individuals with medical contraindications, persons under the age of 18, and pregnant and nursing women are not permitted to work with pesticides. The duration of the workday should not exceed six hours, and contact with acute pesticides should not exceed four hours.

REFERENCES

Gar, K. A. Khimicheskie sredstva dlia bor’by s vrediteliami i bolezniami sel’skokhoziaistvennykh kul’tur. Moscow, 1970.
Poliakov, I. M. Khimicheskii metod zashchity rastenii ot boleznei, 2nd ed. Leningrad, 1971.
Rudnev, D. F., and N. E. Kononova. Priroda i iadokhimikaty. Moscow, 1971.
La Brecque, G., and C. Smith. Geneticheskie metody bor’by s vrednymi nasekomymi. Moscow, 1971. (Translated from English.)
Berim, N. G. Khimicheskaia zashchita rastenii, 2nd ed. Leningrad, 1972.
Zhurnal Vsesoiuznogo khimicheskogo obshchestva im. D. I. Mendeleeva, 1973, vol. 8, no. 5.
Mel’nikov, N. N. Khimiia i tekhnologiia pestitsidov. Moscow, 1974.
Spravochnik no pestitsidam: Gigiena primeneniia i toksikologiia. Kiev, 1974.

N. N. MEL’NIKOV and V. A. KUDASHEVA

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