In this article we will discuss about:- 1. Definition and Salient Features of Pesticides 2. Illustration of Biological Magnification 3. Effects 4. Control 5. Fate 6. Integrated Pest Management.
Definition and Salient Features of Pesticides:
According to the Oxford English Dictionary, a pest is described as a troublesome or destructive organism and the suffix ‘cide’ implies “slaughter of”. Thus a pesticide may be defined as any substance intended for killing pests. In other words, a pesticide is any substance or mixture of substances intended for preventing or controlling any unwanted species of plants and animals.
Actually, pesticides are one of the biocides. A biocide is any substance used with the intention of killing living harmful organisms. Generally pesticides are also known as plant protective agents or crop protection products.
Human beings, along with their domesticated and other animals, are directly or indirectly exposed to the growing use of pesticides. The persistent pesticides, however, increase the chances of bio-magnifications.
Studies on effect of pesticides on non-target animals can be of the following type:
a. Acute toxicity — Oral, dermal, inhalation, irritation etc.
b. Sub-acute — Oral, dermal, inhalation etc.
c. Chronic toxicity — Carcinogenicity, mutagenicity, reproduction, neurotoxicity etc.
d. Occupational exposure — The sub-acute and chronic exposures are the direct effect of contaminated food, air and water.
Certain important characteristics of pesticides as explained below, determine their mode of action, potential etc.
Since the pesticides finally find their way into the aquatic bodies, their stability and persistence in aquatic system is a function of their chemical structure. Pesticides range in stability from few hours to few years. For example Malathion (an organo-phosphate) breaks down in few hours whereas Diaginon and Methoxychlor takes few days; Comphechlor few months and DDT persist for years.
Persistence in the aquatic system is highest for organochlorine insecticides, intermediate for organophosphate and carbamate insecticide and least for herbicides. The highly persistent pesticides produce a potential hazard to the productivity of aquatic systems, because the aquatic biota will be exposed to residue for considerable periods even after only a single contamination. However, if such contamination is repeated or occurs continuously, there is a potential for accumulation of more persistent pesticides in certain components of an aquatic system.
Shukla, J. P. and Pandey, K. (1982 and 1988) reported that juvenile stages of fishes in general and planktons (zoo and phyto) in particular are sensitive indicators to a variety of chemical residues in aquatic environment.
High toxicity to the aquatic fauna or flora is an important criterion for a pesticide to be considered as a toxic pollutant of aquatic systems.
Pesticides differ in toxicity to different aquatic organisms and even different development stages of the same species. Some pesticides are extremely toxic to fish at a low concentration while to aquatic invertebrates even at a lower concentration. For example, LC50 (96 hours) of DDT to rainbow trout is 0.018 ppm; LC50 (24 hours) value of Malathion to Channa gachua is 0.525 ppm; LC50 (96 hours) value of suqin to Rasbera daniconius and ticto is 0.35 and 0.171 ppb, respectively.
Toxicity may be acute and may kill the organism relatively quickly, or chronic — showing gradual effects on activity, feeding, development, reproduction and general physiology. The former is much more obvious because mortality of a large number of fishes soon attracts attention, while the latter does not become obvious.
Differential water solubility is an important factor which can increase or decrease the potential of a pesticide as pollutant or toxicant of aquatic systems. For example, solubility of organochlorines in water is about 30%, organophosphates 20-60% and of herbicides like 2,4-D is 65%. Herbicides like Diquat and Dalapon are approximately 70-80% soluble in water.
Most pesticides tend to carry electric charges that enable them to be adsorbed onto soil particles in much the same way as inorganic ions. Herbicides are much more strongly adsorbed into soil with a high exchange capacity.
In addition to soil adsorption, pesticides undergo major structural changes that completely change their toxic properties. Some of these changes are biochemical, photochemical and inorganic chemical reactions with water or other inorganic substances in soil.
Undoubtedly all the organic pesticides degrade — some very slowly and others rapidly.
6. Bioconcentration and Biomagnifications:
Most of the organochlorine insecticides are capable of accumulating into the tissue of animals and plants from the water-body. They often reach to a concentration many thousand times greater in the biota than in water itself. However, the process of bio-concentration is quite complex and based upon such factors as the time of exposure, rate of uptake, metabolism within the organism, rate of excretion, potential for storage and physiological condition of the organism.
The phenomenon of bio-concentration from the medium in which organisms persist is sometimes confused with that of biological magnification.
The eventually accumulated non-biodegradable pesticides in an organism at a particular trophic level with a greater concentration than its preceding trophic level is referred to as biological magnification. Significantly, animals at the top of the food chains accumulate the maximum residues. This concept is widely accepted. The publication of ‘silent spring’ by Carson (1962) is an oversimplification of the whole process, and there are many other possible explanations for the presence of higher residue in predators than in their prey, food chains being only one factor.
Biological magnification may be explained through following example.
Many pesticides applied to control harmful insects are either chlorinated hydrocarbons viz., Endrin, Toxaphene, Dieldrin, Aldrin etc. are more stable than organophosphorus insecticides like Parathion, Malathion, Chlorothion etc. For that reason they may contaminate the aquatic environment with sublethal amounts which tend to accumulate in the tissues of aquatic animals.
This accumulation of chlorinated hydrocarbons may reach such high levels in organisms at the summit of the aquatic food web that they are unable to reproduce or they may be poisoned outright. The widespread use of one such compound, DDT, has already aroused considerable concern in recent years and some countries, including India, have legally banned its use.
Fig. 3.3 shows the biological concentration of DDT through an aquatic food chain. Animals belonging to the higher trophic levels in the food chains such as seals and birds may have up to 55 g/m3 of DDT in their body’s fatty tissues. Estimation of DDT in human body fat was made in 1964 and these varied from 3.39 g/m3 in England to 25 g/m3 in India.
Application of pesticides has now become a necessary evil of the modern agro-technology and, perhaps, man cannot stop their uses. Although the use of pesticides is directed against pests, they also exert effects on a wide range of non-target organisms, including desirable animals and plants.
The adverse effects of pesticides are:
Varieties of pesticides such as chlorinated hydrocarbons, viz., DDT and others, are highly persistent. They can rapidly pass through a single ecosystem or from one ecosystem to another. For example, DDT may be washed away from a sprayed cotton field by water run-off and eventually be carried to a river and finally to the ocean.
Pesticides like DDT, endrin, dieldrin etc., may gradually slip downward into ground water and eventually contaminate the drinking water supplies. When water evaporates from an irrigated field or from a lake or river, the pesticide molecules may co-distill, i.e., may be carried into the atmosphere along with the evaporating molecules of water.
In a spraying field, DDT and other chlorinated hydrocarbons adhered to soil particles may be carried away along dust erosion and then eventually return to land, lake, river and ocean many thousands of kilometers away from the site of application. As the different components of the biosphere are functionally interrelated, pesticide pollution has spread into all of them. In the fatty tissue of seals and penguins of the Antarctic, Moran et at (1973) reported the presence of trace amounts of pesticides.
Although pesticides furnish several benefits, they entail a number of risks and problems. One is their toxicity or their potential for inducing tumors, cancers or birth defects. The World Health Organization estimates that 500,000 pesticide poisoning cases occur annually in the world and that 1% are fatal (5,000 deaths/year).
In the early years of modern pesticides (1940 – 1960), pesticide manufacturers and users evaluated the safety of a pesticide almost solely in terms of its short term effects on people or on plants and animals of obvious economic interest. Some pesticides, however, have been found to cause cancer or tumors in laboratory animals.
Many scientists believe that positive animal test results indicate a possible cancer hazard to people. Moreover, the tumor-causing agents must be considered to be cancer-causing agents as well; this view is supported by many scientists but questioned by others.
When it appeared that some pesticides might cause cancer, another pesticide-related problem emerged. The property of persistence was desirable, provided people were not killed or made ill, since the more persistent a pesticide, the less frequent the need for applications.
Long persistence, however, also means more exposure of a population to any harmful effects and more time to destroy non-target organisms. The worldwide antimalarial programme still finds DDT its most valuable weapon, because DDT is inexpensive, easily made, non-lethal to humans, and highly persistent.
Because of the magnification, a number of non- target animals accumulate non-biodegradable pesticides from the corresponding environment. The impact of pesticide on non-target organisms may be direct or indirect, but almost always deleterious. Pesticides may produce morphological, behavioural, biochemical and physiological alterations. The most common pesticidal impact on non-target organisms are behavioural and physiological sterility, some other impairments or even death occurs.
The biocides affect the non-target populations similar to that of a target population. This is particularly significant when the non-target population proves to be a beneficial insect such as pollinator or the natural enemy of any important pest are often more susceptible to insecticides than the pests themselves.
In many cases, harmful effect of a pesticide is difficult to detect and demonstrate, because the pesticides do not seriously affect the adults but at certain embryonic stages during development. Mckim and Benoit (1971), Pandey and Shukla (1984) reported that the juvenile stages of fishes are very much sensitive to a variety of chemical toxicants. Burdick et al (1964) observed that as long as the DDT concentration in trout’s egg was 3.0 ppm or less, some fries could not survive, but above 5.0 ppm, mortality of the young fry was 100%.
Many pesticides may adversely affect the reproduction of fish-eating birds and other higher animals.
The herbicides viz., 2, 4 4-D and 2, 4, 5-T at a relatively high doses lower the reproduction in chickens, while chickens exposed to Thiram produce soft shelled eggs of abnormal shape.
Biocides can adversely alter the growth rate of numerous organisms. For example, Dieldrin has been reported to decrease the growth rate of female white-tailed deer.
Besides, the pesticides may also produce some behavioural alterations in non-target organisms. Sublethal concentration of DDT causes trout to forget most of their learned evidence responses.
Mosquito fish exposed to relatively lower concentration of DDT (01 – 20 ppb) tend to prefer more saline water than usual for the species. Even the fingerlings of Colisa fasciata were reported to produce increased mucus secretion, fast movement and gulping of more air under a sublethal concentration of an organophosphorus insecticide — Malathion.
Pesticide does not selectively kills the pest only, but also causes deleterious impact on predatory forms that have been keeping the pest species at reasonably low levels.
A drawback of repeated applications of a given pesticide results in the development of resistant or immune strains of the target pest species. The development of such genetic resistance in pest population is well-known. For example, when DDT was first applied in 1945, the mosquito population appeared to be under control.
However, within 6-7 years, a DDT resistant population developed, so a new chemical, ethylparathion, was applied. By 1961, ethylparathion was no longer effective, and another new pesticide, methylparathion, was introduced, which also became ineffective in 1963, as did Flenthion in 1968.
Of great significance is the fact that this resistance is passed on from generation to generation. It is, therefore, suggested that earlier pesticides should be replaced through a new pesticide each year to minimize the hardship for pest control.
Strobel et al (1981) reported that common man who is not occupationally exposed to DDT and other pesticides opts DDT into his body when eating DDT- contaminated food items. Thus almost all human beings are probably taking DDT into their bodies every day with food they eat and the air they breathe and some DDT is stored in body fat (3 ppm-90 ppm or bit more). Small amounts of DDT may also be stored in other tissues.
It may be stored in liver, tissues of brain, gonads, kidney and blood at concentrations of only 1% of that in fat. Miller (1980) reported that pesticides, when stored in organs, may lead to cancer, softening of the brain tissues, high B.P., stroke, and cirrhosis of the liver. Clayson (1962) suggested that low levels of DDT in our bodies sooner or later may cause cancer.
The observed DDT residue in the fat of Indian population ranges from 12.6 ppm for Gujarat to 31.0 ppm for Delhi.
There are several reports of mass poisoning due to careless handling of pesticides in different parts of India. In 1957, pesticide poisoning in Kerala caused death of 102 persons. In Indore, out of 35 cases of Malathion poisoning reported during 1967-1968, five died. In a foggy and windy night on December 3rd 1984, thousands of men, women and children died due to MIC from Union Carbide factory at Bhopal.
Actually, Methyl Isocyanate is an intermediate or raw material for the manufacture of several manufactured by Union Carbide. A report revealed that within a week total human mortality was about 10,000, one thousand people became blind while more than one lakh people continue to suffer from various respiratory disorders. The most vulnerable parts of the body to MIC were eyes and respiratory organs.
A medical survey held hundred days after the exposure revealed that out of 250,000 people exposed, 65,000 were subjected to severe medical disability (respiratory, eye, gastrointestinal, neuromuscular symptoms) and 45,000 to mild to moderate medical disability.
People suffering from breathlessness, sleeping and digestion problems were incapable of carrying on even light physical labour. Women were badly affected, the worst victims being pregnant women. In addition to the loss of human life, a large number of cattle and birds also met the same fate.
Further, high infant mortality has been reported from places where high residues of DDT were found in human milk. A study from Punjab state of India revealed high levels of DDT and BHC in breast milk samples.
The chronic impact of pesticides is much less obvious but much more widely spread throughout the human population. Different people respond to pesticides in different ways. Many are not affected adversely, while some people are very sensitive and may be adversely affected even by very low doses of pesticides.
Common symptoms of pesticidal poisoning in man are:
11. Neuromuscular Disorders
12. Paralysis etc.
Control of Pesticide Pollution:
The following methods must be followed in using pesticides:
1. The non-selective persistent pesticides such as DDT must be phased out of use.
2. Only selective pesticides must be used.
3. Pesticides must be used in small quantities.
4. Repeated pesticide application should be stopped.
5. Pesticide education should be given to public and farmers.
6. Researches on pesticides should be in progress.
Fate of Pesticides:
From the public health view-point the biochemistry of pesticides is of considerable concern. Biochemical processes constitute the mechanism by which pesticides in the environment are degraded and detoxified.
Among the pesticides, the biological action of DDT on the environment has been most extensively studied. Like many other insecticides, the central nervous system is its main target. DDT dissolves in fat tissue and is accumulated in the fatty membrane surrounding nerve cells. Most likely, it interferes with the transmission of nerve impulses along the axons, which are projections connecting nerve cells. Finally, it leads to the disruption of the central nervous system killing the target insect (pest).
While DDT is fairly stable and persists in the environment, the other groups — organophosphate and carbamates — degrade quite rapidly in the environment. The latter react with O2 and HOH and is decomposed within a few days in the environment. The end products are, however, not toxic.
Among the Third World countries India was the first who imported DDT and BHC for the control of insects pests, soon after its independence. In India DDT was first manufactured in 1952. Among the south Asian countries our country is the largest producer of pesticides with the production of 88,751 MT in 1998 – 99. It has second place to China and 12th place in the world. Up to 2000,155 pesticides have been registered under Insecticide Act, 1968. It includes 57 insecticides, 44 fumigants, 3 acaricides, 1 each soil sterilant, molluscicide and nematicide.
Our country has 4% of the world cropped area with a share of 1.7% of the world pesticide consumption. The world’s pesticide consumption is about 3.1 million tones, of which 24% is consumed in USA, 45% in Europe and 20% in developing countries.
During the last four decades, the consumption of pesticides has increased tremendously, from 154 MT in 1953 – 1954 to 80,000 MT in 1994 – 95. After this the pesticide consumption declined steadily to the present level of 54,135 MT (1999 – 2000). This decline in the use is mainly due to banning of hard insecticide like that of chlorinated hydrocarbon, availability of high potency insecticide and adoption of IPM strategies by the farmers.
Highly potent insecticide also played very important role in the reduction of pesticide consumption. The insecticide of chlorinated hydrocarbons were required in kilograms 1-1.5/ha, organophosphate in grammes (250-500/ha), synthetic pyrethroids (50 g/ha) in case of fenvalerate and cypermethrin) and still lower (10g/ha) dose is required in case of deltamethrin. This indicates the 100 – fold reduction in the dose of newer pesticides.
In our country maximum share of the pesticide is highest in cotton (39%) followed by rice (35%), cereals/millets (17%), and others (9%). During 1979, this consumption pattern was cotton (54%) followed by rice (17%), cereals/millets (6%) and others (23%). Globally the pesticide consumption pattern is slightly changed.
Though cotton has disproportionately higher share, even globally. The world consumption of pesticide share is maximum (27%) in horticultural crops followed by cotton (24%), rice (17%), maize (7%) and others (25%). It is clear that rice, vegetables and fruits are also getting good share but the crops like barley, gram, jute, mustard, soyabean, sunflower and tobacco is even less than 1%.
Up to 1995-96 the major groups of pesticides in agriculture were insecticide (80%) followed by fungicides (10%), herbicides (7%) and others (3%). This consumption pattern has changed in 1999- 2000. It was 60% in insecticides, 21% in fungicides, 14% herbicides and other 5%. From 1995 to 1999- 2000 the type of insecticides also changed.
The percentage of chlorinated hydrocarbon from 40 to 14.5%, carbamates from 15 to 4.5%, synthetic pyrethroids from 10 to 5%, whereas the use of organophosphate increased from 30 to 74%. Increase in the consumption of natural pesticides 2% has also been registered during this period. Natural insecticides include mainly neem and Bt formulations.
In Indian agriculture the average pesticide consumption was very low in 1953-54 (1.2g/ha) which increased to 377 g/ha in 1985-86 and to 431 g in 1992-93. After this a decline trend was noticed. It was 392 g/ha in 1995-96, 320 g/ha in 1997-98 and 288 g/ha in 1999-2000.
These values are much lower than the consumption of 17 kg/ha in Taiwan, 12kg/ha in U.S.A. In India the average consumption of pesticides is very low but several States/Union Territories show very high consumption of pesticides than the national average.
Among them are Haryana, Punjab, Delhi and Pondicherry where consumption is more than 800 g/ha. A significant decline in the consumption pattern has also been observed in some of the states. Tamil Nadu, which used 10,500 tonnes in 1993-94 is now using 2,882 tonnes (1999- 2000). Similarly, in Andhra Pradesh, the pesticide consumption decreased from 14,500 to 7,000 tonnes during this period.
Among the 53 insecticides registered monocrotophos, endosulfan, Malathion, methyl parathion, phorate, quinalphos, dimethoate, chlorpyriphos and carbaryl account for 80% of the total insecticides being used. Mancozeb, sulphur, copper, oxychloride, copper sulphate, carbendazim and thiram among 44 fungicides constitute 86.8% share. Sulphur and copper fungicides still constitute 49.6% of the total fungicides used in the country.
Mancozeb is the major organo compound which accounts for 25% of the total fungicides used followed by Carbendazim (7.4%) and Thiram (3.8%). In case of herbicides, out of 33 registered, only 14 are being used. The top five herbicides (isoproturon, butachlor, 2,4-D, anilophos, atrazin) account for 86.4% of the total herbicides used in the Indian.
Excessive and injudicious use of insecticide has created lot of problems like development of resistance in insects to insecticides. So far more than 504 arthropod species have developed resistance to various insecticides in the world. In our country about 7 insect pests of agricultural importance have developed resistance to insecticides. In Andhra Pradesh the injudicious use of broad spectrum insecticides, particularly synthetic pyrethroids, have created havoc to cotton-growers.
The American boll worm, a major insect pest of cotton, have developed resistance to these chemicals. To kill these resistant species higher doses cannot be used due to several consequences.
Other problems due to misuse of insecticides are biological magnification, particularly of highly persistent insecticides of chlorinated hydrocarbons at each trophic level of food chain, pest resurgence (due to death of natural enemies, improvement in nutritional status of the host plant and direct effect of sub-lethal doses of insecticides on the body of the insect), outbreak of secondary pests like mite due to use of pyrethroids/carbaryl, minor pests becoming major pests (sorghum midge), environmental pollution, deleterious effects on non-target organisms like parasites/parasitoids, predators, pollinators etc. and health hazards to men and animals.
In IPM there is minimum use of pesticides which is used when absolutely necessary for effective control and restoration of the environment. Secondly, there is no need of 100% insect control to prevent the crop losses. The 100% control is neither desirable, possible nor feasible. The best time of pesticide application is based on the Economic Threshold Level values.
For key pests these values have been computed. It has been reported that 10-15% of the dust and 25-30% of the spray are deposited on plant surface and only less than 1% reaches the target body. Thus there is vast scope for decreasing the amount of pesticide by improved method of application.
The misuse of insecticides can be avoided by using them as seed treatment for the control of insects attacking seeds/seedlings, seedling dip for the control of rice pests at seedling pests, root zone application for rice and other pests, whorl application for the termite in wheat/sugarcane etc., infusion and injection methods for the control of mango shoot gall maker in mango and other pests, attract and kill devices, electrostatic spraying, controlled droplet application, need base, timely application etc.
In addition to these improved application technology, proper selection of pesticides is of utmost importance. Pesticides are selected on the basis of some parameters. They are selectivity ratios of different insecticides.
In addition to this, different insecticides are rated for different health hazards. They are, in general, known as Pest Management Rating. In these ratings, toxicity to human being, persistence of insecticide in soil, toxicity to bees, fishes, birds etc. are pooled together and averaged. The overall rating indicates the suitability of that insecticide in IPM.
Pesticides are a viable component of IPM. Sustainability in agriculture for feeding the increasing human population cannot be achieved without the input of pesticides in agriculture, but their use must be judicious.
One may come to the conclusion that Integrated Pest Management (IPM) is a philosophy of pest management rather than a specific, defined strategy. It combines physical, cultural, biological and chemical control and the use of resistant varieties.
It is ecologically based, relying on mortality factors, including natural enemies and weather, aimed at controlling pests below the economic injury level (EIL) and is based around monitoring of pest and natural enemy abundance. A considerable investment of time and effort is required to determine the optimum strategy, but there are long term environmental and economic benefits.