Mon Aug 3

Environmental fate of pesticides

ENVIRONMENTAL FATE OF PESTICIDES

A.Soltaninejad, Ph.D

E-mail:This email address is being protected from spambots. You need JavaScript enabled to view it.
INTRODUCTION
Pesticides have become an essential input for increasing crop production. In a pesticide we look for selective activity against a given set of pests or individual species and that too at very low concentrations. Over the past fifty years many pesticides have been developed. They are grouped under insecticides , fungicides , bactericides , herbicides , nematicides , acaricides , etc. With the increased use of pesticides in agriculture and in animal and human health care, the concern for environmental pollution has also been increased. Several studies on the persistence and residues of pesticides in the environment and their harmful effects on man and animal has been made. As a natural consequence, there have also been studies at the fate of the pesticides in nature and their biodegradation.
PESTICIDE USE IN ISLAMIC REPUBLIC OF IRAN:
We grow a large variety of crops under varying agro-climatic conditions in the Islamic Republic of Iran. Our cropping systems and also the climatic conditions favor the occurrence and spread of a large number of pests. The damage caused to our crops, both in the field and in storage, is not clearly known but some estimates indicate its high range.
Over thousands of pesticides are known in the world; about few hundred are registered for use in the Islamic republic of Iran. They are either organic or inorganic substances. A large majority of the organic substances fall under the category of organochlorides and organophosphates, and a limited few such as pyrethrin and natural plant products .While the pesticides act somewhat specifically against a few pests, none of them is active against all the crop pests. We need several pesticides to combat the large number of pests attacking our crop plants. All of them available in the market are considered harmful to human beings when consumed in large doses and hence the governmental regulations for marketing and use of the chemical are of prime importance .

ENVIRONMENTAL FATE OF PESTICIDES
A.Soltaninejad, Ph.D
E-mail:This email address is being protected from spambots. You need JavaScript enabled to view it.
INTRODUCTION
Pesticides have become an essential input for increasing crop production. In a pesticide we look for selective activity against a given set of pests or individual species and that too at very low concentrations. Over the past fifty years many pesticides have been developed. They are grouped under insecticides , fungicides , bactericides , herbicides , nematicides , acaricides , etc. With the increased use of pesticides in agriculture and in animal and human health care, the concern for environmental pollution has also been increased. Several studies on the persistence and residues of pesticides in the environment and their harmful effects on man and animal has been made. As a natural consequence, there have also been studies at the fate of the pesticides in nature and their biodegradation.
PESTICIDE USE IN ISLAMIC REPUBLIC OF IRAN:
We grow a large variety of crops under varying agro-climatic conditions in the Islamic Republic of Iran. Our cropping systems and also the climatic conditions favor the occurrence and spread of a large number of pests. The damage caused to our crops, both in the field and in storage, is not clearly known but some estimates indicate its high range.
Over thousands of pesticides are known in the world; about few hundred are registered for use in the Islamic republic of Iran. They are either organic or inorganic substances. A large majority of the organic substances fall under the category of organochlorides and organophosphates, and a limited few such as pyrethrin and natural plant products .While the pesticides act somewhat specifically against a few pests, none of them is active against all the crop pests. We need several pesticides to combat the large number of pests attacking our crop plants. All of them available in the market are considered harmful to human beings when consumed in large doses and hence the governmental regulations for marketing and use of the chemical are of prime importance .
The rate of chemical pesticide application varies very widely depending upon the technical advancement of the country and adoption of intensive means of crop production .On an average about 1881.00 kg of pesticide was applied in one hectare of cultivated land in year 1996 in the Islamic Republic of Iran, and the comparable figures for Japan was 7592.07 kg and for South Korea was 13829.00 kg in the same year. Hence the employment of an IPM system with minimum chemical pesticide usage has proven to be an effective pest control method in the Islamic Republic of Iran.
DISSIPATION OF PESTICIDES IN THE FIELD:
The pesticide applied through seed, foliar sprays and dusts and soil get dissipated in the air, water and soil
Dissipation of pesticides in nature
(Table 1).

In the air it may remain volatilized or suspended with other organic particles for various lengths of time, but it would ultimately reach soil or water.
The particles reaching water will if there remain suspended or get dissolved , depending upon its physico – chemical properties .
The ones reaching the soil may get admixed with soil particles , absorbed onto clay and minerals , get dissolved in soil water or get leached out of soil into the grain and ultimately reach stagnant run – off water .
The ones reaching inanimate objects in the vicinity of crop fields may also take same course of dispersal.
On the other hand, when applied to the plants, some may get deposited on various plant surfaces or get absorbed and translocated inside the plant system.
The persistence of the chemical on or inside the plant depends on various qualities of the chemical and the plant. Some of the persistent chemical would reach the animal system as feed or fodder and some either directly or through the animals may reach human system as food. The persistence of pesticides over the various steps involved in the plant –animal – man food –chain varies considerably.However,one would desire to have a long persistent pesticide for use in plant pest control, where as it should be short lived through easy disintegration inside animal and human systems, so that it does not leave any toxicity.
POTENTIAL HAZARDS FROM PESTICIDES:
The potential hazards from pesticides to men due to pesticides may fall under different categories
Potential hazards due to pesticides
(Table 2)

1) Direct contact with the toxic chemical leading to dermal toxicity and when ingested or inhaled inside the body system, various types of toxic symptoms, which in acute cases may lead to death.
2) Indirect hazard due to residues reaching through food, causing toxicity of different intensities.
3) Another form of indirect effect such as killing of economically important species of insects, microbes and ever larger biological forms such as fish.
4) Interference with the ecosystem causing environmental pollution of different categories, including development of resistance against pesticides in the target pest species, leading to more severe pest problem than in the beginning.
While there are sporadic reports on accidental and wanton deaths through consumption of some pesticides of high potency, the number is net alarmingly large when compared with the numbers due to traffic accidents , ets .
Adequate pre-cautionary measures would certainly help in bringing down this number of deaths due to pesticides.
There are several reports on the presence of pesticides in agricultural products marketed in the Islamic Republic of Iran.
Analysis of market samples collected from urban areashave shown more than tolerable limits of certain pesticides like DDT and BHC.
Of ten these pesticides applied for controlling household pests and fruits have been shown to contain malathion and some organochlorine insecticides at levels higher than the maximum residue limits prescribed by FAO/WHO.
Pesticide residues have been also found in animal feed, bovine and breast milks, meat, egg, fish, etc.
Bioaccumulaticn of pesticides is an important area which needs immediate attention.
The residues present in plant parts used as feed and fodder get concentrated in the animal tissue and when such material is consumed as human food, if not destroyed in the cooking process, may cause different forms of harmful effects.
When the pesticides present in water are taken up by fish and other aquatic species, they get highly concentrated up about 50 to 100 folds in some parts of their body and they form potential danger to human system when consumed as food.
There are a few reports on inhibition of nitrogen-fixing bacteria,
azotobacter and azospirillum due to appliction of high concentrations of certain pesticides, though there are also reports or harmlessness or stimulatory effect on a few nitrogen , fixing and nitrifying bacteria by some insecticides, at low concentrations .
Devolopment of resistance to pesticides in pest is more dangerous than the problem of pesticide residues.
There are instances where houseflies exposed constantly to DDT had become resistant to the pesticide and on further continued exposure became dependant on DDT for its metabolism.
There are several instances where microbial pathogens of plants, animal and human being developing resistance qualities against the chemicals to which they were originally sensitive.
Development of resistance in such microbes has been shown to vary with the species and the nature of chemical to which they are exposed.
Cross protection resistance among the pests against the chemicals is also known in a few cases. Use of combination of pesticides to combat the pest and periodical change over of pesticdes of different chemical nature to protect crop plants are some measures commonly recommended to overcome these hazards.
Thus there exists potential danger in different forms and intensities due to pesticides in Iran although the average dose of pesticide applied per unit area of land is far below the levels obtaining in Japan and S.Korea.
DEGRADATION OF PESTICDES:
The pesticides reaching the natural substrata of soil and water are subjected to physical, chemical and biological interactions
Degradation of pesticides
(Table 3).

Physically they may get adsorbed to other molecules or destroyed through heat, light and other physical factors such as irradiation.
Some chemicals are thermoliable, while the others are liable to sunlight and therefore when applied to crops under tropical conditions get readily degraded. Some pesticides readily interact with other organic and inorganic substances they come in contact in the natural substrata, including plant parts and get changed in their molecular structures and chemical properties, including pesticidal effect. However, both the physical and chemical changes of the pesticides in nature result only in incomplete transformation and do not lead to complete mineralization.
More complete transformation of the pesticides in the natural substrata is mediated through microbial actions.
Soil is a living biological system possessing its own characteristics, physico-chemical and biological properties.It is a store house of a large number and variety of microorganisms whose number may vary from a few thousands to a few millions in a gram of soil.
It is also the ultimate sink and the natural scavenger of waste material of plant and animal origin. Invariably everyone of the organic substance, whether as plant debris or animal excreta reaching the soil, is acted upon by one or more microbial species present in the soil. These organisms possessing exoenzyme which are excreted outside their body cells attach and transform the pesticide molecules from one form to another and also use them for their metabolism. In the process the pesticide molecules get disintegrated and detoxified, through a series of chain reactions, several forms, groups and species of microorganisms get involved in stages, to attack and breakdown the molecule into different units and molecular structures, often resulting in almost complete mineralization to produce the end products of CO2,H2O,NO3 ,CL, P2O5 ,etc.It is only very rarely certain organic compounds persistently defy microbial attack and they may remain for more than one crop season or for several years in soil, when they are called recalcitrants.
Insecticides which undergo the same fate as most of the organic substances mostly lose their pesticidal properties, though in a few instances the intermediary breakdown products also possess the same or added vigor against the pest. In some cases the pesticidal spectrum may also get altered due to the action of the microbes on the chemical, resulting in pesticidal action on entirely different groups or species of the pest than the original target pest or pests.
TYPES OF CHEMICAL REACTIONS:
The important reactions mediated by the microbes are: 1) hydrolysis,
2) reduction ,3)addition of hydroxyl group,4) addition of oxygen to double bond to produce epoxide ,5) oxidation of amino group,6) oxidation of sulphur,7) removal of methyl group,8) addition of methyl group,9)removal of chlorine,10)migration of chlorine ,11)metabolism of side chain,and12)cleavage of ether linkages are common ,13)reduction of nitro group,14)replacement of sulphur with oxygen.
Often more than one reaction takes place simultaneously. Repeated application of the same pesticide may encourage growth of certain groups of microorganisms to cause ready destruction of the molecule and as alternate different pathways of degradation may also result. When combinations of pesticides are added, the pathways of degradation of a given pesticide may differ from when it is added in isolation into the soil.
ROLE OF MICROBES:
The microbial population in a given soil under a given set of agro-ecological conditions is in a dynamic equilibrium. Any change in the equilibrium due to extraneous factors is short-lived, since the population reaches its original level within a short period.However, certain permanent changes in the number and type of bacteria, actinomycetes and fungi could be brought through high levels and repeated applications of such amendments like farm yard manure, fertilizers.etc. But such changes are more difficult under tropical conditions than under temperate conditions.
Organo-mercurial fungicides suppress certain bacterial actinomycetes and fungi, while a few are unaffected. In general, herbicides suppress soil algae, where as they do not effect much the other soil microbes.
Organochlorines enhance bacterial population, where as fungi and actinomycetes are not much affected.
Organophosphates also have a similar influence. Where certain changes in the microbial population have been reported, doses several folds higher than the recommended dose against the pest, which have brought out the changes.
Pre-treatment of seeds with pesticides cause changes in the rhizosohere effect of the germinating seedling.
The microorganisms which are capable of attacking the added pesticides in the soil are common dwellers in most soils. The bacteria which degrade pesticide more commonly are: pseudomonas, escherichia, acromobacter, acrobacter, bacilius, and clostridium.
Corynebacterium, flavobacterium and micrococcus. Among the streptomycetes, streptomyces and thermoactinomycetes and among the fungi aspergillus, penicillium, rhzopus, tricoderma and yeasts species are commonly found to attack pesticides, whether it be an organochloride, organophosphate, carbamate, organomercurial or other chemical forms
Microbes metabolizing pesticides
(Table 4).

FACTORS INFLUENCING PESTICIDE DEGRADATION:
The rate of degradation of different pesticides varies with their chemical nature, the soil types, and the factors influencing the soil- pesticide interaction.
(1)Soil moisture is an important factor, influencing microbial degradation of pesticides several organochlorines, such as BHC and DDT which are known to persist for longest periods in certain soils are reported to disappear within a period of few months when the soil is flooded with irrigation water to bring microaerophilic or anaerobic conditions.
(2)Soil pH is an important factor influencing microbial action on pesticides. Some pesticides disintegrate through microbial action more rapidly in alkaline soils than in acid or neutral soils. The microbes which are responsible for the disintegration under such conditions vary widely.
(3)Soil organic matter which is directly proportional to soil micro flora plays an important role in microbial degradation of the chemicals. Besides the organic matter influence soil properties such as texture and structure, etc, which in turn influence pesticide stability in soil.
(4)Soil temperature is a very important factor affecting the life of pesticide in soil. Higher the temperature as prevailing under tropical conditions, quicker is the microbial degradation of the pesticide. There are several reports to show that some of the long-persistent DDT and BHC, etc, in temperate soil conditions, got disintegrated within a period of 60-120 days under tropical conditions. Higher microbial activity under high soil temperature is correlated with such activity.
(5)Various tillage operations such as ploughing, harrowing, irrigation, crop rotation, mixed and multiple cropping systems which directly and indirectly influence the physico-chemical and biological properties also influence the persistence of pesticide in soil.
(6)Soil amendments with organic matter such as farm yard manure, paddy straw and fertilizers have been found to reduce the life of pesticides in soil
Half-life pesticides in soil
(Table 5).

Stability of insecticide in soil (in flooded soil,under tropical conditions)
(table 6)

persistence in days

80-90

80-90

80-90

80-90

3-10

8-20

It is possible that through a combination of various agronomic and soil factors, we could work out ways and means of substantially reducing the life of pesticides in soil and thus minimize their ill effects on the environment.
PESTICIDE DEGRADATION IN PLANTS:
A great number of pesticides when applied at higher concentrations are toxic to plants. At lower levels they interact with plant parts and provide protection against pest attack, the duration of such protection depends on pesticide-plant interactions. Topical application of the chemicals cause certain interaction with the phyllosphere and other plant surfaces, resulting in physical, chemical and biological changes which may be beneficial or harmful to the plant. In the case of systemic pesticides, they may be absorbed through the root system or foliage and become incorporated in the plant sap and other parts. The translocation and distribution of the chemicals occur in two distinct vascular system, viz.passive transport and active transport. In the former case, the movement is from the root to all the transpiring leaves, and in the later case from mature leaves to center of high metabolic activity such as growing points of shoots and roots.When once inside the plant the pesticides are subject to the metabolic pathways of the plant, however, the rate of metabolism is dependent on the nature of the chemical, the plant part in which it accumulates and the time of residence in the plants as also the phase or age of growth of the plant. The pesticide may be subject to a range of different metabolic activities and may lead to the formation of terminal residues that may persist in the plant part even after harvest. In general, Pesticides are only partially degraded in plants through oxidation, reduction, hydrolysis and conjugation, complete breakdown to CO2 and H2O being uncommon.
PESTICIDE DEGRADATION IN ANIMALS:
Pesticide enter the animal body through ingestion, inhalation and absorption when once it is inside the animal system, it is subject to metabolism by a variety of enzymes.
The types of biochemical transformations a pesticide may undergo in an animal or human body depends on the nature of the chemical and the species, age and sex of the animal.
The functional groups in the molecules of pesticides would undergo these reactions result in compounds that are more polar than the parent molecules, and such compounds get excreted from the body. While most pesticides get detoxified in the animal system, in a few cases they may be metabolized to form more active compounds. The metabolism in the animal body takes place in two phases. Phase one involves the production of metabolite through biotransformation reaction such as dehydrohalogenation, dehalogenation, hydroxylation, isomerization, oxidation,
Reduction,hydrolysis,etc.Phase two metabolism involves the formation of conjugates through glycoside sulfoconjugation,glutathion conjugation ,amino acid conjugation or methylation.The resultant conjugated metabolites which are other forms of pesticide derivatives, become polar products which eventually get excreted from the animal body. While this is the general pattern of pesticide metabolism, in animals when present in minimal doses, at higher doses certain toxic symptoms are evident. The LD50 values of the chemicals and the data from acute, sub acute toxicity,neurotoxicity,carcinogenicity,teratogenicity,etc,tests reveal not only the toxic levels, but also the various types of toxic symptoms caused on experimental animals and birds which are indication of possible human intoxication .The LD50 values of various pesticides differ considerably. For example, the most potential of the antibiotic range from 2 to 5 mg/kg body weight, the fungicidal mercuric compounds at 7 to 9 mg/kg,the herbicidal,2,4-D at 300 to 1000 mg, the insecticides,DDT at 113 mg,lindane at 110 mg,malathion at 56 mg,endosulfan at 240 mg, cypermethrin at 2400 mg,etc.However,the approved tolerable or permissible limits of the pesticide for human consumption is at much lower levels in the range of 0.01ppm for fenitrothion in rice,100 ppm for carbaryl in tomato,1 ppm of methyl parathion in rice,etc.

Tolerance limits of some pesticides
(Table 7)

Bioconcentration of pesticides taking place in plants and animals may lead to accumulation of toxic levels in human body. In this respect leafy vegetables, consumed raw, are most harmful as against the ones processed or cooked before consumption.
CONCLUSION:
The pesticides applied to plants and in soil and water get sooner or later degraded into non-toxic inert material through physical, chemical and biological processes, the microbiological processes playing a more dominant role. As in the most biological processes, various environmental factors influence the action of the microbes on pesticides in the natural sub-strata of soil and water.Therefore, the rate of breakdown or persistence of the chemical varies considerably under different ecological conditions prevailing in different regions of the world. The disintegration is certainly more rapid under the prevailing tropical conditions in Iran. In order to derive the maximum benefits from the pesticide, with least interference with the environment, including the biosphere and its plants, animal, soil, water and air, we need more of specific pesticides against the target pests, and the chemicals should be more effective at low concentrations and relatively more persistent without much affecting the environment. We have also to evolve an integrated method of pest management with the inclusion of biological agents.
Chemicals and agronomic practices against a given crop pest or a group of pest attacking the sequential crops in a cropping system. We have to recognize the basic fact that we need pesticides for controlling pests not only as an immediate remedial measure, but also as a long range means of combating pests. We have to learn as how to live with the pesticides safely and comfortably. We therefore, need to carry out intensive and extensive
Inter-disciplinary research work to evolve suitable, effective and economic measures to suit different agro-climatic conditions and agro-ecological situation.
References:
Agnihothrudu , V . and M .S.mithyantha , 1978,
pesticide            Residues: a review of Indian work. Rallis (India) pp169.
Attr, B.S., N.L. Ramanathan and C.R.Kirshnamurti, 1982.
Proceeding of indo- us workshop on biodegradable pesticides, April 16-19, 1979, Lucknow, U.P, India, Govt, of India, dept. of environment –New Delhi, pp .350.
David, B.V. (Ed) 1982.indian pesticide industry, vishas publication. Bombay, India, pp.377.
Edwards, C.A, G. K.veeresh and H.R. Kruger, 1980.
Pesticide residues in the environment in India: proceedings of a symposium held in Bangalore, Nov .1978, university of   agricultural. Science-publication pp524.
Kenneth D.Racke (Editor) Joel R. Coats (Editor) Hardcover, May 1990. Enhanced biodégradation of pesticides in thé environment.
Rangaswami, G, 1982, principal diseases of food crops. Proceedings of the international conference on chemistry and world food supplies – the new frontiers, dec .b-10, 1982, manila, Philippines (in press).
Rao, V.S.1983. principles of weed science, Oxford and IBH publishing co .New Delhi, India, pp-540
www.pesticideinfo.org.iran

مقالات انگلیسی

Environmental fate of pesticides

ENVIRONMENTAL FATE OF PESTICIDES

A.Soltaninejad, Ph.D

Stability of insecticide in soil (in flooded soil,under tropical conditions)
(table 6)

مقالات انگلیسی

ENVIRONMENTAL FATE OF PESTICIDES

A.Soltaninejad, Ph.D

Stability of insecticide in soil (in flooded soil,under tropical conditions)(table 6)

Stability of insecticide in soil (in flooded soil,under tropical conditions)
(table 6)