Toxicology: Agriculture And Environment: Pesticide Chemistry And Toxicology

Dr. Dileep K. Singh is an Associate Professor in the Department of Zoology, University of Delhi, Delhi and is working on pesticide toxicology, bioremediation of pesticide molecules, soil microbial ecology, and soil health and soil fertility. He has about seventeen years of teaching experiences in Postgraduate Departments of the University i.e. in the Department of Zoology and Department of Agrochemicals and Pest Management and has similar years of research experiences. He has completed nine research projects funded by IAEA/FAO, NATP-CGP (World Bank), ICAR, DBT and DST. Four ongoing research projects are funded by DST, DBT, IAEA/FAO and DU-DST Purse Scheme.

He has published 27 international papers in journals like J. Agricultural Food Chemistry, Environmental Science and Technology, Chemosphere, Canadian J. Microbiology, Biodegradation, J. Environ. Science and health and Bull. Environ. Contam. Toxicol. Impact factor of these journals ranges between 0.9 to 4.36. About 9 national papers are published in journals like Pesticide Research Journal, Indian J. of Microbiology and Indian J. of Entomology. He has been an Executive Editor of Indian J. of Microbiology since 2006. He has been elected Treasurer, Association of Microbiologists of India (2011-2013). Reviewed many international research papers i.e. FEMS Microbiology Letter, J. Environmental Science and Health (B), Bull. NRC, Applied Soil Ecology, International Journal of Analytical Chemistry etc. and book on microbiology. About 84 abstracts were published in proceedings of conferences, symposium and workshops. He has published two popular books on pesticides in Hindi and has contributed two chapters in book. He has been associated in writing chapters on Pesticide in e-book by NISCAIR (CSIR), New Delhi, India (2005). E-Chapters on Toxicology of insecticides and “Insecticidal method of pest control” are available on the website http://nsdl.niscair.res.in/, posted on 28th September, 2007. He is also a member of working group for creating e-content for ACPM constituted by NISCAIR, New Delhi , India (2006) Member of Content Advisory Committee for Biology, constituted by MHRD New Delhi for the preparation of e-Biology Book (2006) on http://sakshat.gov.in/

Ten students have obtained their Doctorate Degree under his supervision and six students are working for the same. Four Post Doctorate students have also worked with him. About 22 students have obtained their M. Phil. Degree under his supervision and about 25 students of M.Sc. (ACPM) have submitted their projects. He is a member of The National Academy of Sciences, Allahabad. India (2008), Pesticide Society of India, the Association of Microbiologists of India, and Entomological Society of India. His research group is working on pesticide residue analysis, soil microbial ecology, soil health and microbial bioremediation of pesticides. He was an elected member of Academic Council (2002-2006) of University of Delhi for two terms and had four years administrative experiences as Resident Tutor in one of the PG hostels of the University. He is a member of many committees in the university and in other organizations. More information about author is available on website http://people.du.ac.in/~dksingh/

I am thankful to my students for providing me valuable information and queries related to this eBook. I am thankful to my wife Ms. Saroj Singh, my daughter Ms. Shagun Singh and my son Mr. Kartikay Singh for their constant support in writing this eBook. I am highly indebted to Bentham Science publishers for providing me an opportunity to disseminate this knowledge to my students in the form of an eBook. I am highly thankful to different sources that are used as information in this eBook to fill the gaps in knowledge, these are mentioned at different places; without acknowledging them my liabilities towards the society and science will not be completed.

Pesticide ; Insecticide ; Classification of pesticides, Toxicology ; LD 50; LC 50; Dose response relationship; Pesticide Metabolism ; Phase I reactions; Phase II reactions; Insecticide formulations; Insecticide resistance; Antidotes.

Pesticides are biocides especially designed to kill, repel, attract and mitigate the organisms, which are nuisance to humans and their agricultural and hygiene activities. The pesticides may be the organic molecules or inorganic synthetic molecules or the biopesticides. The toxicological activities on pests depend on their chemical structure and different life stages. These pesticides can be classified as organochlorine compounds, organophosphorus compounds, carbamates, pyrethroids and neonicotenoids. In organochlorine compounds, the number and position of Cl in molecule decides the toxicity. They are nonpolar and lipophilic in nature. Organophosphorous pesticides (OP) are neutral ester or amide derivatives of phosphorous acids carrying a phosphoryl (P-O) or thiophosphoryl (P-S) group. OP pesticides are identified by single characteristic i.e., they act by inhibiting cholinesterase enzyme. Carbamates are anticholinesterase inhibitor pesticides and are synthetic derivative of physostigmine, also known as eserine, which is a principle alkaloid of plant, Physostigma venenosum, Calabar bean. Chemically, they are esters of carbamic acid, HOOC.NH2, with insecticidal (and related) properties. Pyrethroids are esters formed by the combination of two acids i.e Chrysenthemic acid and Pyrethric acid, with three alcohols namely Pyrethrolone, Cinerolone and Jasmolone. They are nerve poisons and affect the nerve axon, causing repetitive discharge of nerves which results in eventual paralysis. The neonicotinoids, are the newest major class of insecticides, derived synthetically from nicotinoids. Mode of action of neonicotinoids is that they act as agonists at the insect nicotinic acetylcholine receptor (nAChR). In this chapter, we will study the chemistry of different group of pesticides.

In this chapter, individual representative pesticides are described, taking as example DDT, DDD, HCH, ᵞ-Lindane, Endosulfan, Malathion, Parathion, Chlorpyrifos, Carbaryl, Carbofuran, Aldicarb, Cypermethrin, Allethrin, Diflubenzuron, Teflubenzuron, Fenoxycarb and Imidacloprid. Here, the pesticides are described by using their chemical structure, molecular weight, physicochemical properties and fate in the environment.

In this chapter, toxicity of molecules is described. The toxicity of pesticides to an organism is usually expressed in terms of the LD50 (lethal dose 50 percent) and LC50 (50 percent lethal concentration). And the interaction of toxic chemical with a given biological system is dose-related. So, it is the dose which makes substances poison. The right dose differentiates a poison and a remedy. At high doses, all the chemicals are toxic, at appropriate intermediates doses they are useful and at low enough doses they do not have a detectable toxic effect. There are certain pesticides (e.g. DDT) known today, because of the long term exposure to them at doses that do not immediately kill the organism showed severe effects like Carcinogenic, Mutagenic and Teratogenic effects.

Insects administered chemicals by several methods including topical application, Injection Method, Dipping Method, Contact or Residual Method, Feeding and Drinking Method. The susceptibility of insect population to a certain poison is assessed by constructing dosage-mortality curve in which the logarithmic scale of dosages is plotted against the probit units of percent mortalities at a given period of time.

In this chapter pesticide metabolism has been described. Pesticide Metabolism is typically a two stage process. These are Phase I and Phase II reactions. Phase I reactions normally add a functional (polar reactive) group to the foreign molecule which enables the phase 2 reaction to take place. These reactions are catalyzed by the cytochrome P450 group of enzymes and other enzymes which are associated with endoplasmic reticulum. Phase I reactions includes, Microsomal oxidation and Extramicrosomal oxidation reactions. Phase II reactions are conjugation reactions and involve the covalent linkage of the toxin or phase I product to a polar compound. In general, conjugated products are ionic, polar, less lipid soluble, less toxic and easily excretable from body.

All reactions of pesticide metabolism are described using chemical structure with possible target sites and enzyme involved.

In this chapter, we have described the path of exposure to toxicants, various toxicological symptoms and their antidotes. Persons who are frequently involved with pesticides application should become familiar with the signs and symptoms of pesticide poisoning and get immediate help from a local hospital, physician, or the nearest poison control center. Treatment for poisoning with OP and carbamate insecticides involves the use of atropine which counteracts the muscarinic effects, keeping the individual alive. Antidotes are remedy or other agent used to neutralize or counteract the effects of a poison. Medical antidotes are available to neutralize the poisoning effects of the pesticides. However, if it is taken improperly then these antidotes can be more dangerous than the effects of the pesticide itself. Both carbamate and organophosphate pesticides attack cholinesterase in the blood and make it ineffective. Physician can determine the patient’s base level of cholinesterase by a simple blood test. If the cholinesterase level has decreased, the patient has been overexposed to either organophosphate or carbamate pesticide. One should avoid further contact with these pesticides until his cholinesterase level has returned to normal. In severe cases, medical antidotes must be given to the patient.

Pesticides are a diverse group of substances with a potential for varied toxic effects. They can enter the human body in three ways i.e. by absorption, through the skin or eyes (dermally), through the mouth (orally), and by breathing into the lungs (inhalation).

In this chapter different pesticide formulations and its method of application has been described. Pesticides are utilized in the form of formulations. Formulation is final physical condition in which the insecticide is sold commercially. Formulation is the processing of a pesticide by such methods that will improve its properties of storage, handling, application, effectiveness and safety to the applicator and the environment, and profitability. Many formulations are made like emulsifible concentrate, water-miscible liquid, wettable powder, water soluble powders, oil solutions, flowable powders, aerosols, granular, fumigants, ultra low volume concentrates, fogging concentrates, dusts, impregnated materials, poison baits, slow release insecticides etc. A technical grade of pesticide is the pure form (purity up to 95-98 per cent) of the chemical.

Pesticides are applied in the field with the help of suitable equipment. A variety of sprayers are available in the market. Granule applicators as well as dusters are used for soil treatment. Ultra low volume application became possible through the development of equipments that allow application of very narrow range of droplet sizes. The application patterns, day timing and temperature are important considerations while applying the pesticides. The type and level of pest infestation decide type of the pesticides to be use as well as the pesticide application method.

In this chapter, the behavior of pesticides in soil, water and air environment has been described. When the pesticide is introduced in the environment it transferred to different components of the ecosystem and move in different trophic levels. Transfer includes adsorption, leaching, volatilization, spray drift, and runoff. Pesticide also degrades in environment by microbes (microbial breakdown), chemical reactions (chemical breakdown), or light (photo degradation).

Extensive use of insecticides may lead to the development of resistance in insect. Resistance is defined as the ability to develop tolerance to doses of an insecticide which would prove lethal to the majority of individuals in a normal population of the same species. It is a dynamic phenomenon that has multiple factors which are broadly classified as biochemical, physiological, morphological, genetic and ecological factors.

Development of resistance means a major economic loss as well as increased hazards due to higher quantities of pesticides used. So, to prevent pesticide resistance as well as to protect the environment a number of strategic management practices are recommended.

In this chapter, the main theme of pesticide usage and its regulations are summarized in general to make some understanding about the pesticide toxicology.

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