Agrochemical pesticides can be divided into four main groups:
1. Traditionally compromising inorganics, e.g. arsenate, sulphur and copper.
2. Biologically selective agents — plant extracts such as nicotine and pyrethroids . More recent developments include synthetic pyrethroids which are stable when exposed to light and water.
3. Organochlorine compounds — DDT, dieldrin and aldrin.
4. Organophosphorus products.
A classified list in terms of usage:
Acaricides; Algicides; Antifeedants
Avicides; Bactericides; Bird repellents
Chemosterilants; Fungicides; Herbicide Safeners
Herbicides; Insect attractants; Insecticides
Insect repellents; Mammal repellents; Mating disrupters
Molluscicides; Nematicides; Plant activators
Plant growth regulators; Rodenticides; Synergists
Each major group may divide into chemical or other classes:
botanical and other biological products carbamates-insecticides
chlorinated hydrocarbons; organo-phosphates; pyrethroids;
benzimidazoles diazoles dithiocarbamates diazines inorganics
amides bipiridils; carbamate-herbicides; dinitroanilines
phenoxy hormone products; sulfonyl ureas&; triazines; uracil; urea derivatives
The frequency, amount and types of agrochemicals applied varies from year to year.
1. The use of agricultural chemicals has contributed to a plentiful and inexpensive food supply. The real cost may be environmental.
2. Large volumes of pesticide liquid are sprayed and applied throughout the world, and have entered every food chain.
3. In the United Kingdom pesticide labels must conform to the Data Requirements for the Control of Pesticide Regulations which cover the classification, packaging and labelling of dangerous substances.
4. Agricultural chemicals are designed to act on the intended target and be innocuous to other creatures. Following metabolism or passage into the food chain this may not be the case. Side effects may be acute, cumulative and chronic. A toxic substance can accumulate in a living organism until a concentration is reached at which toxicity is manifest. For example, the organochlorines are toxic, readily stored in fat and reduce the fertility of many species.
5. Metabolism has an important role in dictating the selectivity of action of agricultural chemicals and protects some species, e.g. animals and humans. Enzymes that metabolise foreign compounds catalyse reactions which: (i) alter molecular structure to produce a less toxic product; and (ii) increase the polarity and water-solubility to facilitate urinary and biliary excretion. Most enzymes responsible for the primary metabolism of foreign compounds are hydrolases and oxygenases. The extent of metabolism dictates the persistence of pesticides in soil, plants and animals
6. Oxidation or hydrolysis are the primary metabolic reactions in pesticides, with the insertion or exposure of polar groups. Occasionally, products of primary actions undergo secondary changes which are species-dependent.
Further Reading —Bingham, S.A. (1989) Agricultural chemicals in the food chain. Journal of the Royal Society of Medicine, 82, 311–15.
Chaudhry R, Bala Lall S, Mishra B, Dhawan B ( 1998) A food borne outbreak of organophosphate poisoning. British Medical Journal. 317, 268-9
Chavasse DC, Shier RP, Murphy OA, Huttly SRA, Cousens SN, Akhtar T ( 1999) Impact of fly control on childhood diarrhoea in Pakistan: community-randomised trial. Lancet 353, 22-25
Cherry N, Mackness M, Durrington P, Povey A, Dippnall M, Smith T , Mackness B ( 2002) Paraoxonase (PON1) polymorphism in farmers attributing ill health to sheep dip . Lancet 359, 763-4.
Eichholzer M, Gutzwiller F ( 1998) Dietary nitrates, nitrites, and N-nitroso compounds and cancer risk. A review of the epidemiological evidence. Nutrition Reviews 56 95-1105.
Hassall, K.A. (1990) Biochemistry and Uses of Pesticides, 2nd edn, Macmillan Press, New York.
Hoyer AP, Grandjean P, Jorgensen T, Brock JW, Hartvig HB ( 1998) organochlorine exposure and risk of breast cancer. Lancet 352, 1816-20
Ismail K, Everitt B, Blatchley N, Hull L, Unwin C, David A, Wessele S ( 1999) Is there a Gulf War syndrome? Lancet 179-82
Lang, T. and Clutterbuck, C. (1991) Pesticides, Ebury Press, London.
Longnecker MP, Klebanoff MA, Zhou H, Brock JW ( 2001)Association between maternal serum concentration of the DDT metabolite DDE and small-for-gestation babies at birth. Lancet 358, 110-14
Mocarelli P, Gerthoux PM, Ferrari E, Patterson DG, Kieszak SM, Brambilla P, Vincoli N, Signorino S, Tramacere P, Carreri V, Sampson EJ, Turner WE, Needham LL ( 2000) Paternal concentration of dioxin and sex ratio of offspring. Lancet 355, 1858-63
Ott WR, Roberts JW (1998) Everyday exposure to toxic pollutants. Scientific American 72-77.
Pesticides 2001. Your guide to approved pesticides. Pesticides Safety Directorate UK and Health and Safety Executive.
Pesticides Usage Survey Report 159: Arable Farm Crops in Great Britain. MAFF and Scottish Executive Rural Affairs Department.
Stephens R, Spurgeon A, Calvert IA, Beach J, Levy LS , Berry H, Harrington JH ( 1995) Neuropsychological effects of long-term exposure to organophosphates in sheep dip. Lancet 345, 1135-39
www.pesticides.gov.uk website of the Pesticides register UK
http: //www.foe.co.uk Friends of the Earth
www.panna.org/panna pesticide appraisal site
www.epa.gov/pesticides USA government site for pesticide information
www.pesticides.gov.uk classification of terms a UK site
http://plantprotection.org/?HRAC/moa2002.htm herbicide action
http://ipmworld.umn.edu/chapters/bloomq.htm insecticides: chemistries and characteristics
http://www.agcom.purdue.edu/AgCom/Pubs/WS/WS-23.html herbicide mode of action summary