Agricultural Chemicals

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:

Pesticides —

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

Virucides

Each major group may divide into chemical or other classes:

Insecticdes —

botanical and other biological products carbamates-insecticides

chlorinated hydrocarbons; organo-phosphates; pyrethroids;

Fungicides —

benzimidazoles diazoles dithiocarbamates diazines inorganics

morpholines triazoles

Herbicides —

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.