Pesticides are substances that are used to kill or block the reproductive processes of unwanted organisms. Synthetic pesticides are of concern to us, because of the possible effect upon human health through eating of food, or drinking water, contaminated with these chemicals. Most pesticides can be put into one of three categories
Control of insects by insecticides helps to curb disease (for example malaria and yellow fever) and protect crops. Organochlorines are a group of compounds, which have been developed and used as insecticides since the 1950s. The best-known organochlorine compound is DDT*(dichlorodiphenyltrichloroethane) Organochlorines are stable in the environment, toxic to insects in small amounts, but much less so to humans; and, because they are organic compounds, not very soluble in water.
The advantage of these insecticides is that, being persistent; they show their biological activity for long periods of time. On the negative side, these insecticides by accumulating in the environment affect many non-target organisms, not just the target pests. It is primarily because of their persistent nature that many of these insecticides have been phased out of use.
Herbicides are used to kill weeds. Sodium chlorate, NaClO3 and sodium arsenite Na2 AsO3 were commonly used as weed killers in the first half of the last century, but inorganic arsenic compounds, in particular, are toxic to mammals.
Organic herbicides are, therefore, now used. They are much more toxic to certain types of plants than to others. Organic herbicides such as triazines are presently widely used to kill weeds in cornfields.
Fungicides are used to check the growth of fungi. Fungi, as you know, are the plants without chlorophyll. They therefore cannot use solar energy to convert carbon dioxide and water into energy-rich carbonhydrates. They live as saprophytes on decaying organic matter or as parasites at the expense of living organisms. Hence they are deemed as a threat to human interests.
Fungicides are important because they counter the growth of fungi. Organic compounds of mercury have been used as fungicides. These compounds break down in soil and this has had disastrous consequences. Many human deaths in Iraq (1971 - 1972) resulted from the people eating bread made from grain that had been treated with the fungicide methylmercury.