Products are effective against more than one biological class:
Agro-pesticides can be divided into inorganic compounds, synthetic organic chemicals and bio-pesticides.
Inorganic compounds are chemicals used for pest control, such as application of sulphur, lead arsenate, copper and lime mixtures, borax and chlorates, and mercury compounds. Inorganic pesticides are based on chemical elements that do not break down, and therefore many of them have very severe environmental and toxicological effects in their use. For example, some accumulate in the soil; lead, arsenic and mercury are very toxic.
Most synthetic organic chemicals are chemically derived from mineral oil products. After the introduction of insecticides and herbicides in the 1940s, their use spread rapidly throughout the world and continued to increase during the 1950s and 1960s. Increasingly sensitive tools for chemical analysis of the residual effects on crop parts, the environment, and test animals were developed from 1960 to 1980, enabling detection of very small amounts of pesticide residues in food and the environment, down to less than one part per ten million. This exerted a strong influence on pesticide development, its use and regulation.
Bio-pesticides are substances derived from plants or animals. These can even be the organisms themselves, and include fungi, bacteria, viruses and nematodes, plant-derived chemical compounds and insect pheromones. Some biological pesticides, such as nicotine, can be very toxic and their use is as hazardous as many inorganic or synthetic pesticides. Less toxic to man are the flowers of Pyrethrum, a root extract of Derris elliptica (Rotenone) and leaves and flowers of the Neem tree (Azadirachta spp.), which have been used for generations as effective insecticides. Other naturally occurring substances that are used include cow’s urine and garlic juice.
Pesticide manufacturers have now made synthetic versions of many naturally occurring botanical pesticides, by identifying the essential chemical mechanisms that kill harmful organisms for crop protection.
Farmers have to know how a pesticide affects the pest. Insecticides, for example, can kill through dermal (skin) contact, act as a stomach poison, inhibit growth or repel the insect, and thus prevent it from feeding on the crop or stored product.
Leaf-eating caterpillars become sufficiently contaminated with insecticide residues when they crawl and feed on leaves. Boring insects inside leaves and stems as well as certain sucking insects are more protected against direct contamination. They are, however, poisoned by feeding on sap and tissue inside plants that have been sprayed with systemic insecticides.
The aim of chemical control is to bring the toxic active ingredient in contact with the target pest or disease agent in such a way to kill them, or inhibit their growth and development. Application of a crop protection product is effective if the physical and chemical formulation of the active ingredient kills or harms an insect, fungus, bacteria or other harmful organism which causes crop damage. Effective application meets the following conditions:
Correct dosage is not only dependent on pest level, but also on potential crop damage or expected harvest loss, as well as on economic costs and benefits of crop protection. Potential damage to the crop may justify the application of the pesticide, but this must always be done in accordance with the instructions on the label.
Application is economically efficient if the avoided damage and crop loss justifies application. Farmers should always estimate and compare the cost of applying pesticides with the rate of loss in yield or with the quality deemed acceptable.
Different aims of pesticide treatment: preventive or curative
Pesticides can be distinguished according to their effect: