AgriGuide

Pest and Disease Management

Pest and Disease Management

Pesticides

Pesticides and fertilizers (chemical and organic), with information about the different types, and identifying the negative aspects of each. It shouldn't be very extensive, just a short bit of information about each.

African farmers strive to achieve highest yield and the best quality product possible to ensure food security at the household level. This would be done with a minimum investment of energy and resources, without being affected by all kinds of harmful organisms (pests) that threaten to reduce the quality and yield of crops. Protecting crops from pests is extremely important, but it is difficult to achieve maximum results with minimum effort. Farmers have to look not only at a measure’s immediate effect, but also at its long-term effects.

In best practices for organic production, non-chemical protection measures are always to be preferred against any threat of pest infestation or diseases to crops.

However, major challenges still lie ahead. The risks and hazards related to toxicity of pesticides remain as serious as ever, despite of programs for enhancing safety use and wide distribution of practical extension materials.

It is important for farmers to use pesticides, after being informed correctly. Moreover, farmers should combine their knowledge gained through experience with the information they receive on proper use of pesticides.

Pesticides classification

Pesticide is the name used to indicate agrochemicals used for food and cash crops protection. A pesticide is a substance intended to prevent, destroy, repel or control any animal pest or disease caused by micro-organisms, as well as unwanted weeds. Pesticides can affect harmful pest animals and microorganisms through direct contact, feeding or other kinds of effective exposure during stages of growth.

Bio-pesticides can play an important role in pest management. These consist of beneficial micro-organisms, and can be bacteria, viruses, fungi and protozoa, beneficial nematodes or other safe, biologically based active ingredients. Benefits of bio-pesticides include effective control of insects, plant diseases and weeds, as well as human and environmental safety. In some areas pesticide resistance and environmental concerns limit the use of chemical pesticide products.

Pesticide names

The full chemical name of a crop protection product is often difficult to pronounce as well as to remember. The coded name is referred to as active ingredient (abbreviated as a.i.). This is generally a shortened version of the full chemical name. The active ingredient is the compound used to control the harmful organism. Its ability to kill, harm or deter a certain pest or disease has been proven and its use is authorized through a national or international registration process.

Ways to categorize pesticides used in agriculture

Chemicals or agro-pesticides available can be classified according to type of pest or disease against which they are effective (Table 1).

Table 1: Agro-chemicals including pesticides (P) and their activity

Category Activity
Algicide Kills algae e.g. on wood
Anti-feedant Prevents animals feeding on a crop or stored product
Attractant Attracts pest animals
Bactericide (P) Kills or inhibits bacteria growth
Fungicide (P) Fungus disinfection
Fumigant (P) Gas or smoke against pests or fungi in stored products
Herbicide Kills or inhibits growth of weeds
Insect growth regulator Modifies insect development stages or growth
Insecticide (aphicide) (P) Kills or harms insects (e.g. aphids)
Miticide / acaricide (P) Kills or harms mites (or spiders)
Molluscicide Kills snails and slugs
Nematicide (P) Kills nematodes
Repellent Keeps away pest animals Repellent
Rodenticide Kills rats, mice, rodents
Sterilizant Sterilizes insects in chemical way
Termiticide (P) Kills or harms termites

Products are effective against more than one biological class:

  • Some insecticides also kill mites or nematodes;
  • Some fungicides are also effective against bacterial diseases;
  • A few pesticides kill both nematodes, insects, fungi and weed seeds.

Chemical origin

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.

Effective application and aims of pesticide application

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:

  • The right choice of crop protection product;
  • Applied in the right dosage, at the right time and using an appropriate technique.

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

  • Preventive pesticide treatment aims to protect crops or stored products beforehand against infection by diseases, infestation by animal pests or competition from harmful weeds.
  • Curative treatment aims to destroy or limit population development of harmful organisms.

Pesticides can be distinguished according to their effect:

  • Contact pesticides need to reach the harmful organisms directly to be effective. The finer the spray mist, the better it will penetrate the crop and thus kill the organism.
  • Systemic pesticides attach to and penetrate the plant surface, and then disperse through the whole plant. Pesticides that persist for some time in the soil and subsequently penetrate through contact with roots, are also systemic. In order to be effective, these products do not need to be dispersed in a fine mist like a contact pesticide. Bigger droplets, smaller in number, can thus be dispersed over the crop, making the treatment easier and less costly.
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