The control of the various insect pests affecting crop plants is a major problem for crop production. Insect pests may be controlled by means of various cultural practices, the use of chemicals known as insecticide and, biological methods of insect control.
1. Cultural Methods of Insect Control
Hand picking:For example, fully grown adult grasshoppers and caterpillars of some insects may be partially controlled by hand picking.
Crop rotation: Since insects are generally selective in the choice of crops they attack; a rotation of crops can result in a reduction of insect numbers when new crops are planted.
Tillage practices: Ploughing by harrowing normally reduces the population of soil pest by exposing them to sunlight and desiccation, and to predators and parasites.
Weed control: Some weeds act as host to insect pest, timely control of weeds would deprive them of their host.
Adjusting time of planting to avoid period when insect’s population is at its peak. For example sorghum midges can be effectively controlled by planting early so that flowering is complete before the adult midge population reaches damaging levels.
Resistant varieties: Pest may also be controlled by planting pests resistant varieties. New varieties, resistant to an increasing number of insect pests have been produced in recent years by the research institute.
Timely harvesting: Prompt harvesting is known to help protect maize and beans from damage by maize weevil and bean bruchid.
Observance of a closed season: Some pest cannot survive in the absence of a specific host plant. Observing a closed season for the cultivation of this plant provides effective control.
A good example is the pink bollworm (pectinophoragossypiella) provided no cotton is grown during the closed season, this pest is deprived of a carry-over site for the next season, with the result that its population is kept below the level at which it causes serious economic loss.
Trap cropping: A trap crop is used to divert the pest from the main crop. The pest usually prefers it to the main crop for feeding or egg laying. The trap crops are grown in strips at appropriate intervals within the field. The pest population concentrates on the trap crop, while the main crop suffers little damage.
Optimum plant density: The biology of both pest and their natural enemies can be affected by plant density. For example, bean fly infestation in kidney bean is less severe in densely planted crops than in thinly planted ones (Abate, 1990).
Similarly, populations of aphid Aphidcraccivora are lower and the spread of rosette virus, of which this insect is a vector, is less rapid on more densely sown fields of groundnut (Farrell, 1976a).
Crop sanitation: Cleaning crop fields after harvest and burning crop residues to destroy over wintering pest populations are important cultural practices.
Cropping patterns: For example intercropping check the spread of pest and is less frequent than in monoculture.
2. Chemical Method of Insect Pest Control
The most effective method of controlling insect pest is by spraying or dusting crops with insecticides. The choice of insecticide will depend on the feeding habits of any particular insects.
Biting and chewing insects are usually controlled by the use of stomach poisons. These are chemicals which, although they do not harm the crop, will poison the insects.
They are sprayed or dusted on to the crop usually before the rains, to avoid them being washed off the leaves, or may be applied directly on the insect. Example of stomach poisons are lead arsenate and Paris green.
Contact poisons: This would kill insects as a result of direct application and are most effective when they are sprayed onto the body in a form of a fine mist which completely covers the insect.
Contact insecticide may also be sprayed on the leaves or stem of the crop as a preventive measure before the arrival of the insects. This type of insecticide is particularly useful in controlling sucking insects.
Examples of contact poison include DDT, Gammexane, Lindane, nicotine sulphate, Gammalin 20, Didimac 25, etc.
Systemic poisons: These chemicals are absorbed either by the leaves, stems or roots of crops without harming them but the chemical will poison any insect which feeds on the treated crop. Systemic insecticides are particularly useful in controlling sucking insects such as mealy bugs and aphids.
Suffocation: Some insects have a protective covering which make their control difficult. One method of controlling insects such as scale insects and mealy bug is by suffocating them, which is cutting off the air supply to their spiracles with chemicals such as kerosene emulsion which is sprayed directly onto the insects.
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Fumigation: Insect pest of stored products are generally controlled by fumigants – that is insecticides which kill by poisonous vapours or fumes. Fumigants may either be gaseous, liquid, or powder forms. Examples are carbon disulphide, hydrogen cyanide (gas) sulphur and methyl bromide.
Host Plant Resistance
Host plant resistance works in three major ways: Antixenosis, Antibiosis, and Tolerant plants.
Antixenosis: Or non-preference refers to the innate qualities of the plant that render it unsuitable to the insect for oviposition, feeding and shelter.
Examples of antixenonsis are resistance of pubescent varieties to the potato aphid (Aulacorthumsolani) and to leafhoppers and of nectar less cotton to plant bugs.
Antibiosis: Plants possessing antibiosis have deleterious effects on the pest feeding on them.
Seeds of some pulses contain substances that inhibit the growth or reproduction of storage pest.
Tolerant plants: Are those plants that suffer little damage in spite of supporting a pest population capable of inflicting heavy yield losses. Example is the introduction of resistant variety of cotton to cotton leafhopper (Empoasca lybica) in some parts of Africa, which relegated it to minor pest status.
3. Biological Method of Pest Control
Biological control refers to the use of living organisms for the control of pests; it involves the use of predators, which feed on harmful insects and reduce their numbers to a minimum. Broadly speaking, biological control also includes the use of pathogens (bacteria, fungi, viruses, protozoan, and nematodes).
Biological control using parasites and predators has been effectively utilized in the control of Kenya mealybug (Planococuskenyae) by Anagyrus spp.
4. Regulatory Method of Pest Control
Regulatory methods depend on legislation to enforce the quarantine of plant material. The legislation requires that propagating material (seeds cuttings, whole plant) imported from abroad be accompanied by a phytosanitary certificate stating that they are free from pests and diseases.
Materials are inspected by a trained quarantine officer at the port of entry and if dangerous organisms are found the whole consignment may be destroyed.
5. Insect Growth Regulators
Insect growth regulators (IGR) are substances that interfere with the growth and development of insects. These compounds do not kill the insects immediately and thus do not prevent pest damage on a current crop.
6. Sterile Insect Technique
In the sterile insect technique (SIT), insects (male or female) are mass reared in the laboratory, sterilized (by radiation or chemosterilants) and released in the field.
They mate with the wild population and produce sterile progeny. Because of its technicality and high cost this method has no application in sub-Saharan Africa.
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