Insect damage to crops growing in the field and in storage is a serious problem in most tropical countries and many techniques are now being adopted to reduce this loss. It has been estimated that insects may account up to 20-25% of total food and export crop loss and, if this figure could be significantly reduced, the national economy of many countries would benefit to a considerable degree.
Insects can damage crops by boring holes in seeds and fruits, eating leaves, reducing the quality of produce by staining it with the products of their feeding processes which may also be toxic, destroying young plants by eating their stems and roots and reducing the growth rate of crops by interfering with the natural growth processes of plants. Many insects can also transmit fungal, viral and bacterial diseases. Some insects are, however, beneficial in that they transfer pollen from one plant to another which leads to cross fertilisation, other insects are predators and feed on smaller insects which may be harmful to crop growth.
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Pests and Parasites, And It Examples
INSECTS ARE USUALLY CLASSIFIED INTO TWO MAIN GROUPS,BASED ON THEIR FEEDING HABITS.
BITING INSECTS
These are insects which have well developed mouth parts which are well adapted for biting and chewing leaves or other plant tissues such as seeds, fruits or flowers. They may be destructive in either the young or adult stages.
INSECTS WHICH BELONG TO THIS GROUP INCLUDE THE FOLLOWING.
BEETLES: These feed on the leaves, flowers or fruits of many crops. Some kinds may also bore into stems and fruits e.g. coffee.
WEEVILS: These are mainly pests of stored products, particularly seeds and their feeding normally results in severe damage to the value of the crop. Examples are the weevils which feed on stored maize, sorghum, groundnut and cocoa.
GRASSHOPPERS: These may be large, such as the variegated grasshopper and locusts or fairly small such as some forms of cricket. They can be very destructive, particularly when they move from place to place in swarms and can consume large quantities of leafy material very rapidly. They appear to be able to feed on a wide range of farm crops.
BUTTERFLIES AND MOTHS: In the young or larval stage of the life-cycle, the caterpillars of these insects can do a great deal of damage by feeding on the leaves of the plants on which they develop from the egg stage. Some types feed on the growing shoot and follow this by tunnelling under the leaf epidermis. Many insects in this group produce large numbers of eggs and whole plants may be rapidly defoliated unless control measures are applied soon after the young larvae appear.
SUCKING INSECTS
This group of insects feed by inserting a long, narrow, needle-like sucking proboscis or mouthpart into the tissues on which they feed. They are then able to extract the liquid content of the tissue and use it as a food material. This results in a reduction in the food, mainly in the form of soluble sugars, which is available to the plant and a slowing in the growth rate results. A heavy infestation of these insects may well cause the death of young plants but an equally serious factor is the ability of these insects to transmit viral and bacterial diseases from infested to healthy plants. The infection is carried via the proboscis and the liquid content of the insect body tissues, since such virus organisms are ultra-microscopic in size. Toxic or poisonous products which are produced in the body tissues of these insects may also be injected into the plant. Examples of sucking insects include the following.
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TYPES OF INSECTICIDES (Stomach and contact poisons)
CAPSID BUGS: These have a proboscis during both the young and mature stages of growth. They can feed on both the stems of young plants and the fruits of mature crops, and can cause serious injury. An example is the cocoa capsid which feeds on both the shoots and pods of cocoa.
APHIDS: These are usually small insects, some have an active flying stage which enables them to move from plant to plant when feeding. They are generally produced in large numbers when the environmental conditions are favourable and are very destructive since they usually prefer to feed on the younger plant tissues such as growing points, buds and young leaves.
Aphids are particularly likely to spread plant virus diseases of garden crops such as tomato and pepper but serious farm crop viruses such as groundnut rosette virus are also carried from plant to plant by aphids.
MEALY BUGS: These are small and slow moving and have a partly waxy or woolly body covering which makes it difficult to kill these insects by spraying or dusting. They are most often found feeding on perennial crops such as cocoa and citrus but may also be serious pests of annual crops which have a fairly long growth period. Mealy bugs are the main vectors or carriers of swollen shoot disease of cocoa and may be responsible for transmitting virus diseases of citrus.
They are often carried from place to place by ants, which use them as a source of food, but this increases their mobility.
SCALE INSECTS: These are very similar in structure to mealy bugs but have a harder body covering which is resistant to many types of insecticides. They also move very slowly, but are also sometimes carried to different parts of a tree by ants. Scale insects feed on the stems and leaves of plants such as oil palm, cocoa and citrus and also transmit virus diseases.
FRUIT PIERCING MOTHS: These are particularly damaging to developing fruits such as those of citrus since they have a proboscis which can penetrate fairly thick plant tissueş. The damage they inflict on young fruits by making holes in the outer skin is often added to by the development of secondary diseases due to bacteria and fungi.
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