Damage caused by insect pests


Damage caused by insect pests

Insects belong to the phylum arthropods. Arthropods form a major group in the animal kingdom and constitute one of the largest and most important pests of crop plants. It is a group of animals with segmented bodies and hard skin.

The following classes under which are other minor divisions are identified:

1) Class insect: These have one pair of antennae, tracheal respiration, and three pairs of true legs, three body regions (head thorax and abdomen). Examples of animals belonging to this class are grasshopper, bee and beetle.

2) Class arachnida: These are characterized by possession of two body regions (cephalothorax and abdomen), four pairs of legs, absence of antennae, and respiration by means of tracheae, diffusion through the body walls. Examples include the spider, mites and ticks.

3) Class chilopoda: Insects in this group have elongated body with 15 or more pairs of legs with only 1 pair occurring on each body segment e.g. the centipede.

4) Class diplopoda: These have elongated body with many legs usually two pairs per body segment except for the first few at the back of the head. E.g. millipede.

5) Class crustacean: These are usually aquatic, possess gills and have five or more pairs of legs. Examples include crabs, crayfish and lobsters.

In this article, you should be able to discuss the damage caused by insect pests

Damage caused by insect pests.


Damage caused by insect pests

Damage caused by insect pests

Damage by insects can be grouped into three major categories:

1. Biting and chewing: Some insects consume part of the plant with the aid of their biting and chewing mouth parts. Grasshoppers, lepidopterous caterpillars and beetles all consume whole portions of leaves, stems, flowers, fruits or roots of plants. Locust and army worm consume whole plants. The quantities of vegetation consumed can be quite high.

For example, a single female locust Schostocerca sp. can consume 1.5g of vegetation per day and a swarm of locusts covering 10km2 can eat up to 2,000 tons of vegetation per day.

Lepidopterous caterpillars are defoliators and can completely eat up the leaves of plants. Removal of leaves and other vegetative plant tissues interferes with growth and development of the crops, reduces photosynthetic surface of plants and reduces yield. Damage to the flowers and fruits leads to drastic reduction in the yield of crops.

2. Piercing and sucking: The Hemiptera and Thysanoptera have mouth parts which are modified and adapted for piercing and sucking plant tissue. The Siphuculata and some Diptera have mouth parts for piercing and sucking animal tissue too. Piercing and sucking insects do mechanical damage to the tissues they pierce. They cause loss of plant sap or blood and seriously affect the growth and development of the host. In some cases, parts of the plants attacked may be distorted and rendered unfit for sale and for human consumption.

Sucking insects also inject toxic saliva into plant tissues and this may cause death of such tissues. When fruits are attacked, blemishes may result and this lowers the quality of that fruits. Thrips pierce and suck cowpea flowers, cause flower abortion, and reduce fruit formation and yield. Piercing and sucking pests cause secondary damage through the introduction of pathogenic organisms, such as fungi and bacteria. These invade the wounds resulting from insect attack.

For example, cocoa mired feeding lesions formed on cocoa stems are invaded by the weakly pathogenic fungus Calonectria rigidiuscula which causes die – back of the cocoa stem. Piercing and sucking insects also directly transmit various diseases of crops. E.g. maize streak, virus on maize leaf curl of cotton, cassava mosaic disease, ground nut rosette virus and cocoa swollen shoot virus disease. Tsetse flies (Glossina spp) transmit the protozoan parasite trypanosomes, which cause sleeping sickness in humans and nagana in cattle. Mosquitoes transmit the malarial parasite in humans.

3. Boring: Some insects tunnel into the stems and fruits of crops and remain inside the tissue where they consume large quantities of the tissue. Such insects have mouth part adapted for biting and chewing.

E.g, larvae of most stored products insects like Callosobruchus maculates on cowpea, Tribolium spp on cereal grains, Ephestia cantella on maize, Dermestus maculates and Necrobia rufipes on dried fish and Cylas puncticollor on sweet potato. 

Maruca vitrate (testulalis) is a major pest of cowpea where it bores into the unripe pods, while Sesania spp are important stem borers of maize and sugar cane plants which are bored and died quickly. Boring insects reduce the quality of timber and stored produce, thus lowering the farmer’s income.


Also read: Definitions and Importance crop pests

Control of insect damage

The historical objective of the entomologist was primarily to develop and introduce modifications into the environment in such ways that diseases will not be spread by insects and crops will not be damaged by them.

This objective has been achieved in numerous cases. For example, in many cities flies no longer play a major role in spreading intestinal infections, and land drainage, improved housing, and insecticide use have eliminated malaria in many parts of the world.

Massive outbreaks of the Colorado potato beetle in the 1860s led to the first large-scale use of insecticides in agriculture.

These highly poisonous chemicals (e.g., Paris green, lead arsenate, concentrated nicotine) were used in large quantities. The continued search for effective synthetic compounds led in the early 1940s to the production of DDT, a remarkable compound that is highly toxic to most insects, nontoxic to humans in small quantities (although cumulative effects may be severe), and long-lasting in effect.

Widely used in agriculture for many years, DDT was not the perfect insecticide. It often killed parasites as effectively as the pests themselves, creating ecological imbalances that permitted new pests to develop large populations.

Furthermore, resistant strains of pests appeared. The environmental longevity of many early insecticides was also found to cause significant ecological problems. Similar difficulties were encountered with many successors to DDT, such as Dieldrin and Endrin.

In the course of developing effective insecticides, the primary emphases have been to reduce their potential to cause human health problems and their impact on the environment

Biological methods of pest management have become increasingly important as the use of undesirable insecticides decreases. Biological methods include introducing pest strains that carry lethal genes, flooding an area with sterile males (as was successfully done for the control of the screwworm fly), or developing new kinds of insecticide based on modifications of insects’ growth hormones.

The sugar industry in Hawaii and the California citrus industry rely on biological control methods. Although these methods are not consistently effective, they are considered to be less harmful to the environment than are some chemicals.

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