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Thursday, 20 September 2018

Pests of Crops

Pests attack crops and decrease their production. To affect production appreciably, a given pest  must be present in large numbers.

   Pest may be plants or animals.  Plant pest are  known as weeds. The most Important animal pests are insects. Other animals pest includes mites, nematode's, snail, birds and rodents.

    Pest of crops feed on various parts of a crop plant, there by affecting their growth.

Some crop pests
My previous write up lists some common pests of crops. We shall now generally discuss the pests that attack various part of crop plants.

Stem borers

Stem borers of cereal crops like maize are the larvae of certain moths. The moths lays 3000 to 4000 eggs at the junctions of the leaf sheaths and stem of a maize plant. After a week, the eggs hatch into a larvae which bore into stem and feed on it. This weakens the stem causing the maize plant to break even in slight wind. Once the larvae mature, they drop to the ground and pupate in the soil. Adults moths emerge from that pupae, mate, lay eggs and die. For maize, the peak period of stem borer infestation is in the latter part of the year.

Root Feeders

Root feeding insects may be insect larvae or adults. An example is the yam beetle which lives mainly in the soil. Mature yam beetles breed and lay their eggs (November to December) in damp places like river banks. The eggs hatch into larvae which feed at first on decaying matter in the soil and then on grass roots, before changing into pupae. The adult beetled emerge (March) and set off on a feeding areas where they burrow into the ground and feed on the yam tubers. The beetles continue feeding on the yam for some time (July to October) before they SRT off on a breeding migration (November to December ) to the river banks.

Leaf feeders

Leaves of crops are eaten by snails, leaf feeding beetles, caterpillars of various moths and butterflies, grasshoppers and locusts.

Young shoot feeders

Insects like aphids, mealy bugs and scale insects Pierce and suck juices from young shoots of crop plants. An example is the cassava mealy bug. The bugs first settle on the shot apex and feed on the young tissue. Gradually they move to the leaves and then spread to all parts of the plant. The bugs rapidly multiply: the females lay up to 450 eggs and the life cycle from egg to Adult only takes about 23 days.

     Many sucking pests also transmit disease causing fungi, bacteria and virus to the plants. For instance, the cassava mosaic is a virus disease which is transmitted to the plants by white flies. Such pests which transmit disease causing agents are known as "vectors"

Fruit and seed feeders

Fruit and sees feeders include fruit piercing moths , fruit flies, cotton Stainer's and certain beetles. Examples include the red boll worm, the larva of a moth, eats its way into cotton bolls, causing them to drop before time. Cotton Stainer's are small reddish insects that feed on unripe Cotton seeds in the cotton bolls. This may cause the bolls to turn brown. When cotton stainers feed on the seeds of half open bolls, they pass out excreta which stains the lint yellow. Contain stainers do the greatest damage when they insect the  spores of a certain fungus into the cotton bolls while feeding on the seeds. The fungus grows on the lint staining it yellow.

   The female cotton stainer lays its eggs in batches of about a hundred in most soil. The eggs hatch into nymphs which moult five times emerging as adult. The development period takes about three to five weeks. Adult cotton stainers also have other host plants such as tomato, millet, maize and okra. This makes it difficult to eradicate them.

Pest control

This is mainly aimed at reducing the pest population and protecting crop plants. To effectively carry out pest control , a knowledge of the pests' habits and life cycles is important. from this , we can deduce the peak periods of infestation, the resting stages, and the vulnerable stages where control measures can be implemented.

  The various methods used In pest control are given below. Usually a combination of pest control methods are used to control a given crop pest.

Physical methods
These involve collecting pests by trapsand then destroying them. Snails and caterpillars are often picked, while light traps and sticky traps are used to trap adult insects. Other physical methods include burning, flooding (to destroy soil pests) and using deterrents such ad bird scarers to keep away birds that feed on grains and fruits.

  Generally, physical control methods cans only be used successfully with a small area and at low infestations.

Cultural methods
In these methods , the normal farming practice is modified so that environmental conditions do not favour the development of the pest. This requires a through knowledge of the pest's life cycle and living habits.

  Crop rotation, closed season practice, fallowing and the changing the time of planting make sure that susceptible crops are not  around during the development of the pest so that the pest starves to death. This is difficult if the pest has several alternate hosts like the cotton stainer. Planting resistant or tolerant crop varieties and early maturing varieties also helps to control pests. Many pests breed and develop in the Bush around cultivated fields. Clearing or burning these areas destroys the pests. Tilling land brings soil pests to the surface and  kills them.

Biological methods
In these methods, natural enemies of pest are used to destroy pests. Such enemies may be predators, parasites or diseases of pests. For example, many wasps feed on stem borers. Sometimes, the natural enemy is brought in from another country.

Chemical methods
Chemical s known ad pesticides are used to kill or reduce the pests population, or to protect the crop from attack by pests. They are applied as dusts, granules, liquids, suspension and emulsions. This is a widely practiced method of pest control. Yam beetles are controlled by dusting yam sets with aldrin 2.5% dust or Gammalin   A at planting. The beetles are killed when they feed on such treated yam setts.

Modern trends in pest control
In the sterile male techniques, male insect pests are sterilized by chemicals or radiation, and then released into the pest population. This reduced the pest population.

   Some chemicals attract insect pests. These are used to lure the pests to come in contact with insecticides or chemicals that cause sterility.

These are undesirable plants that compete with crops for space and nutrients. Weeds are usually hardly plants with extensive root systems and efficient methods of propagation and seed dispersal.

   Weeds reduce crop yield and increase the cosy of farming  operations. Some weeds reduce the quality of crop products by imparting bad odour and bad taste to the products. Livestock grazing on weeds may produce poor quality hide, wool or milk. Certain weeds are toxic to animals. Weeds may also harbour crop pests and diseases.

   Weeds may be annuals, biennials or perennials. Example of weeds includes:

Grasses , e.g carpet grass and elephant grasses;
 members of the sunflower family , e.g . goat weed, Emilia and Tridax, and 
others  like mimosa pudica ( Sensitive plant) , 
Desmodium and Cyperus.

Weeds may be controlled 
Mechanically by hand pulling, hoeing , tilling, mowing or burning; 
by crop rotation ; 
biologically by using the natural enemies of weeds; and
Chemically by using herbicides. Some herbicides are specific and only destroy a particular type of weed and not the crop plant.