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Lesson Note

Subject: Agriculture Science

Topic: Crop Pests.

Lesson Objectives: by the end of the lesson, the learners should be able to:

  1. Define crop Pests;
  2. Mention and explain three categories or classes of crop Pests;
  3. State the effects of these pests on crops.

Lesson Summary / Discussion


Crop Pests can be defined as animals which causes damages to plant and plant produce by feeding on them there by reducing their quantity and harvest quality.

Insects, the most important pests of crop are very many indeed. They
are found in all parts of the world. The amount of damage they do to crops varies from one county to another and from season to season. The damage also varies depending on the number of insccts that attack a crop from year to year. Insects pests spread much more quickly in areas where only one type of crop, for instance, yam is
grown. This explains why the yam beetles spread quickly in Niger State
of Nigeria and caused serious damage to the yam crop before control methods could be worked out. Some times too, Insects get together in their
thousands and attack crops thus causing an outbreak.

Classification of Insect Pest of Crops Based on Mouth Parts

1. Piercing and Sucking lnsects
These insects have mouth-parts wilh which they pierce and suck juice
from the leaves, stems and fruits of plants. This type of mouth-part is
called a proboscis Insects in this gruup include buttertlies, moths,
capsids, aphids, mealy bugs, scale insects, white flies etc.

Damage by Piercing and Sucking Insects

Piercing and sucking insects cause two types of damage to crops:
a. by removing juice from plants, they reduce the food inside the attacked
plants, thus making them weak. Some of the sap-sucking insects (bugs)
inject poisons (Loxins) into the plants after piercing through them. Such
poisons kill the parts of the plant where they are dropped. The most
common sap-sucking insects are capsides eg. Sablbergella singularis
(cocoa bark sucker), the cotton staincr Dysdercus spp), aphids, mealybugs, scale insects and white flies.

b. The second type of damage that is done by sucking insects is that, they
allow the entry of disease organisms such us viruses into the attacked
plants when they pierce them. disease causing oganisms can only be
introduced to a healtlhy plant if the insect had picked up the disease
organisms from an infected plant on which it fed before coming to the
healthy plant on which it was later feedıng. Serious damage and even
death can occur to such attacked plants. Sap-sucking insects often
introduce fungal disease to plants on which they feed.
A gioup ot butterflies and moths have long, thin proboscis with hardened points. They pierce through flowers to suck nectar and through he skin of fruits such as oranges, grapefruits, mangoes and pawpaw to suck juice from them. Ripe fruits are usually attacked. Such fruits rot around the area where they have been pierced. They later drop and cannot be easily sold resulting to wastage.

2. Biting and Chewing Insects

Insect larvae or caterpillars and some adult insects bite and feed on plant leaves especially vegetables. Some of these are locusts, grasshoppers, crickets and biting beetles.

Damages Caused by Biting Insects
Biting insects such as grasshoppers, feed on vegetables like Amaranthus
caudalus (tete) and Corchorus olitorius (ewedu). Attacked vegetables are
sometimes totally defoliated (all leaves are eaten up). Since the leaves of vegetables are the valuable part of the plant to be sold, this may mean a
total loss of income to the affected farmer.

Insect pests which attack stored produce do a lot of damiage. Losses to stored produce are usually high if the insects are very many. The
commonest insect pests of stored grains are the maize and rice weevils
Sitophilus spp), while the bruchid beetles (Callosobruchus spp) also attack stored cowpeas (bean) Sorghum and wheat grains are however attacked by Sitophilus spp and Sitotroga cerealella (a moth). ‘The groundnut seed beetle (Caryedon spp) attacks groundnut stored in the shell.

All insect pests of grains and groundnuts eat the endosperm and embryo of the seeds. In this way, the stored items are reduced in food value for human consumption, attract poor market value and such seeds cannot
be planted as they will not germinate. The yam bectle (Heteroligus meles)
also daes a considerable amount of damage to yam tubers in storage.

Birds too cause serious damage to fruits and high seed losses to rice, maize sorghum and millet. Bird damage to rice crops leads to heavy grain losses in west Africa espccially in the Cambia, Sierra Leone, Senteg
Guinea, Chana, Liberia and Nigeria, especially in Northern Nigeria where more grain crops are grown.

3. Boring Insects
Another type of inscct that causes damage is the ‘Boring beetle’ whose
larvae enter the crop stems and eat up their juicy centres. The stems
become so weak that strong winds can break them. Examples of boring
beetles are the palm weevil, the rhinoceros beetle ofl coconut palm and the stem borer.

Agricultural Pests

Effects of Crop Pests on Crop Yield
lnsect: the attack of insects accounts for about ten percent of crop losses yearly. Some crops suffer higher losses. For instance, the cowpea (bean) weevil eats up to fifty per cent of the cowpea seeds hat are produced every year. You must have observed that most of the beans sold in the market two to three months after they have been harvested have holes in them. Each hole was made by a bean weevil (beetle) after it had eaten portions of the bean seed. For crops such as Amaranthus spp (lete) which are grown for their leaves as vegetables, insect attack which leads to complete loss of leaves means a total loss ol money to the farmer. The grain eating birds suth as the notorious ‘village weaver (Quelea), doves, pigeon, etc reduce the annual yield of rice by upwards of 20 percent. Birds which feed on ripening fruits such as pawpaw, mangoes and bananas, peck at them and reduce their market value. Losses can be reduced by prompt harvesting of ripe fruits.

Rodents: these account for crop losses are rats, mice, squirrels, grass cutters and rabbits. They biytr all the young seedlings of maize, rice and oil palms and thus kill them. Some dig up root crops and eat them up, while some others feed on fruits and seeds. Rodents attack crops both in the field and during storage of the produce.

Monkeys: these eat fruits and reduce the yield ol fruit trees. They also eat fresh maize cobs. They invade farms in flocks which action accounts for a
considerable loss of farm revenue. In Sierra Lcone, monkey attack on cocoa pods resulted in as much as 20% loss of the annual yield of cocoa.
In Akure, Ondo State, monkeys eat a lot of cocoa pods. ln the savannah
areas of Oyo and in Kwara and Niger States, the monkey is a menace to
maize farmers.

Done studying? See previous lessons in Agricultural Science

Lesson Evaluation / Test

  1. What are crop Pests?
  2. Mention and explain three classes of crop Pests based on mouth Parts.
  3. What are the damages caused by Insects, birds, rodents and monkeys?

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