Biology

PEST AND DISEASES OF CROP

DEFINITION:

A pest is an organism, insect or small animal which cause damage to crop in the field and to plant products stored in barn and warehouse and serve as a vector of disease.

CLASSIFICATION OF PEST

Pests are classified into two major types:-

1.   According to the parts of plant they attack.

 2.   According to the type of animal.

  

1.   According to the parts of plant they attack:-    

A.   Pests of leaves:-  the common leave feeder among insects are grass hoppers, beettles and caterpillar of moths and butterflies. Grasshopper cause great damage to crops such as alfalfa, maize, cassava, soya bean, and spinach.

 B.   Pest of young shoots:-   Many insects such as aphids, mealy buds and shoot borers feed on the shoot of plants. The shoot borer lays its eggs on the leaf surface. When the eggs hatch the larvae bore into shoot. Eg. Mango shoot are being attacked by shoot borers, causing wilting of leaves and the entire branches may die.

 C.   Pest of stem:-  Stem borers cause damage to crops in the farm/field such as cereals, maize and rice. Adult insects lay its egg on the leaves surface or in the sheath of leaves. When the eggs hatch, the larvae bore into the stem of plant and feed on the sap and stem tissues. Sometime the damage can be so much that the nodes dry up and dies. Extensive damage by these larvae may cause stem to break up.

 D.  Pests of roots:-  These are pest that attack the root of plants. They are also called root borers. Examples include gnats, the larvae of beetle as well as nematodes. The underground nature of this pest make it difficult to detect the damage to the roots until it is extensive. The beetle is a pest that attack cucumber, water melon and maize roots. It lays its eggs on the underside of leaves. When the larvae hatch, they fall onto the soil where they tunnel into the roots and damage to plant root thereby causing low yield. 

Root knot nematodes feed on and spend most of their life cycle within the roots of plants. The female root knot nematodes lay eggs on the root surfaces of plant such as tomato. The eggs hatch in the soil and the larvae penetrate the roots and feed on the root tissues. This causes the root cell to enlarge resulting in swollen structures called knots. This decreases the ability of the plants to absorb water from the soil and may lead to lower yield and poor quality crops.

 E.   Pest of Fruits and Seeds

 Fruit flies, weevils and cotton stainers are some examples of insects that feed on fruits and seeds. It has mouthparts that are well adapted for piercing and sucking. Cotton stainer:- cotton stainer pierces through the cotton fibre and feed on the seeds of the cotton plant, it transmit the fungus to the fibres of the cotton boll, causing it to be stained yellow. A punctured cotton boll may fall to develop and grow and shrinks fibre also have low economic value.

 Fruit flies can pierce through the pulp of fruits. The female fruit fly may attack papaya fruit by piercing into it and laying eggs there. When the eggs hatch, the larvae eat the fruit tissues causing damage to the fruit. The fruit then falls. The larvae then move into the soil where they will grow into adult fruit flies.

 Pests of grains:- Some insect pest infect and multiply among harvested crops such as rice, wheat, maize and beans. Examples of such pests include beetles, moths and weevil. The adult usually lay its eggs on the surface of a grain. Some insects may bore a hole in the grain or seed and lay its eggs. When the eggs hatch, the larvae feed on the grain tissues. At pupa stage, feeding stops. But when the pupa metamorphous the adult pest continues to feed on the remaining grain tissues causing more damage. 

 

2.   According to type of animal:- 

A.   Invertebrate Pests:- These are animals that do not posses back bone. They include; insects, snails and nematodes. Insects pest are adapted to various environmental conditions. They can reproduce quickly and cause extensive damage to crops in farm land as well as stored crops. Nematodes are also invertebrate pest that attacks leaves, stems or roots of plant.

 B.   Vertebrate Pests:- These are animals that have back bone. Vertebrate animals which act as pest also cause damage to crops. These include birds, rodent, monkey, elephants and rabbits. They cause extensive damage to field crops as well as harvested crops. Rodent multiply rapidly in number hence they also transmit diseases to human.

 Life cycles of some crop pests

 1.   Aphids:-  Aphids feed on the sap in the phloem vessel of plants. They use their proboscis to pierce into the plants, causing damage to plant tissues. 

 The reproduction in aphids is complex. They can switch from sexual to asexual mode of reproduction depending on the environmental conditions.

 In spring aphids hatch out from eggs. These newly hatched aphids are a population of wingless females. There are no males at this stage. Each of the female reproduces by parthenogenesis (reproduction without their eggs being fertilized by a male). The multiplication of these females may cause extensive damage to host plants, if conditions become very unfavourable, e.g in dry conditions in tropics; the female aphids start to give birth to both male and female aphids. The male and female aphids now reproduce sexually and the female, rather than give birth to live young now lay eggs. The fertilized eggs are hardy and survive the hash environmental conditions which would have killed the adult aphids.

            

Yam beetle (Heteroligus meles):- destroy tubers, gives tubers low market value. Mating of male and female takes place. Eggs are laid. Eggs hatch into larvae which feed on decayed organic matter. The larvae moult three times to develop to pupa. The pupa forms adult which dig into the soil to search for yam tubers.                          

 Control:- Aldrin

2.   Grasshopper (Zonocerus variegates):- Grasshoppers feed heavily on the leaves of plants, causing great damage to crops. Grasshoppers reproduce sexually. The female grasshoppers lay its eggs in the soil near the base of plants. When the temperature is warm enough, the eggs hatch into nymphs which have to burrow their way out of the ground and crawl up the plants. The nymphs feed voraciously and grow very quickly. They moult about three to five times before reaching adult size. Grasshoppers have a lifespan of about two months.

3.   Rice Weevils (Sitophilus Oryzae):-    Rice weevils are pests which feed on stored crops such as rice grains and maize grains. The weevils reproduce sexually and have a lifespan of seven to eight months. The female weevil lays one egg in a grain of rice. Upon hatching the Larva feeds on the grain for about three to five weeks and then pupates. Pupa stage lasts for about one week, after which an adult emerge from the grain.

 Methods of Pest Control

 Methods of Pest control include physical, cultural, chemical biological methods, Quarantine method.

 1.   Breeding pest resistant varieties of crop

 2.   Physical methods:- These include using trapping devices or exposure to high temperature. The most direct method of removing insects is by hand-picking of caterpillars from fruit trees and killing them physically. Trapping devices such as nets, light traps and sticky boards can be used to trap pests. Physical methods may be useful in a small area, but may be less practical or economical if used on larger scale.

 3.   Cultural Methods:- These include irrigation schemes and early  harvesting which serves to cut off food supply for the pests. The methods also include crop rotation, mixed cropping and tillage which reduce pest attacks.

 4.   Chemical methods:- Application of pesticides and insecticides may be carried out in form of spray, granules or dusting. On a larger scale, spraying of chemicals over fields of crop can be done using spraying equipment from planes and trucks.

 5.   Biological methods:- Biological methods involve deliberate introduction of natural agent such as predators, parasites and disease causing pathogens to reduce or eliminate the pest population. Natural predators are seen in the lady birds which selectively feed on the aphids. Sterile male technique: this method is applied to insects that reproduce sexually. Exposure of male insect to X-ray or gamma rays will make the male sterile therefore no offspring will result during mating. Also chemicals known to have sterilizing effect such as chemo-sterilizer can be used to sterilize male insect. Pathogen and parasites such as bacteria, viruses and fungi can be used to cause diseases on the pest.

	Plant Pest	Diseases cause/transmitted
1.	Aphids	Transmit virus such as groundnut rosette, swollen shoot of cocoa.
2.	Mealy buds	Mealy buds  disease of cassava
3.	Nematodes	Root knot
4.	Yam beetle	Tuber rot of yam

       

Crops and major pest

S/NO	CROPS	PEST
I.	Beans	Beans weevil(Callosobrucis maculatus)
II.	Rice	Rice weevil(Sitophilus oryzae), stem borers, army worm and leaf roller
III.	Maize	Maize weevil (sitophilus zae), stem borers and army worm
IV.	Yam	Yam beetles, nematodes and Rodents
V.	Cocoa	Stem borers, root mealy bug, Aphids, Black tea thrips
VI.	Groundnut	Leaf worms, Aphids, boll worms, snail and grasshopper.
VII.	Cotton	Cotton stainer, and boll worms.
VIII.	Sorghum	Weevil, boll worms,  Aphids and sorghum midge.
IX.	Stored grsin	weevils
X.	Vegetables	Caterpillars
XI.	Cassava	Mealy bugs
XII.	Guinea corn	Stem borers and Moth
XIII.	Coffee, cocoa, Kola, Orange	Leaf mines

DISEASES OF CROPS.

This is the harmful deviation and alteration of crop from the normal functional and physiological process.

Below is the table summarizing the diseases of various plants

Name of crop	Name of disease	Causative organism	Mode of transmission			Effect on crop		control
cassava	Cassava mosaic	Virus	Bemisia fly intros virus as it sucks plant sap.			Pale yellow and greenish patches on leaves. Stunted growth, poor yield.		Plant resistant varieties.
Tomato and Okro	Root knot	Root knot Nematodes	Nematodes attacks plant root			Swollen root cell, stunted growth, poor yield		Crop rotation, use of nematicides, growing resistant varieties.
groundnut	Rosette	Virus	Transmitted by insects(Aphids).			Curled and Molted leaves, yellowish leaves, stunted growth, poor yield		Plant resistant varieties. Early planting, uproot affected plant.
Yam	Yam tuber rot	Nematodes Fungi	Attacked in barns by fungus.			Soft yam tuber, rot.		Use of Aldrin dust during planting. Store in moist free environment.
Onion	Downy mildew	Fungus	Spores are airborne deposited on leaves			Death of leaves from white fungal coating, no formation of bulb.		Spraying with lime and surfur before planting. Or sulphide of potassium
cocoa	Black pod	fungus	Spores spread by rain and insects or animal			Black spores on pod which dry up over time. Causing seed to rot.		Spraying with fungicides, regular clearing of farmlands,destruction of infected plant
Banana	Sigatoka (leaf spot)	fungus.	Spores deposited on leaves while growing.			Yellow or brown spots on leaves. Low yield		Spraying on copper fungicides.
Maize	I.   Maize streak virus II.   Corn smut III.   Maize rust	Virus Fungus(Ustilago maydis) Fungus(Puccinia sorghi)		By sucking and piercing insects Spores Dispersed by wind	·         Streak on leaf running parallel to the vein. ·         Causes low crop yield. Renders crop valueless Formation of brown rust-like spots and stripes on the leaves.		Early planting, spraying with insecticides, destroy affected crop. Treat grain with fungicides before planting.

EXERCISE:

1)      What do you understand by the word pest.

2)      List five types of pest that you know

3)      What are the  methods used in pest control?

4)      List plant diseases that you know.

 

 

SUBJECT TEACHER: MR. ABAH PHILIPPIAN.

Phone no. 08160640879

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