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2. Vegetable nursery

2.1 Nursery

Almost all vegetables crops are propagated by seeds, except a few like sweet potatoes,

asparagus, garlic, potato, which does better if propagated vegetatively. Among the

vegetable crops which are propagated by seeds, most of them like cucurbits, beans, peas,radish, turnip, carrots and leafy vegetables are sown directly in the fields. Other

vegetables like cabbage, cauliflower, tomato, onion, chilli etc. are first sown in nursery-

 beds where seedlings are raised and then transplanted. A nursery could be considered as

a location where plants are cared for during the early stages of growth, providing

optimum conditions for germination and subsequent growth, until they are strong

enough to be planted out in their permanent place.

Production of vegetable transplants/nursery raising

Wide ranges of vegetable crops are raised in nurseries. Some of the commonly nursery

grown vegetables are-

cabbage cauliflower broccoli

 brussels sprouts lettuce chilli

leeks tomatoes brinjal

onion celery

Importance/Advantages of nursery raising in vegetable production

1: It is convenient to look after the tender seedlings.

2: Eliminates the problem of emergence in difficult soils.

3: Weed control easy

4: Can achieve close to 100% plant population

5: Shorter cropping cycle therefore can get earlier planting and harvest

6: Reduced field management costs7: More uniform crop possible

8: Higher yield possible

9: More optimal use of expensive hybrid seeds.

Factors to be considered for raising nursery

1: Location of the nursery

2: Near the farm house

3: Well exposed to the sun but protected against severe heat

4: Well protected against animal damage, strong winds.

Water1: Near the water source

2: Continuous supply of good water

3: The seed bed should be kept moist not continually wet.

Soil

1: Soil should have a large quantity of organic matter

2: Soil texture should be neither too coarse nor too fine

3: Sufficiently porous and is adequately aerated

4: Has a fair degree of water holding capacity

5: Normally rich in all the necessary elements, but can be further improved by the

normal cultivation and with addition of manure

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2.2 Types and characteristics of nurseries

2.2.1 Open field

Beds are raised or basin prepared after the soil is brought to a fine tilth. No structures

are required. Open field nurseries are raised only when the natural conditions are

favorable for growth and development of the plant.

2.2.2 Thatch roof  

Beds are raised or basin prepared after the soil is brought to a fine tilth. Over the

 basins/beds thatch roof is constructed. The thatch roof raises the temperature; protect

the seedlings from frost damage (modified environment).

2.2.3 Green house 

A green house in the real sense is a transparent roofed glass or plastic house.

i) Polytunnel (See skill card on polytunnel) Raised beds are covered with plastic material supported by a small sticks or bamboo

splits to form a tunnel. This structure raises the temperature, protect from frost damage

and conserve moisture. It is more efficient than the thatch roof.

ii) Glass house 

House is constructed out of glass frames fitted with glass or plastic. Such structures are

 provided with adequate ventilation and may even have temperature-controlling facilities.

Seedlings are raised inside the house on raised beds or plots. Most congenial conditions

can be created for the growth and development of seedlings.

2.3 Germination

For a seed to germinate certain conditions must be met. The germination temperature

requirement for specific crops is given in table 2. These are supply of sufficient water,

air, and temperature. If seeds fail to germinate there are many reasons:

1: Temperature is too cold or too hot2: Too dry or too wet

3: Seeds were old so not viable or had been eaten by pests 

4: Disease like pre-emergence damping off. 

5: A hard layer on surface meant emerging shoot was unable to reach surface. 

6: Sown too deeply or too shallow. 

7: Dormancy 

Table 2. Temperature requirement for germination of viable seed

Crops Minimum(oC) Optimum (

oC) Maximum (

oC)

Cabbage 4 29 38

Carrot 4 27 35Brinjal/chilli/tomato 16 29 35

Pea 4 24 29

Radish 4 24 35

Spinach 7 29 29

Onion 7 24 35

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3. Nursery management

3.1 Land preparation 

Land preparation refers to making the soil suitable for sowing or transplanting of crops

in the field. The soil should be fine, moist and firm to provide better germination and

excellent medium for seedling growth. Prior to seeding, the field should be levelled andworked to obtain a fine textured soil free of clods and debris. Enough of well

decomposed farm yard manure should be mixed thoroughly in the soil.

Why to prepare the land? Land preparation is necessary to facilitate the growth of the crops. A proper land

 preparation must:

1.  Get rid of the weeds from the field

2.  Kill the insects by exposing them to the natural hazards and predators

3.  Provide sufficient support to the growing plant

4.  Supply with well balanced nutrients

5. 

Provide with optimum soil moisture

6. 

Facilitate good soil aeration

3.2 What are the operations involved in land preparation 

3.2.1 Irrigation

Irrigation is necessary firstly to facilitate the germination of weed seeds so that they can

 be removed during land preparation. Secondly irrigation facilitates the other field

 preparation operations like ploughing/digging which otherwise is difficult in hard, dried

soils.

3.2.2 Ploughing/digging1.  Ploughing/digging turns the soil from lower layer to the surface exposing the

insects in the soil to the predators and natural hazards.

2.  Ploughing/digging also exposes the weed seeds to the birds and insects.

Simultaneously, it mixes the soils of different layers and ensures that the

nutrients are well distributed in the soil

3.2.3 Manures and fertilizers application (see detail on soil nutrient

management)

Application of manures and fertilizers adds the nutrients that are removed through

harvesting of the crops, through erosion and leaching.

3.2.4 Clod crushing Clod crushing is required to break the soil particles into finer grains which can facilitate

well mixed nutrients, well aerated soil mass and good water holding capacity.

Fine soil particles allow the seeds/roots of the seedling to get good contact with the soil

 particles.

3.2.5 Bed preparation

Bed preparation facilitates the ease in the cultivation operations like sowing and

transplanting and later in the intercultural operations. The width of a bed should not

 be more than 120 cm and the length 150 cm or more. This width facilitates weeding

and watering without trampling the bed. The bed is kept raised about 15 cm high so as

to provide proper drainage of excess water and the level of the bed surface is alsomade slightly raised in the centre with a little slope on the two sides.

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3.3 Sowing: The common practice is to broadcast seeds in the nursery-bed but line

sowing is preferred so as to check proper germination and to facilitate weeding,

hoeing and plant protection operations. The rows are usually about 5 cm apart. Small

seeds are sown mixed with a little sand and covered with soil. The soil covering

should be lighter in heavy soils. General rule for sowing seeds is to sow seeds 2-3times their own thickness deep. If seeds are sown too deep nutrient reserves will be

exhausted before the plant emerges or emerging plants will be weak or liable to die, if

sown too shallow then it is likely to be eaten by birds or washed away by water. Spacing

allows for seed losses. However if you think the seed losses will be higher than normal

then test the seed viability beforehand (see germination test above). A week before

transplanting, the seedlings may be exposed to full sunshine and moisture stress to make

the seedlings sufficiently hardened for field settings.

3.3.1 Transplanting

1.  Some vegetables grow better when transplanted (cabbage, tomato, chilli) and

some does better when directly sown (root crops).2.  While there are some vegetables which can be either direct sown or

transplanted depending on circumstances as such as land availability and

labor. eg. mustard green

3.3.2 Rules for transplanting

1. 

Transplanting should be done as soon as seedlings are about 4 to 8 weeks old,

10 to 15 cm tall and have formed about 3 to 4 true leaves.

2.  The nursery bed should be watered 24 hours before uprooting the seedling for

transplanting so that they may not suffer from desiccation and minimize root

damage.

3.  The seedlings should be dug up not pulled up.

4. 

When the seedlings are uprooted it experiences transplanting shock.

Therefore, it is essential to water plants immediately after transplanting and till

the plant has recovered.

5.  Always transplant under cool conditions so that plants may establish

themselves in the cool weather in the night and may recover from the shock of

transplanting before sunrise.

6.  Avoid seedlings which have grown too tall. Such seedlings become weak and

may start flowering very early.

7.  During transplanting, care should be taken to protect seedlings against wilting

 by frequently sprinkling water on them and covering the roots one with moist

soil or leaves.8.  Setting the seedlings to a depth of first true leaves when transplanting in know

to result in earlier fruiting and larger fruit size in some crops.

3.4 Irrigation

Irrigation refers to artificial application of water. Irrigation is done to supplement the

soil water, to keep plant body cool and to facilitate nutrient up take by the plants.

Vegetables are 80 to 95 percent water.

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3.4.1 All plants need regular supply of water to

1.  Absorb nutrients from soil

2.  Replace losses from transpiration

3.  Maintain turgor pressure to support plant

4.  Photosynthesis

3.4.2 Actual requirement will vary from crop to crop but generally

1.  For germination and at early seedling stage

2.  After transplanting

3.  Under hot conditions

4.  Depends upon the water holding capacity of soil.

5.  Irrigation is most important during critical period of growth. e.g. for peas

during flowering, for cabbage head development.

3.4.3 General rules for irrigation

1.  Always water before water stress symptoms occur

2. 

Water under cooler conditions3.  Avoid watering larger plants under strong sunlight

4.  Apply water on soil surface not on to the plants. This helps to reduce risk of

diseases developing which require water on leaf surface to get inside plants

e.g. blights.

5. 

The amount of water to be applied at a given time will depend on the soil

moisture status, and water holding capacity of the soil. For in-depth

knowledge on irrigation in vegetables refer irrigation chapter in this course

handout.

3.5 Intercultural operations. are those operations undertaken in a standing crop for

various reasons. The following are some the important intercultural operations

commonly undertaken.

3.5.1 Weeding: 

Weeds reduce yield and quality of vegetables through direct competition for light,

water and nutrition as well as by interference in harvest operation. Weeds also

harbour pests and diseases which can infect/infest the vegetables. Weed competition

is very critical and major emphasis of control should be done in the early stages of the

 plant growth. Incorporation of the several of the following management practices into

vegetable production practices increases the effectiveness of controlling weeds.

1: Crop competition2: Crop rotation

3: Mulching

4: Mechanical control

5: Herbicides

3.5.2 Hoeing: Hoeing stands for opening the soil in a standing crops with the help of

a hoe or a pointed stick. Hoeing facilitates in breaking the soil crust (in some soils),

aeration of the soil, movement of water in the soil mass and bringing the needed

nutrients closer to the roots of the crops. However, hoeing should not be too deep

thereby injuring the roots of the crop.

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3.5.3 Top addressing:

Addition / application of fertilizers in the standing crop is termed as top dressing. Top

dressing is done to provide nutrient when needed most and to avoid the loss ofnutrient through leaching.

3.5.4 Earthing up

Piling up of the soil in the standing crop is called earthing up. This operation is

required for tuber and root crops and facilitates the growth and development of roots

and tuber crops.

3.5.5 Mulching is covering of the soil with organic matters like grasses and crop

residues or with artificial materials like plastic sheet. Mulching is done to prevent loss

of soil, loss of moisture, to increase soil temperature, inhibit weed growth, addsorganic matter if the mulching material is organic matter, reduces fruit and reduces

fertilizer loss

3.6 Training and pruning

Like the fruit trees some vegetables also require training and pruning.

Providing support to the trailing types of vegetables is an example of training while

removal of parts which are not useful or diseased is an example of pruning.

3.7 Pests and Diseases (Details given in specific crops)

All vegetables will be attacked by some pests and disease at some stage. The insectthat affects vegetable crops can be grouped into four major groups.

1: Soil insects (example: cut worms, white grubs, red ants, wire worms etc)

2: Stem and foliage feeders (example: caterpillars, loopers, Diamond back moth,

 beetles etc.)

3: Mites and sucking pests (example: Aphids, whiteflies, plant hoppers, thrips, bugs

etc)

4: Insects that consume seeds, pods and fruits (example: pod borer, fruit borer, fruit

flies, etc)

Similarly the diseases which affect vegetables can be grouped as

1: Root diseases (example: Club roots, damping off, root rot, wilts etc)2: Foliage diseases (examples: Blights, rusts, leaf spots, etc)

3: Fruit diseases (examples: fruit rots, anthracnose, fruit blight etc.

4: vascular diseases (examples: wilts)

These diseases could be caused by either fungi, bacteria, viruses and nematodes.

Often unfavorable conditions can also cause diseases, which are normally termed as

disorders. However, disorders are not contagious and can be corrected by changing

the unfavorable condition in question.

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Any practices which produce strong and healthy seedlings will reduce the risk of pest

and disease attack.

1.  Maintain soil fertility and use adequate manures to maintain soil structures to

 provide aeration and supply of micronutrients.

2.  Use only healthy seeds and seedlings of the desired variety.

3. 

Varieties with resistance to specific pest or disease are also available.4.  Rotate your crops to prevent build up of soil-borne problems. This also makes

efficient use of nutrients.

5.  Practice good crop hygiene. Remove crop debris and weeds that acts as an

alternative host for pests and diseases. Destroy diseased plants and collect

insect infested fallen fruits and bury it.

6.  Monitor your crops regularly for early detection of problems. Small numbers of

 pests especially egg masses and larvae can be destroyed by hand

3.8 Pest control methods. The following are some of the pest control tactics which

could be employed to manage a pest on vegetables.

3.8.1 Cultural methods

The term ‘culture’ here refers to ‘cultivation’; and when we modify or manipulate our

cultivation practices, it normally leads to a change in the agro-ecosystem within

which the pest develops. This change affects the lifecycle of the pest and thus pest

 problem reduces. A control method that reduces pest problems by manipulating the

cultivation practice is therefore known as cultural control.

Cultural control has also some disadvantages: it is labour intensive, and its beneficial

effects are not seen immediately like in the case of chemical control. It is for this

reason that with the advent of modern synthetic chemicals, farmers in countries where

labour is expensive abandoned cultural control and chose to use cheap and effective

synthetic chemicals. However, there are now attempts to resurrect some cultural

control measures as part of the pest management programme because of the

environmental concern, health hazard and pesticide resistance associated with the

 prolonged use of pesticides. In the developing countries where labour is relatively

cheap, and chemicals are not readily available, cultural control is still the best pest

control option. By and large Bhutanese farmers – whether knowingly or unknowingly,

depend on cultural control methods.

Operations under the cultural control method include: tillage practices, field

sanitation, crop rotation, adjustments in date of sowing and planting, adjustment ofsowing or planting distances, water and fertiliser management etc.

3.8.1.1 Tilling, or turning of the soil at appropriate times, can destroy soil-inhabiting

insects at different stages, either by crushing, burying them deep, occluding

emergence tunnels, or exposing them to the full radiation of the sun. In addition, it

turns under crop residues that may be the habitat for insects living above the soil.

Tillage also destroys weeds that may serve as alternate host plants. Following tillage,

many insects are exposed on the soil surface for natural predation by other animals

such as the birds and the pigs.

3.8.1.2 Crop sanitation. Farm hygiene often has a good pest control purpose.

Destruction or elimination of crop residues by burning, removal of diseased or insect

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infested fruit and plant parts is a good sanitation practice whereby the residual pest

 population is destroyed; and those surviving remain without shelter for hibernation.

The elimination of weeds around field margins has also the same purpose. Use of

disease and pest-free planting material is also an important prerequisite for clean

cultivation.

In case of high stem borer incidence in paddy it is good to burn the stubbles in the

field. Removal and destruction of fallen fruit is an essential sanitation practice for the

control of fruit flies in citrus.

3.8.1.3 Crop rotation. Planting the same crop in the same area, year after year, often

contributes to the build up of pest population. Rotating crops in the field, or allowing

an area to lie fallow, may act as a deterrent to the increase of pest populations. This is

 because of the food supply being cut off and pests facing starvation. It is most useful

against diseases caused by fungi and nematodes and against insects with limited host

range. Care, however, must to be taken when the pest involved is polyphagous, like

 Helicoverpa armigera  the American Bollworm, as unsuitable choice of crops mightlead to a build up of the same pest. Crop rotation normally reduces and delays attack

rather than giving complete control.

3.8.1.4 Polycropping or mixed cropping. Crop mixtures are a characteristic of many

traditional agricultural systems. Pest attack is theoretically reduced because

'apparency' of the host plant is less, provided that the mixture is chosen to avoid crops

attacked by the same pest. The procedure allows for a greater diversity to the

agroecosystem by increasing the checks and balances of parasite and predator

 populations. Further, many combinations are supposed to provided protection from

attacks by nematodes. Mixed cropping may, on the other hand, present a preferential

food source for a pest.

3.8.1.5 Adjustment of sowing or planting time. Many plants are susceptible to

attack of pests during a limited period in their life and serious attack will result if the

 population or inoculum build-up takes place during that period. Adjustments of

sowing or planting can be done to avoid such critical periods when pest pressure is

high and the plant susceptible. However, to put this concept into practice, the

knowledge of the conditions favouring appearance of pest and vulnerable stage of the

 plants is essential.

Synchronized (same time) planting of crops over a large area is another strategy thatcould prevent build up of pest population. Otherwise pests can move from one field to

another depending upon the availability of the right crop stage. Moreover there is also

the advantage that pests remain distributed over a large area instead of concentrating

and inflicting heavy damage on few fields.

3.8.1.6 Trap cropping  involves planting a suitable crop (attractive to the pest) in a

small area to attract the pest. The trap crop can be grown earlier or along with the

main crop. After it has attracted the pests, it would need a heavy chemical spray -- or

alternatively, it may be ploughed in or destroyed by any other suitable means. By this

method natural enemies are not destroyed on the larger crop desired for harvest. If

grown alongside the main crop,

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3.8.1.7 Fertilization  and water management practices  alter the nutrients and

environmental conditions available in a crop for a pest species. Fertilizer is beneficial

and usually necessary for a health crop growth. A healthy crop is better able to

withstand pest damage. Excessive use of fertilizer also makes the plants susceptible to

 pest and diseases. It encourages the growth of weeds which may act as alternate hosts

for pests. Therefore optimum use of manures and fertilizers is important for pestcontrol.

Flooding is a practice used to control soil insects, while drainage is, for instance, used

for control of Rice Case Worms. Mite population on some commodities can be

suppressed by overhead irrigation.

3.8.1.8 Avoidance. Observations sometimes reveal that certain areas (and fields) are

constantly "at risk" from particular pests, while others are pest free or have a low pest

incidence. In such cases it would be advisable to avoid planting crops in such areas.

This practice is especially effective against soil-borne diseases and nematodes, but

can also be effective against various insects.

3.8.1.9 Planting density For getting maximum yield it is necessary to have as many

 plants as possible per unit area within the permissible limits of space and nutrient

availability. But planting density has a bearing on pest incidence. While dense

 planting is preferred to compensate for the loss during pest damage, it might also

contribute to the creation of suitable micro-climate that favours the development of

fungal diseases. While it has both advantages and disadvantages, it is important to

maintain a balance between the objective of maximizing yield and the consequent

effect on microclimatic conditions favouring pest development.

Let's take a look at an important control tactic which is receiving a great deal of

attention by researchers and holds promise for future control of insects that damage

agricultural products. This involves the manipulation of the genetic material of an

organism.

3.8.2. Biological control There is some confusion about the extent of what constitutes biological control.

Sometimes non-manipulated natural control is included and sometimes also genetic

control (sterilization, plant breeding) and use of pheromones are included. Here we

will explore the biological control tactic in a narrower sense by defining it as "the

manipulation of parasites, predators, or pathogens to manage the density of

insect populations." All pests have natural enemies, so why are not all pests undereffective biological control? One answer to this is that natural enemies, although

active, may not reduce pest populations to the low levels which we require in some

agricultural systems. For instance, we may require that produce be entirely free of

 pest damage, as in some high-value fruits. It is unrealistic to expect natural enemies to

maintain pest populations at such very low levels - natural enemies themselves cannot

reproduce if the pest is too scarce.

The natural habitats where natural enemies have evolved contain a great diversity of

 plant and animal species. By contrast, many modern crops represent monocultures of

only one plant species and this creates, in turn, a low diversity of animal life in the

crop. These large monocultures of similar age provide ideal conditions for the rapid

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development of pest populations, but they may be lacking in several factors which

natural enemies require.

Crops are often interrupted by periods of fallow, which may make it difficult for

natural enemies to persist in the crop habitat. On the other hand, pest populations

decrease over fallow periods too, and in some agricultural systems where continuouscropping is made possible by irrigation, pest problems may become worse, possibly

 because continuous cropping allows pests to build up populations more rapidly than

they can be controlled by natural enemies.

Practices such as cultivation and burning of croplands are also known to decrease

numbers of natural enemies, but may affect pests as well. For cropping practices such

as these, it is difficult to generalize whether effects on natural enemies will be more

severe than those on pests - each case must be carefully examined separately.

For other cropping practices, a negative effect on natural enemies is easier to predict.

This is true for the use of insecticides. In general, insecticides are more toxic to insectnatural enemies than to the insect pests against which they are applied, and the many

examples of pest resurgence, where the elimination of natural enemies by insecticides

leads to greater pest problems after spraying, shows how severely this cropping

 practice can limit biological control.

From the above example, it is not surprising that natural control in crops is often

ineffective. To rectify this situation, and improve the biological control component of

integrated pest management, there are three general methods which can be applied.

These are: conservation-enhancement or augmentation-colonization and importation.

3.8.2.1 Conservation  refers to any practice that avoids the destruction of natural

enemies and enhances the population density of biological control agents.

Modification of cultural practices may conserve and enhance natural biological

control agents. Examples are, for instance, 1) Strip cropping and 2) Banding of fruit

tree trunks so that ants cannot protect aphids and mealybugs against predators and

 parasites. Modifying pesticide use to cause less mortality to important natural enemies

is another example.

3.8.2.2 Augmentation. Often natural enemies of pests can be increased by releasing

additional biological agents which have been reared and colonized in laboratories.

Such a manipulation of a controlling agent is called augmentation or colonization.Augmentation may take the form of innovative releases that depend on natural

increases of the agent for control in later generation or inundative releases designed to

 provide immediate control of the target pest. An example of augmentation is the

release of laboratory reared populations early in the crop season, before field

 populations of the same natural enemies appear, in order to establish the natural

enemy on the pest population before it grows too large to control. Because the

 production of natural enemies can be expensive, this approach should be pursued only

if conservation and introduction are not effective or feasible. On the other hand,

inundative release of pathogen on predatory mites is becoming a common method of

control in developed countries as these natural enemies are often inexpensive to

 produce.

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3.8.2.3 Importation  is a third biological control method. Importation refers to the

collection of a biological agent from one geographical area and importing it to another

geographical area where it will be used as a control agent. Once successfully

colonized stock is collected in the field, it is distributed throughout the area infested by the host. Importation is employed against pest species that have been accidentally

imported into an area and have few or no natural enemies in their new environment.

This form of introduction is often called classical biological control.

 

There have been many instances of complete or partial success in control of pests

through introduction of natural enemies. The classic illustration of success was the

importation of the Vedalia Lady Beetle to control the Cottony Cushion Scale. The

Cottony Cushion Scale was accidentally introduced into California from Australia. In

the absence of natural enemies it quickly spread throughout the California citrus

industry. Investigations found that Cottony Cushion Scale in Australia was being

controlled by a small lady beetle called the Vedalia beetle. One-hundred-twenty-nine beetles were shipped from Australia to California.....and in six months 10,000 beetles

were released in the state through collection and release of beetles from the initial

release sites. The spectacular result was that within two years the scale outbreak was

completely controlled by the Vedalia beetle. The beetle has subsequently been

shipped from California to Florida, and to many foreign countries, where it has

reduced Cottony Cushion Scale to non-pest status.

Biological control not only involves matching insect against insect for control

 purposes, but uses other organisms as well. The Japanese beetle was accidentally

imported into the northeastern United States. This pest has been found to damage over

250 species of plants. Bacteria associated with the beetle were found to produce a

disease called milky disease. These bacteria were mass reared and the spores released

to substantially control the beetle populations.

Another bacterium, Bacillus thuringiensis, which is abbreviated "BT" produces spores

and crystals. When ingested, the crystals cause paralysis and rupture of the gut in

larvae of Lepidoptera, Coleoptera and Diptera. After the crystals cause gut rupture,

the spores invade the insects' tissue and reproduce. The spores and crystals are

 prepared commercially under various brand names like Bactospeine and are used

much as a synthetic chemical insecticide.

There are several other biological control agents which have been used against insect

 pests and are undergoing additional research. Fungus diseases of arthropods are in the

 process of experimentation to control rust mites and insect pests. Tiny viruses are also

 being studied in an effort to biologically control mosquitoes, aphids, sawflies,

immature Lepidoptera and rhinoceros beetles.

Any study of biological control in entomology would be remiss if it did not mention

the use of insects to control weeds. Often weed plants are accidentally imported into

an area where there are no natural enemies. The classic example of weed control

concerns the use of a moth against the prickly pear cactus in Australia. In the mid

1800's a single cactus was imported into Australia. Fifty years later it had spread over10 million acres. Six years later it had spread over 23 million acres and was

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continuing to spread at the rate of 1 million acres a year. The importation and rearing

of a moth from Argentina, Cactoblastis cactorum, solved the dilemma. The moth

larvae tunneled the cacti, causing them to dry up and desiccate. There are additional

classic stories of weed control by insects, but it will suffice to say that to date, more

than 25 serious weed pests have been controlled by insects in some way.

3.8.3. Chemical control 

The control of pests with the use of chemicals constitutes the chemical control.

Depending upon the target pest against which the chemical is used, they are classified

as detailed below.

Classification of pesticides

By definition, pesticide is a pest-killing agent (the Latin word cida means to cut or

kill). It includes those products used to control insect pests, rodents, plant diseases,

nematodes and weeds. Also included under pesticides are compounds used as

repellents, attractants, ant-feedants etc. In legal terminology pesticide may be defined

as “any substance used for controlling, preventing, destroying, repelling, or mitigatingany pest”.

Pesticides can be classified based on the type of pest controlled

Class or Type Target pest controlled

Insecticide insects

Fungicide fungi

 Nematicide nematodes

Rodenticide rodents (rat, mice, porcupine etc)

Herbicide/weedicides weeds

Acaricide mites, ticks

Bactericide bacteria

Viricide viruses

Miticide mites

Classification of insecticides by mode of action - the ways they enter or affect the

pests a: Contact poisons 

As the name implies, this type of insecticide kills by contact with the insect, and must

 be applied in such a way and at such a time as to ensure a direct contact with the

insecticide, either by being sprayed or by walking on the sprayed plant. Caterpillar

 pests would be best controlled by such chemicals because their body is not protected by scales as in adult beetles and are therefore more vulnerable to contact poisons.

Examples of insecticides with contact action: Malathion, Cypermethrin (also a

stomach poison), Dimethoate (also systemic), chlorpyrifos (also stomach poison and

has vapour action)

 b: Systemic insecticides

Systemic insecticides are highly water soluble and as a result they are easily absorbed

 by growing plants through the roots, stem or leaves. Once absorbed into the plant,

they are then translocated to untreated parts through the vascular system thereby

rendering the whole plant poisonous. Systemic insecticides are particularly effective

against sucking, boring and mining insects. E.g. Dimethoate.

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c: Stomach poisons

Stomach poisons generally enter a pest’s body through the mouth during feeding

(ingestion) and are absorbed through the digestive tract.

d: Fumigant insecticide 

The fumes or vapour given off by the insecticide usually gain entrance via the

 breathing mechanism of the insect, and frequently affect the nervous system.

Fumigants are not usually used on growing crops, but are used for soil treatments, and

in grain stores.

E: Suffocating materials

These materials are usually oils that clog the respiratory mechanism of pests, eg. Tree

Spray Oil (TSO) used to control scale insects.

Negative effects of pesticide use A variety of pesticides  such as herbicides, nematicides, insecticides and acaricides,

have been, and continuously are being developed for control of pests.

Several major problems may arise from total use of pesticides. First, there

may be outbreaks of secondary pests. Sometimes, chemicals used to kill a pest may

result in the outbreak of another organism which previously was not considered a

major pest. For example, an insecticide not only may kill a plant-feeding insect, but

also its natural enemies. If these natural enemies also attack a second plant-feeding

insect, this insect may increase in numbers and eventually become a major pest.

A second problem associated with use of pesticides is that total reliance on pesticidescan selectively kill the susceptible pests, and leave those with genotypes conferring

resistance. Generally, higher rates of pesticides and frequent applications lead to

more rapid selection of resistance pests.

A third problem which may arise from total use of pesticides is resurgence.

There is evidence that populations respond to severe mortality by rapidly increasing

their birth and survival rates. This compensates for the mortality. Increased birth and

survival may occur in pest populations that have been subjected to severe pesticide

stress.

And finally, pesticides cause concern in that they cause health hazards and

cause environmental pollution.

3.8.4. Regulatory and Legislative control 

One method of crop protection is to prevent the entry of pests into an area in which

host plants are growing. This method of exclusion of pests is enforced through certain

legal measures commonly known as quarantine. Knowledge and methodology of

exclusion are utilised and practised by a legally constituted authority to:

•   prevent the introduction of foreign pest,

•   prevent or retard spread within a country,

•  enforce control and eradicative measures,

•  enforce certification of the materials

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When restrictions apply for the movement of pests between nations it is called foreign

 plant quarantine and when such restrictions exist within the country it is called

domestic quarantine. Quarantine Regulations or Acts are of comparatively recent

origin. The adoption of such steps have arisen out of the fact that extensive damages,

often sudden in nature, have been caused not by indigenous pests, but exotic ones

which have been introduced along with plant materials in the normal channel of tradeor individual interest. Examples of pest and disease spread in this manner in India are:

San Jose Scale (Aspidiotus perniciosus) of apple, potato tuber moth.

One basic category in this form of control is formed by the quarantine laws.

The establishment of exotic pests can be avoided by enforcing quarantine control of

introduced germplasm. This has enabled some developing countries to avoid serious

 pest problems; for example, Malaysia maintains strict control of imported planting

material, especially from South America, chiefly to protect the rubber industry from

South America leaf blight,  Microcylus ulei. Similarly, Mauritius restricts imports of

sugarcane to protect the sugar industry from entry of the Fiji disease virus. However,

quarantine measures cannot guarantee to exclude pests permanently and so the

Malaysian Rubber Research Institute has an active research programme to developresistant cultivars so that if the disease does arrive its impact will be minimized.

Quarantine measures can be expected only to delay and not prevent the arrival

of new pest problems. However, they are valuable in providing time to develop

measures to prevent a major catastrophe when the pest does gain entry in instances

such as rubber leaf blight or Fiji disease, where the threat is known. Unfortunately,

many problems cannot be predicted.

3.8.5. Genetic control

Genetic control refers to the control of pests by manipulating the genetic make-up of

either the pest or the host. An example of manipulation of genetic material in the pest

is such as the one where an insect is subjected to radiation, chemicals and other

 procedures. This process is often called autocidal control. With regard to the

manipulation of the genetic material of the host plant, it involves the conferring

resistance to the plant by breeding. Such a tactic is referred to as host plant resistance.

3.8.5.1 Autocidal control 

Radiation or chemosterilants can be used to sterilize insects which, if released in

sufficient numbers, can reduce reproductive success and lower or even eliminate pest

 problems. Lethal genes may be used in the same way.

This concept was pioneered in the USA in the campaign to eliminate a cattle

 pest, the screw worm fly, Cochliomyia hominivorax.  Male flies were sterilized byexposing puparia to gamma-rays and flooding the population with sterile males. Since

the screw worm mates only once, females mating with sterile males produce no

offspring. This is an expensive operation, requiring massive factory scale production

of sterile insects to be effective. Screw worm was successfully eradicated from the

USA and now efforts are being made to eradicate it from Mexico. This same method

will be used in Africa, following the accidental introduction of screw worm there in

1988. The screw-worm project is often referred to as the sterile insect technique

 because it employed a technique of sterilizing flies by radiating them.

A similar programme in Mexico is preventing spread of the Mediterranean

fruitfly, Ceratitis capitata, northwards from Central America. In Africa, considerable

research has been done on autocidal methods for control of tsetse, Glossina spp., butno practical programme has yet resulted from this.

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  The high cost of these campaigns makes them viable only under special

circumstances and where the target pest is amenable, as with flies which mate only

once. As a means for continual application for widespread pests it is too costly and

not practicable.

3.8.5.2 Plant breedingThe second type of genetic manipulation involves the host plants of insects and is

called host plant resistance. In this control tactic the genetic make-up of a plant is

altered by selective breeding to make it less susceptible to pest damage.

The growing of crop varieties which are less vulnerable to pest damage or

which give satisfactory yield in spite of pest attack has many advantages. Once such

varieties are available, pest control requires no extra labour and is therefore

economical; moreover, the environment does not suffer from side effects of the

chemical control.

Plant breeding is a long-term strategy which takes years to develop new

varieties, bulk them and make them widely available to farmers. Thus, it does not

 provide quick solutions to pest problems and may be effective only for a short time.Further, it depends upon the existence of genes which can be bred into crop varieties

and in some instances sources of resistance have not been found or are incompatible

with producing an acceptable product.

3.8.5.3 The nature of plant resistance

There are three basic modes of plant resistance: They are antixenosis or non-

 preference, antibiosis and tolerance.

a: Antixenosis  is associated with insect behavior. "Anti-" means against while

"xenosis" refers to a guest. Therefore, antixenosis means against a guest or 'to keep

away a guest.' In this case the guest is the unwelcome insect pest. The insect pests are

kept away by breeding plants that possess characters which are unattractive to insects.

Examples of such characters are: colour, palatability, hairiness, waxiness, gummosis

etc.

b: Antibiosis  literally means "against life." Here, the host plant manufactures a

chemical that interferes with the normal physiological functions of the pest. The

chemical is ingested by feeding and the effects may be: death of immatures,,

abnormal growth, failure in metamorphosis, reduced fertility of adults, or abnormal

reproductive behaviour. Examples of plant characters that confer the antibiosis mode

of resistance are: toxins and deterrants, hairness and waxiness etc.

c: Tolerance refers to the ability of a host plant to repair damage incurred by the pest

or to increase growth and produce a crop in spite of pest attack. Soya beans are one

example where pest feeding produces a thinning that is compensated for by additional

 plant growth. Many traditional, long-term rice varieties have a high tillering capacity

that enables them to compensate for stemborer damage. The mechanisms of resistance

shown are: compensation,

A plant may possess more than one of these modes of resistance and they may

 be interrelated in various degrees to give a plant resistance capabilities. In addition,

the expression of these three methods of resistance are dependent on environmental

factors such as light, temperature, humidity, soil, and pest population density andcompetition.

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3.8.6. Mechanical and physical control 

Of all the control tactics, mechanical and physical measures are some of the most

ancient. However, they are usually expensive in time and energy and are rather

inefficient.

3.8.6 Hand destruction, as the name implies, it is simply to destroy an insect by

hand. It includes picking larval forms or egg masses from a plant and destroying them

 by some physical means. In Bhutan hand-picking of Papilio larvae from citrus

seedlings forms an example for this type of control. Other examples are : swatting of

clustered nymphs of citrus shield bug (Rhynchocoris) on citrus trees and poking for

trunkborers with a springy wire (e.g. bycycle spoke) up the borer- holes in apple and

citrus trees.

3.8.6.1 Mechanical exclusion  involves excluding insects from an area by some

mechanical device. Good examples of mechanical exclusion devices are: windowscreens which exclude flies and mosquitoes and placing collars at the bases of young

trees and plants to prevent insect attack. Banding of fruit trees can be effective against

ants which protect and encourage aphids and mealy bugs. In that way predators get a

 better chance to control these pests. Bagging of fruits like jackfruits to deter fruit flies

from oviposition is another example of mechanical exclusion.

3.8.6.2 Mechanical traps  or collecting devices are usually employed for insect

collecting or monitoring, rather than control measures, and depend on additional

means to attract insects such as lights or chemicals. In some localized instances they

may serve to suppress insect populations. Use of pheromone trapping in isolated

citrus orchards can be a good way of exterminating fruit fly populations. Flying

 beetles like chafers can be collected in large numbers with light traps, but a sounds

knowledge of flying time is essential.

3.8.6.3 Temperature extreme is a physical factor that may be manipulated in small

areas to manage insect populations. Excessive heat or excessive cold for long periods

will eliminate some insects, especially when they are in vulnerable stages. This

method is applicable to stored product pests. Drying of grain reduces moisture content

and results in lower infestation rates.