Weed control methods in crop production.
Chemical weed control
Integrated weed control
Insect control methods in crop production.
Preventive control of plant diseases.
People have been combating weeds, insects, and plant
diseases throughout history (#1). Pests of the plants
man use for food are abundant. Pests (insects and weeds),
and diseases, developed "hand in hand" with the plants
they attacked (#2-3). Pests are plants, animals or viruses
that are detrimental to humans and crops. The main categories
of pests considered in this lessons are weeds, insects,
diseases, nematodes, and vertebrates.
Cultivation of one or a few crops species on the same
land for several years or decades shifts the population
balance of soil microorganisms, plant pathogens, insect
pests, and weeds to those strains favored by the crops
grown. Most crops are subject to attack from a large
number of pests (insects, weeds) and diseases (#4).
These are considered to be the major limiting factor
in crop production. The susceptibility of cultivars
to attack from specific pests or diseases varies greatly.
Development of resistant lines would seem to offer the
best prospects of crop protection, particularly if this
is combined with the use of healthy, clean seed, efficient
weed control and crop rotation.
It is very difficult to make reliable estimates of
losses due to pests. There is no question, however,
that these losses are of substantial and often staggering
proportions (#5). A conservative estimate suggested
that plant pests destroyed one third of mankind's supply
of food and fiber every year. Losses of much greater
magnitude occur as a matter of course in many less developed
It is only in recent times that man's response to pest
control has been based on any appreciable understanding
of the nature and causes of pest problems. Methods of
pest control change and become increasingly effective
as we gain greater understanding of pests and their
habits. Although there remains much that we do not know,
we can formulate pest control programs on a rational
basis. A control program should be based on an understanding
of the biology and habits of the pest, a consideration
of all effective methods of control, and recognition
of the level of control that is both desirable and possible.
Some of the preventive measures and control methods
for pests and diseases are discussed in this lesson.
Concept # 1:
Weed control methods in crop production:
A weed can be any plant that grows where it is not
wanted (#6). Weeds compete with crops for light, space,
water, and available nutrients, which result in lower
crop yields (#7). Weeds cause losses by reducing plant
yield and quality, decreasing harvest efficiency, and
by harboring insects and diseases. The adverse effects
due to weeds are pronounced more under marginal conditions
than when dealing with fertile soils and abundant rainfall.
Control is necessary at all times of the crop (#8).
Crops are most sensitive to weed competition in their
early stages of growth. Competition during the first
quarter of the growing period does irreparable damage
to the crop, and often results in total crop failure.
Although weeds cause considerable less damage during
the later development stages of the crop, weed control
is still necessary to ensure both the quality of the
harvest and for host plants that harbor disease and
pest growth. Unlike other agricultural pests, weeds
do not attack our crops directly, rather they compete
with them and serve as pest hosts.
Weed control should be the main activity as far as
crop maintenance is concern. Most of the crops in the
production unit call for very good care in this respect
(#9). As a rule of thumb, the quicker the canopy is
closed, the less weeding will be required. Thus, the
reason for row planting vs. hill planting. Weed growth
is also checked by crop rotation, which in addition
reduces the incidence of diseases and pests.
As with all pests, the weed species must be accurately
identified for successful management.
There are various ways to control weeds: preventive
measures, hand weeding, mechanical weeding, chemical
weed control, and integrated control.
Preventive measures. Preventive measures are meant
to control the further spreading of weeds. They include
the use of early cultivation, uncontaminated seed, the
elimination of weeds before seeding is started, keeping
irrigation canals clean, proper composting of manure,
and prevention of soil and water runoff. The producer
must create and maintain ideal conditions for the crop
to develop fast and compete with the weeds for nutrients,
light, and space. Once the crop is established and the
canopy formed, it is very difficult for weeds to develop
and compete with the crop. Unfortunately, many weed
researchers are trained in industrial countries where
labor costs exceed the costs in developing countries.
As a result they are firm believers in the application
of weed killers, well above trying to improve the effectiveness
of hand weeding.
A common stumbling block is the late start of cultivation,
after the first heavy rains with no time for pre-planting
weeding (#10). Such late start results in delayed and
therefore hurried planting, leading to wide plant spacing
and many gaps. Thus, weeding has to be postponed until
planting has been completed, leading to a pile-up of
weeds and work while the weeds compete fiercely with
the crop seedlings. A combination of simple measures
can help prevent this situation:
Deep tillage by plow, cultivator, or hand should
start right after harvest while the soil is not
yet too hard and man and beast are in good condition
(#11). This practice stops weeds from seeding or
A first blanket weeding (or harrowing) soon after
the first rains. If possible a second such weeding
should continue after two weeks (#12).
Planting can start 4-8 weeks earlier, leading to
a close full stand that later on completely smoothers
any late weeds.
The first weeding between crop rows should start
a week after planting when planted rows have just
emerged and the visible weed seedlings, are easiest
to kill (this practice is possible after line or
Mechanical weeding: Weed control by
frequent plowing is a common practice in areas where
the availability of draught animals and time are not
limiting factors. It is also possible to perform mechanical
weeding while the crop is growing, providing it is not
to high. Planting crops in rows is advantageous for
this purpose. When mechanical weeding is performed to
avoid damage to the roots of the crop, it is essential
that the implements (cultivators, ridges) only work
the upper layer of the soil. This especially applies
to early weeding, when the weeds are still small.
Chemical weed control: The use of
herbicides has become the most common method of weed
control in agriculture (#13). Chemical weed control
is economically feasible when labor costs for hand weeding
rise. Also, where there are peaks in labor requirements
or constraints on mechanical weeding. However, rapid
adoption of chemical weed control by farmers in developing
countries is not likely to occur. Often there are problems
concerning crop damage or the failure of the herbicide
due to incorrect application (dosage, time, weather
conditions) to be contended with. Other constraints
to consider are the high cost of chemicals and their
toxicity which can be harmful to animals and/or human
Experience and extreme care are important in chemical
weed control. Thorough cleaning of both people and spraying
equipment after use is imperative to avoid disastrous
effects on other crops and on people.
Very often the claim for needed herbicides is a symptom
of other things being wrong, especially time, method
and depth of soil cultivation, inefficient (usually
late) hand weeding, and uneven plant population.
We need to emphasize that a weed control program should
be based on a thorough knowledge of weed problems and
that chemical control measures should be applied only
when they are truly necessary. If needed, we should
employ the method or combination of methods which will
give the most effective, practical, economical, and
environmentally sound weed control.
Integrated weed control: Weed control programs have
to be adjusted to local circumstances. There are no
definite rules. The most efficient method of control
combines a number of measures, with each contributing
its share to an ultimate goal: the control of weeds.
It is virtually impossible to eradicate weeds; there
will always be re-infestation from surrounding areas.
However, integrated weed control attempts to use all
available tactics or strategies to manage weeds so that
an acceptable yield and quality can be achieved economically
with the least disruption to the environment.
Apart from the preventive and direct control measures,
especially early dry season plowing or hoeing, a number
of activities must be incorporated in an integrated
weed control program for it to be effective. Such activities
Crop rotation: prolonged growing of the same crop
will favor the development of particular weed communities
Narrow rows or interplanting: Planting crops in
narrow row to close the canopy faster and to provide
shadow to suppress the growth of weeds (#15).
Placement of fertilizer: Applying fertilizer to
crop plants reduces the chance that weeds can profit
from their nutrients (#16).
Prolonged weeding operations: prevent the further
spreading of weeds by removing them before they
produce flowers and seed (#17).
Concept # 2:
Insect control methods in crop production.
Harmful insects are insects which cause damage to a
crop during some period of its life cycle, resulting
in a noticeable reduction in yield and/or crop quality
(#18). Since insect populations usually increase rapidly,
early control is needed to prevent excessive damage.
Recognition of insect problems requires both the ability
to identify the pest and to recognize the signs and
symptoms of damage or injury. Insects can cause crop
damage in a variety of ways, according to their eating
1. Chewing and boring Insects These
type of insects damage the crop by chewing on leaves,
fruits, seeds, and roots (#19). Chewing insects actually
eat all portions of various plant parts and their damage
is sometimes relatively easy to identify, e.g. notched
or ragged leaves, holes in fruits and seeds, or damage
to stems, leaves, and roots.
2. Sucking Insects These type of insects
damage the crop by sucking plant juices from leaves,
stems, roots, fruits and flowers (#19). These insects
can also transmit plant diseases and viruses to the
crop. Their sugary honey dew excretions make affected
parts sticky and susceptible of sourly fungus growth.
All sucking pests compete for assimilates and cause
early wilting and shedding of leaves and buds.
3. Others Other insects may cause
scaring and galls on various plant parts which results
from egg-laying and larvae activity (#20).
It is important to note that symptoms such as yellowing
of leaves, wilted or dead plants, and weakened root
systems may result from cultural practices or from any
number of pest problems. Where one or more of these
symptoms are present and you are unsure of the cause,
you should dig up the plant and examine the roots for
clues on the problem. If you are unsure of the cause
of a particular or the identity of a specific insect,
ask for qualified help.
General approaches to insect control include:
Cultural Control: Some insect pests
of field and vegetable crops can be controlled by cultural
or good management practices. These cultural practices
are directed at "weak points" in the insects life cycle
and are generally something the farmer does anyway,
such as plowing, disking, date of planting, etc. Advantage
is taken of the insect's relationship to its host plant.
Crop rotation: There are several requirements
that must be fulfilled for effective crop rotation (#21).
The first of these is that the alternate crop must be
an unacceptable plant to the insect pest. Otherwise,
you have not met the basic requirement of removing the
food source. The insect must also have limited dispersal
activity and a long life cycle. There is no advantage
to crop rotation if a pest species is capable of moving
at will from crop to crop or can complete its development
before the crop can be rotated.
Tillage: Early cultivation after harvest
to incorporate remaining crop residues into the soil
also affects insects by exposing spores, and adult insects
or pupae to weathering and their natural enemies or
by turning under the litter which serves as their habitat
(#22). Other practices which may help control insects
could be to keep a strict crop hygiene that includes
burning or hot composting of affected material and removing
crop residues, stalks, and stumps after harvest.
Plant Population and Date of Planting:
Plant a fairly dense stand to avoid weeds that could
act as host plants for pests and diseases (#23). Plant
at the right date of planting to avoid too much water
or too dry conditions which may increase insect or disease
Balanced soil fertility program. Maintain a good fertility
program to have healthy plants which can compete effectively
with the insects for survival (#24).
Chemical Control: Despite the controversy,
concern, and criticism relating to chemical control,
this method continues to be the most reliable. Alone
or in combination with other techniques, chemical insecticides
will continue to be the major method of controlling
most insects in the foreseeable future. Problems have
arisen because we have relied on chemicals too heavily
in the past years. The types and judiciousness of uses,
and the integration of these uses with other available
techniques to reduce our dependence on pesticides must
always be considered.
Concept # 3:
Preventive control of plant diseases.
Plant diseases can reduce the quantity and quality
of food, fiber, and ornamental crops from the time of
planting through harvest, sale and usage (#25). A reduction
in crop growth may be the result of an insufficient
supply of plant minerals or the activity of toxins produced
by bacteria, fungi, or viruses. The internal nature
of most diseases causes considerable damage before the
symptoms become noticeable, which makes control very
difficult and hardly economic.
Managing plant diseases is a complex problem. Curing
plant diseases is nearly impossible, so management measures
focus on preventing diseases from occurring or limiting
Fungi are the most common plant pest. Fungi lack chlorophyll
and, therefore, cannot manufacture their own food through
photosynthesis. Fungal diseases cause a variety of symptoms.
Any part of a plant's roots, stems, leaves, flowers,
fruits, or seeds may be infected. Fungi also attack
harvested products, such as grain, bulbs, and wood,
while they are in transit or in storage. Fungi can be
spread from healthy to diseased plants by wind, rain,
or irrigation water, soil, machinery, humans, and animals.
Some fungi can penetrate healthy tissues directly. In
other instances they enter through wounds.
Bacteria are perhaps most familiar to us as causal
agents of a number of important human and animal diseases,
e.g. tuberculosis, pneumonia, typhoid fever, brucellosis
and anthrax. However, some bacteria are destructive
Bacteria are microscopic organisms that quickly increase
in number, especially in wet, humid weather. The life
cycle of bacteria may be as short as 20 minutes and
its population may increase tremendously in such a short
period of time. For example, if a bacterium divides
every 30 minutes, a single bacterial cell could produce
281,474,956,710,656 offspring in 24 hours. Their importance
as plant pathogens lies primarily in their astonishing
reproductive capacity. Bacteria live and reproduce in
the spaces between plant cells and/or in the vascular
system (conducting tubes for water and nutrients).
Like fungi, most bacteria cannot manufacture their
own food; they must obtain it either from dead or decaying
organic matter or from living tissue. Bacteria are most
important in post harvest decays and spoilage of fresh
fruits and vegetables while en route to or at the market,
or at home in the refrigerator. Bacteria enter through
natural openings and plant wounds. Foliar diseases caused
by bacteria are often spread by wind and driven or splashing
rain. Insects also introduce certain bacteria as they
feed from the crops.
Like bacteria, viruses are probably most familiar to
us as causal agents of human and animal diseases, e.g.,
polio, rabies, smallpox, and warts. They are, however,
responsible for some of the most destructive plant diseases.
Viral plant diseases include mosaics of tobacco and
vegetables, curly top of sugar beets, and yellow dwarf
of barley among other grain crops.
Viruses are smaller than bacteria and reproduce only
when associated with living tissues. Viruses cause a
variety of diseases and symptoms that most frequently
cause reduced yields and poor quality products rather
than killing the host. Nearly all viruses can survive
only in living cells; their spread from diseased to
healthy plants, therefore, depends on some means of
direct movement from plant to plant. Most viruses are
transmitted by insects, particularly aphids, leafhoppers,
and mites. Viruses are often serious problems in plants
that are propagated by vegetative means (e.g. tubers,
bulbs, roots, cuttings) because the virus is easily
carried along in the propagating material.
Because of the close association of viruses with the
living cells of their hosts, no chemicals have yet been
discovered which are capable of controlling the virus
without doing irreparable damage to the host. For the
present at least, viruses must be controlled by means
other than the use of pesticides. In this case as with
other pests, we can not completely get rid of plant
diseases. Rather, must simply learn to live with them;
the most we can hope for is a reduction in damage to
an acceptable level.
General approaches to plant disease control
Preventive control: Many diseases
are soil or seed-borne. They may be aggravated by a
deficiency (K, Zn, B) or excess of minerals (especially
N), or lack of drainage. Prevention of diseases or a
reduction in disease incidence is usually concerned
with plant populations rather than individual plants.
We cannot view plant disease as we do human or animal
diseases, where the welfare of the individual may be
of paramount importance. We must instead consider the
entire crop and attempt to minimize disease on that
level. At the same time, we must be aware of the fact
that a single plant may serve as a source of infection
for an entire field. Therefore, the main preventive
measures should be crop rotation, crop hygiene, a balanced
plant mineral supply, proper drainage, seed disinfection,
and the use of resistant varieties (#26-27). Copper
compounds are excellent, cheap spore killers and therefore
Cultural and crop management practices (#28):
Crop rotation involves the growing of the same crops
on the same area once every 2-6 years. Disease severity
and economic loss from plant pathogens generally will
increase for most crops if grown continuously. In order
to be successful stubble, straw, stalks, and volunteer
seedings should be removed first. Some plant pathogens
have wide host ranges. A rotation scheme involving crops
which are susceptible to the same diseases should be
avoided. Although three different crops may be planted
in successive years, if all are susceptible to a common
pathogen, severe disease may occur on the third crop
if not all. A general practice is to avoid planting
a legume after a legume or a grass after a grass. Rotation
is particularly valuable in controlling a number of
root diseases, any of which cannot be economically controlled
by other methods. Weed control is also important because
this vegetation may serve as a source of inoculum for
susceptible crop species. Elimination of unwanted vegetation
is an important element in the control of several plant
Proper planting time and methods:
Susceptible crops should not be planted in fields that
would favor disease development unless the grower is
prepared to utilize good control strategies. Seed rot
and seedling diseases are favored by wet and cool soils.
Fields that have a history of prolonged water-logged
soils may be the site of severe root-rot problems. Planting
should be delayed until the soil moisture and temperature
are favorable for seedling development (#29). Optimum
soil moisture and temperature will vary with the particular
crop species. Improper planting depth can lead to poor
stand due to seedling diseases.
Proper Plant Nutrition: Total plant
health begins with proper plant nutrition. Many plant
diseases are less severe on healthy plants (#30). Thus,
to ensure healthy plants we need to maintain a proper
balance of soil nutrients.
Chemical control: The use of chemicals
can be an effective means of plant disease control and,
in some instances, may be the only practical alternative
available (#31). Chemicals are most effective against
fungal pathogens, less effective against bacteria, and
unavailable for control of viruses. Most of the pesticides
and fungicides used in agriculture are toxic to human
beings as well as to livestock and poultry. For this
reason, the instructions which go with the chemicals
should be read very carefully.
List of figures for lesson 1.6
(Click on the numbered links below to view and print
1. Drawing of farmer spraying weeds in crop.
2. Drawing of insects and corn crop.
3. Drawing of weeds.
4. Drawing of diseased plant.
5. Drawing of money $ $ $ lost due to pests.
6. Drawing of crop shading weeds.
7. Drawing of weed competition with crop.
8. Drawing of weed control.
9. Drawing of well cared for crop.
10. Drawing of early planting.
11. Drawing of deep plowing after harvest.
12. Drawing of farmers weeding in the field
13. Drawing of a farmer spraying.
14. Drawing of crop rotation.
15. Drawing of field with different row distances.
16. Drawing of farmer applying fertilizer
to the plant.
17. Drawing of field full of weeds.
18. Drawing of insects and output reduction.
19. Drawing of insects chewing a plant.
20. Drawing of plant with galls, scars, or
21. Drawing of crop rotation.
22. Drawing of farmer tilling the field.
23. Drawing of a field densely planted.
24. Drawing of plants with different fertility
25. Drawing of diseased plant.
26. Drawing of crop rotation.
27. Drawing of good field drainage.
28. Drawing of cultural control.
29. Drawing of a planting calendar.
30. Drawing of a healthy plant.
31. Drawing of a farmer spraying.
Lesson 1.6 Notes
Process for selecting pest control method:
A positive identification of a pest must certainly
be established before any consideration of a pest control
program is either needed or possible. Control of a particular
pest should be considered only when it is believed that
economic damage will occur. Economic damage is simply
the amount of injury which will justify the cost of
applied control measures. An orderly decision-making
process must be followed in order to intelligently and
effectively plan and carry out a pest control program.
The principal elements of that process are outlined
It is very important to detect pest infestation
before they become a problem. Failure to do so will
often result in increased cost of control, less
effective or ineffective management measures, and
significant damage to the crop or site. Proper detection
requires frequent and careful checking of fields
or other sites, a knowledge of the common pests
and predators, an ability to recognize potential
problems, and a thorough knowledge of the crop or
other plant growth characteristics. You must be
able to recognize "abnormal" plants and pest damage.
Positive identification of a pest is essential
in order to determine whether it is harmful to the
crop and, if so, to establish an adequate control
program. In case of plant diseases, identification
can sometimes be based on symptoms rather than actual
identification of the disease agent. In case of
pests knowing their life cycle is essential; because,
in insect management an insecticide should be applied
to coincide with the presence of a susceptible life
stage. In some cases the susceptible stage is only
the adult or larval stage. In most cases chemicals
do not affect the eggs. Weather conditions can also
be monitored to predict how long it will take a
certain insect to develop.
Economic Significance: Control of a particular pest
should be considered only when it is believed that economic
damage will occur. Pest species in low numbers may cause
little injury to a plant's final yield or quality. Greater
populations may cause slight yield or quality loss but
not enough to offset the cost of a management measure.
In other occasions, larger populations can cause significant
damage and their management becomes essential. Therefore,
economics must be a primary consideration in pest management.
Management of the pest should only be considered if
economic damage will occur and the population is at
or above the economic threshold.
Different species in each class of pests (weeds, insects,
diseases) differ in size, reproductive capacity, and
rate of growth. Remember, when making a pest management
decision, consider the market price of the crop, the
cost of application, and the effect on the environment.
Method Selection. Once a pest problem has been identified,
the biology and the habits of the pest understood, and
the economic significance established, then the appropriate
method or combination of methods can be selected to
manage the pest in an effective, practical, economical
and environmentally-sound manner. Proper selection requires
that you be thoroughly familiar with all available management
methods and that you fully evaluate the benefits and
risks of each. The pesticide selection should be considered
based on the less toxic and effective option.
It is very important to evaluate the results of
your pest management program. This can be done in
several ways such as monitoring pest populations
or infection before and after treatment, comparative
damage ratings, etc. Insects and their activity
can be monitored by either or both scouting and
Usually to adequately evaluate a treatment, it is necessary
to leave untreated checks to use as a basis for comparison.
In some situations, it is impossible to leave untreated
checks. Always, record the results from your evaluation
for future reference.
Methods of Pest control.
We will not attempt to discuss in depth the various
methods of pest control. Our intent here is to simply
give you an overview of the available alternatives and
to present some characteristics of each.
Resistant Varieties. Frequently, pest problems can
be avoided or minimized simply by planting resistant
varieties. The degree of resistance to a particular
pest may be either partial or complete. These varieties
possess genetic defenses such as protective physiological
or physical characteristics which reduce their susceptibility
to pests. Selecting resistant varieties makes the environment
less favorable for pests and keeps them below harmful
levels. Frequently, pest problems can be avoided or
minimized simply by using resistant varieties.
Crop Rotation. Crop rotation can be an effective mean
of maintaining pest populations at manageable levels;
oftentimes it is a necessity. If a crop which is susceptible
to a particular pest is grown year after year on the
same land, pest infestations can become devastating.
Rotation to other crops not desired by resident pests
may offer at least a partial solution and may at the
same time provide additional benefits such as increased
soil fertility and a reduction in soil erosion.
Cultural Control. Many pest problems can be avoided
or minimized by using appropriate cultural control techniques.
Cultural control includes a number of practices designed
to create optimal growing conditions for the crop and/
or unfavorable conditions for the pest. Cultural control
involves, normal farming operations such as varying
the planting time, cultivating, fertilizing, irrigating,
and harvesting which may alter somewhat in response
to particular pest problems. Sanitation practices, such
as removal of crop residues which harbor pests, can
also be an effective means of cultural control.
Biological Control. The foundation of biological control
focuses on maximizing the effects of the natural enemies
of pests. Biological controls are most commonly used
to manage insects, mites, and some weeds. These natural
enemies should be preserved in the field, as they are
beneficial. This requires a careful choice of pest control
measures, particularly an informed selection and judicious
use of pesticides. Special consideration needs to be
given to select insecticides that will not harm your
beneficial insects. You can also help to ensure the
continued presence of these species by preserving appropriate
habitats in surrounding vegetation.
Chemical Control. Despite their potential hazards,
chemical are essential components of pest control programs
and will remain so for the foreseeable future. Chemical
controls use naturally derived or synthetic chemicals
called pesticides which kill, repel, attract, sterilize,
or otherwise interfere with the normal behavior of pests.
Chemicals act quickly and are effective against large
pest populations. In many cases, the application of
pesticides may be the most effective and feasible control
tactic. However, pesticides should be used only when
needed and in such a manner that you, your family, your
neighbor, and the environment are adequately protected.