Agronomy - Weed and Pest Control

After this lesson participants will be able to:

  1. Control weeds in the crop production unit.

  2. Control pests in the crop production unit

  3. Prevent diseases in the crop production unit.

Lesson Preparation:

  1. Collect insect and weed samples from around the village to show.

  2. If possible get visual aids of pests.

  3. Arrange for field trip to identify local weeds, insects, and other pests existing in the area.

  1. Weed control methods in crop production.

  2. Hand weeding

  3. Mechanical weeding

  4. Chemical weed control

  5. Integrated weed control

  6. Insect control methods in crop production.

  7. 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 countries.

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:

  1. 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 spreading.

  2. A first blanket weeding (or harrowing) soon after the first rains. If possible a second such weeding should continue after two weeks (#12).

  3. Planting can start 4-8 weeks earlier, leading to a close full stand that later on completely smoothers any late weeds.

  4. 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 row planting).

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 beings.

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 include:

  1. Crop rotation: prolonged growing of the same crop will favor the development of particular weed communities (#14).

  2. 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).

  3. Placement of fertilizer: Applying fertilizer to crop plants reduces the chance that weeds can profit from their nutrients (#16).

  4. 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 habits:

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 attacks.

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 their effect.


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 plant pathogens.

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 include:

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 preventive fungicides.

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 diseases.

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 full-sized figures)

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 with hoes.

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 other malformations.

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 rates.

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 below.

  1. 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.

  2. 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.

  1. 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 trapping systems.

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.


Click on the magnifying glass of any image for enlargement.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
Figure 19
Figure 20
Figure 21
Figure 22
Figure 23
Figure 24
Figure 25
Figure 26
Figure 27
Figure 28
Figure 29
Figure 30
Figure 31
Contact Us    
© Copyright 1996-2004 Benson Agriculture & Food Institute, all rights reserved
Record visit