Module 3: Tools, methods, and tactics of IPM
By this stage of the course you have a good idea of what IPM comprises, and have looked into some of the fundamental concepts underlying this approach to pest management. In this module we would like to introduce some of the major tools that are used in IPM. Modern agricultural science, in combination with indigenous farmer knowledge acquired over thousands of years, provides us with a comprehensive “toolbox” of pest control methods, and agricultural professionals need to have a good understanding of what they are, how they work and the advantages and disadvantages associated with each.
While the list of such tools is quite long, all IPM methods can be classified into 3 main categories, i.e. prevention, intervention and regulation. On the following pages we will look into each of these approaches and tools in more detail.
Preventing the occurrence of pest problems before they can cause economic damage is by far the most preferred approach in IPM, and a key to prevention is crop health and the first rule of IPM is often cited as “grow a healthy crop”. Pest problems in agricultural crops are sometimes linked to inadequate plant health, and may be avoided by good farming practices. Pests may prefer sick, weak, or injured plants, and good crop health may be related to a lower incidence of pest problems.
Some preventative measures that directly inhibit pest problems target specific pests, while others reduce the chance of pest outbreaks more generally. They may act by reducing the carrying capacity of the agro-ecosystem for a particular pest. This can be achieved through increased natural enemy populations, decreased pest shelter or nest sites, decreased food, or fragmentation of the agro-ecosystem. This may appear to be very technical, but put more simply, a key and very effective preventative method is to increase the biodiversity of the farm system.
Increasing the biodiversity of the ecosystem will generally increase system stability and reduce the dramatic population explosions that characterise pest outbreaks. By encouraging many different species to co-exist on his lands, the farmer can reduce the risk that any one of them will become a major pest problem.
Biodiversity can be encouraged in many different ways. Increasing soil organic matter, mulching exposed soils, and reducing unnecessary cultivation can stimulate soil biodiversity. Plant biodiversity can be increased by encouraging hedgerows, inter-planting crops, using living mulches, and planting trees on the farm. Animal biodiversity can be increased by integrating livestock into the production system, providing or protecting existing natural enemy habitats, adding ponds, and increasing plant species diversity, especially flowering plants that provide food for predators and parasitoids species. Finally, crop genetic diversity can be increased by planting multiple cultivars of a single crop, or by using older varieties and landraces, rather than hybrid seeds which are generally very similar genetically.
3.1.2: Good Agronomic Practices
Good agronomic practices can prevent pest problems because they encourage good crop health and bolster crop resistance to pests.
Crops should be grown in the appropriate climate and in the appropriate season. Crops grown out of season, outside their ideal range or planted too close together are often stressed and therefore more prone to pests. Crop rotations, pest-resistant varieties, good sanitation, removal of alternate pest hosts, and disease-free stock and seeds can all be used to break persistent pest cycles. Appropriate fertilization and irrigation tends to result in healthy, pest-resistant crops.
Some specific preventative agronomic practices are detailed on the following pages. These include spatial methods, sequence-related methods and manipulation of planting materials and inputs. Some tried and tested IPM methods such as crop rotation fall into more than one category.
3.1.3: Spatial Methods of Prevention
The way that crops are arranged on a farm can affect their susceptibility to pest outbreaks. Several methods that can prevent pest problems are listed below.
- Cropping Pattern. Many pests have multiple alternative hosts. By avoiding planting alternative hosts near each other, pest build up can be avoided. For example, the cotton stainer bug, Dysdercus, uses volunteer cotton and Malvaceous weeds as an alternate host.
- Plant Spacing. The space between plants affects air flow and sunlight penetration, and therefore moisture and humidity levels. Wider spacing often results in reduced fungal disease, although close spacing tends to repel aphids and thrips.
Wide plant spacing allows more air circulation and sunlight penetration within the crop, reducing understory moisture levels. Generally, this helps reduce fungal disease, although soil splash from ‘bare ground’ during rainfall and irrigation can increase soil-borne disease transmission. Wide spacing also facilitates mechanical weeding, but sowing at lower than the agronomically optimum density can result in a yield penalty. This may be justified if the value of the averted pest damage is greater than the value of sacrificed yield. The question of corn spacing is considered at http://www.agronext.iastate.edu/corn/production/management/planting/row.html.
- Intercropping. Growing two or more crops in the same, alternate, or double-rows often results in reduced pest problems.
Choosing crops with different growth forms and pest regimes is the best way to take advantage of intercropping’s benefits. Mixes of short and tall annuals (e.g. sorghum with cowpeas, maize with beans) are commonly found intercrops in the tropics. Sometimes a third crop, such as a cucurbit, is grown as well. Intercrops can also be used to grow several varieties of a single crop. Resistant varieties of a crop can protect high-value but susceptible varieties when they are intercropped.
- Strip Cropping. Growing two or more crops in broad, alternating strips has many of the benefits of intercropping but allows a broader range of mechanized field operations.
3.1.4: Sequence-related Methods of Prevention
Previous crops can have an effect on pest problems in the present crop. Planting crops in a particular sequence can reduce pest problems in general and soil-borne pest problems in particular. Several sequence-related planting methods that can prevent pest problems are listed below.
- Crop Rotation. Growing different crops in the same field over several seasons can reduce or prevent the build-up of pests, especially non-mobile or soil-borne varieties.
Some crops directly reduce pest problems for the succeeding crop. African marigolds release an anti-nematodal compound called thiopene, which can reduce nematode loads for subsequent horticultural crops. Cucurbits or sweet potato are often included in a rotation because they tend to choke out persistent weeds.
- Multiple Cropping. Growing several consecutive crops in the same field in a single year can addresses pest problems on an annual basis.
- Over-seeding and Undersowing. Over-seeding and undersowing are farming techniques where one crop is sown into another. Planting or seeding a second crop into an established crop can reduce pest problems.
Orchards and plantations are often under-sown with annuals, herbs, and other plants with the aim of reducing pest problems. Flowering herbs are commonly used as cover crops in grapes and apple orchards. Traditional shade-grown coffee is inter-planted with native shade trees, increasing biodiversity and reducing pest problems.
3.1.5: Planting Materials and Inputs
Both the genetic make-up and the health of planting materials can affect crop susceptibility to pest problems. Several preventative measures related to planting materials are listed below.
- Host-plant Resistance. Crop plants that are bred to resist certain pests are an important component of IPM. Many diseases and insects have been managed effectively by breeding genetic resistance into modern varieties.
Crop plants can resist pests in three main ways.
- Nonpreference. The crop plants are avoided by the pest because they do not ‘like’ it.
- Antibiosis. Plants cause a reduction in the biological performance of the pest.
- Tolerance. Plants can tolerate the pest and still provide an adequate yield.
The mechanisms of host plant resistance are diverse. Crops resist pests through colour, small, palatability, hairiness, waxy coatings, gross morphology, gumminess, necrosis, hardness, phenological shifts, toxin production, nutrition, integration with biological control, and compensation. Each mechanism of resistance may be classified under one of the three main types above.
Resistance management plans are designed to reduce or prevent the development of resistance, by planting resistant varieties in a particular way. For example, many maize growers will grow a strip of an older, susceptible variety around a new resistant variety so that the pest has something preferable to eat and will be more likely to maintain its current resistance status.
- Grow Disease-free Plants and Seeds. Many pests and diseases are transmitted along with seeds and other planting materials. Disease-free stock and seeds can prevent many endemic pest problems, particularly in vegetatively-propagated plants. Virus and disease loads on vegetatively propagated stock such as seed potatoes can reduce yields by as much as 50%. Farmers can greatly improve plant health by starting with clean planting materials.
Insect pests can be removed from seed stock by dry heating in an oven. Sunlight, carbon dioxide, steam, and bleach solutions are other low-tech ways to reduce transmission of pest problems through planting stock.
- Crop Genetic Diversity. High crop genetic diversity is a traditional way of ensuring an adequate harvest.
Reducing crop genetic diversity may lead to a uniform crop with desirable characteristics, but it also can leave crops vulnerable to pests and adverse environmental conditions. Often, as in the case of hybrids, the crop is restricted to a single genome. The consequences of massive plantings of single-genome hybrids can be severe, as was the case in the United States in 1970-71 when southern corn leaf blight destroyed 15% of the US corn crop due to broad plantings of a susceptible hybrid. Similarly, massive plantings of pest-resistant rice varieties in Asia have led to development of pests that overcame the original resistance after a few seasons, and led to pest upsurges in the ‘resistant’ rice.
There are several ways in which crop genetic diversity can be increased and genetic vulnerability decreased.
- Promotion of traditional varieties. Traditional varieties that are mass selected tend to have high degrees of genetic diversity. Although these varieties tend to be lower-yielding than modern, high-yielding varieties, they are more dependable and stable under flucutating pest regimes.
- Intercropping multiple varieties. Intercropping multiple varieties of a single crop can increase crop resistance to pests, especially when the varieties are resistant to different pests. For example, the intercropping of a blight-resistant rice cultivar with a blight-susceptible cultivar of rice allowed the blight-susceptible cultivar to be grown successfully in an infested area.
- On-site selection and seed saving. By selecting and propagating crop varieties on farm, farmers can adapt a variety to their local conditions while maintaining crop genetic diversity.
- Plant Appropriate Crops and Cultivars. Most crops and cultivars are adapted or bred to perform best under particular environmental conditions. Crops grown outside of this optimum location are often subject to increased stress and reduced health, and are prone to pest problems.
Wheat is an example of a crop that is limited to certain environmental conditions. While it can physiologically survive in hot, humid conditions, diseases and pests are encouraged by such conditions and this effectively limits wheat cultivation in the tropics. Breeders have been trying to develop wheat strains which are suitable for tropical conditions. Growing wheat in the tropics currently requires heavy use of pesticides, and an IPM-oriented farmer may be better off choosing another crop.
Tomatoes are a widely grown cash-crop throughout the world. In the tropics, they are grown mainly in the dry season, partially because tomato plants need temperatures below 30 degrees Celsius in order to set fruit, and partially because of disease pressure. Tomatoes suffer from various blight diseases in very humid conditions. However because of this, fresh tomatoes can be sold at a premium in the wet season, and many farmers attempt to grow them during this more difficult rainy season. Special cultivars with a degree of blight resistance have been developed for this purpose, and these should be chosen over the ‘regular’ varieties for wet season growing. By growing cultivars adapted to each season, an IPM farmer can effectively reduce potential pest and disease problems. Tomatoes are a good example of a crop where different cultivars are more appropriate for different seasons in the tropics – see Suggested Cultural Practices for Tomato, a publication from the Asian Vegetable Research and Development Center (AVRDC) at http://avrdc.org/?p=51.
Optimizing inputs to crops produces results in healthy plants that resist pests. Fertilization and irrigation are discussed here in the context of pest problem prevention.
- Fertilization. Proper plant nutrition plays an important role in IPM. Over-fertilized plants often suffer pest outbreaks due to ‘soft’ or watery growth, while under-fertilized plants are subject to stress-related pests.
Soils with adequate, balanced fertility result in quick-growing, healthy plants that resist pests. Fast-growing plants reduce opportunities for pests that attack certain growth stages, such as stem-borers, by shortening their period of vulnerability. Fast-growing plants can also compensate for any damage that does occur. Fast-growing plants quickly form a canopy, discouraging pests that disperse aerially or seek bare ground.
There can be negatives if the wrong nutrient regime is used. While high fertility can increase potential yields, balanced fertility that is appropriate to the intended crop is the key to pest prevention. Many diseases and insect pests are associated with too much nitrogen fertilizer in proportion to phosphorous, potassium, and other elements. Similarly, several pest problems have been managed using a disproportion of specific trace elements, such as silicon or zinc.
Building productive soils through the addition of compost and organic matter can help reduce nutrient imbalances. Relying on non-chemical sources for at least part of a crop’s fertility requirements can indirectly reduce pest problems.
- Irrigation. Overwatering and drought can both stress crop plants. Water management in crop production can also have large effects on pest habitat and populations.
Firstly, applying an optimum amount of water to a crop results in quick-growing, healthy, and pest-resistant plants, but overwatering can cause plants to become lush and prone to fungal diseases and insect such as thrips and aphids, while plants stressed from – will be less able to withstand pest attacks.
Secondly, water can be used directly on pests. Strong jets of water can physically knock pests off plants, and thirdly, water can be used indirectly to control pests. Flooding a field can drown non-aquatic pests, and flooded rice-paddies attract fish, frogs, and other aquatic predators to prey on pests. Raising and lowering water levels in a flooded field can augment predation of above-water pests by aquatic predators. Adjusting irrigation can speed or slow crop maturity, preventing a crop from ripening at the same time as a pest population peak.
It is not always possible to totally prevent pests from damaging a crop or reducing its economic value. This means that when pest populations do begin to approach the Economic Injury Level an intervention has to be made to protect the crop and farm profits.
Once a decision has been made that an intervention is required, a range of intervention options are available. These include chemical, biological, cultural, physical and genetic interventions. The following pages describe the various intervention tools methods at the disposal of an IPM practitioner.
Note that many of these intervention methods have also been listed as preventative measures in the previous lessons. While technically an intervention is reactive to a pest problem and a preventative measure is proactive, in practice the distinction between preventative measures and interventions is often blurred. For example, using a pest-resistant cultivar as a preventative measure in one season could be a reaction to a specific pest problem in previous crop.
3.2.1: Chemical Interventions
Chemical interventions introduce organic and inorganic substances into the crop or farm ecosystem to manage pest problems. They can be man-made (synthetic), or collected and derived from organisms (biopesticides, pheromones, allelochemicals, insect growth regulators) or collected from other natural sources (inorganics).
Chemical interventions can be applied in a variety of ways. They may be diluted in water or oil for spraying, left dry for application as dusts or granules, or added to baits or traps. Spraying, dusting, fogging, smoking, and other techniques can be used to apply chemical interventions to crops.
For a summary of chemical intervention formulations, visit one or more of the following sites:
- The FAO Pesticide Management Unit at http://www.fao.org/agriculture/crops/core-themes/theme/pests/en/
- US EPA have additional information and links about chemical interventions at http://www.epa.gov/opp00001/about/aboutus.htm
Categories of chemical interventions are given below, with sources of further information.
- Synthetic Pesticides. These include some of the most notorious, but also the most useful of all plant protection products. They are made from carbon-containing compounds and synthetic (chemical) pesticides are widely used and available throughout most of the world.
Many so-called IPM programs are based largely on the use of synthetic pesticides, but this is not grasping the principle of IPM. Before pesticides are used, IPM practitioners need to calculate economic injury levels and make recommendations. Using synthetic pesticides may often be simpler and more predictable than other IPM interventions, but every effort should be made to first use other interventions. A program which makes limited, intelligent use of synthetic pesticides is often more effective than one that relies entirely on synthetic pesticides for crop protection, or abandons synthetic pesticides completely. Crops can be grown organically, but the risk of pests can be significant, and th cost of production may be higher. If pesticides are used, they should be used effectively to complement other IPM methods, such as biological control. Using a selective synthetic pesticide that kills pests more effectively than it kills predators and parasitoids will increase the proportion of beneficials to pests, which is likely to increase the efficiency of biological control. Understanding the biology and lifecycles of both pests and beneficials is crucial in order to use synthetic pesticides most effectively in IPM.
For more information about synthetic pesticides in IPM programmes, go to the following links:
CropLife Africa and the Middle East produces a newsletter [email@example.com]. To receive a regular update by email, contact firstname.lastname@example.org and ask to join the mailing list.
The Compendium of Pesticide Common Names is useful for identifying synthetic pesticides.
The British Crop Protection Council publishes The Pesticide Manual, which is a standard reference for anyone working with synthetic pesticides, see http://www.bcpc.org/.
- Botanicals. Botanical pesticides are biopesticides derived or refined from plants.
Botanicals are said to be variable in their efficacy. They are therefore not widely accepted, especially by commercial farmers. The extraction, processing and formulation of many botanicals are often carried out at a local level and many are produced and used mainly by small farmers. Rather than incorporate a botanical with a low level, but wide-ranging effect on pests such as neem, into an IPM program, IPM practitioners should first evaluate their efficacy and role before inclusion in their program. Note too that some botanicals such as nicotine, can be toxic, so great care is needed in their use to avoid health risks.
To see some innovative botanicals, visit the Soil Technologies Corp at http://www.soiltechcorp.com/ or the CABI resources at http://www.cabi.org/default.aspx?site=170&page=1016&pid=509.
- Inorganic Pesticides. Inorganic pesticides are substances derived or refined from non-living sources.
The exceptions to this rule are copper and sulphur-based compounds which are used as fungicides. Sulphur is a widely used and safe way to control fungal diseases and mites. Bordeaux mixture, a combination of sulphur, copper, and lime, is an important fungicide used in orchards and vinyards.
- Semiochemicals. Semiochemicals are chemicals produced by organisms that modify the behaviour of animals.
The most commonly used pheromones in IPM are sex attractants. These chemicals are usually produced by females to attract males for mating and are used by IPM practitioners to attract male pests into traps. Pheromone traps are often used to determine population density by sampling the number of males caught in a trap in a certain amount of time. Alarm pheromones can be used to repel certain species from crops. A traditional practice in Mexico takes advantage of the alarm pheromones released by burning beetles. Placing burned beetle pests in bean fields overnight effectively repels living beetle pests by the next morning.
Allomones are produced by many plants to repel herbivores and prevent them from feeding. Many secondary products produced from plants to protect wood, such as tannins, cyaninis, etc. – are in fact anti-herbivore allomones. Some act by repelling, others directly affect the growth and development of the pest organism. Allomones have not been used very much as an applied intervention, but they are often the basis for companion planting and other cultural interventions in IPM systems.
- Hormones. Hormones are chemicals produced in one part of a pest’s body that effect the growth and behaviour of other parts of the body.
Insect growth regulators are usually synthetic versions of naturally-occuring hormones. They are highly selective and have extremely low toxicity to other organisms (including humans). However they may be expensive and/or difficult to apply and use. In some cases their extreme specificity limits their usefulness, but this can be an advantage, e.g., treating large areas of locust hoppers with an IGR that has no effect on other organisms.
3.2.2: Biological Interventions
Biological interventions use other organisms to manage pest problems. These organisms include predators, parasitoids, or pathogens that kill the pest species being managed. Organisms may be directly introduced into the agro-ecosystem, or alternatively, conditions within the farm ecosystem can be altered to indirectly encourage beneficial organism populations.
In this cartoon, three kinds of beneficial insects are being encouraged by provision of hiding places (board and lodging) and flowering plants that many need to complete their life cycle. In an IPM system they will then move into the corn (maize) and destroy insect pests.
Biological interventions may be highly selective, and there are rarely negative side-effects (except in the case of classical biological control, see below). Released organisms may be self-perpetuating, and if used early may breed at a rate that keeps pest numbers within acceptable limits.
While pest resistance is rare, because predators and parasitoids tend to co-evolve with pests, and the relationship between them has stabilised, there are cases of pests developing resistance to frequently applied pathogens. Rearing and release of biological agents may sometimes be simple and inexpensive.
Disadvantages of biological interventions can include slow action, unpredictability, and incompatibility with pesticides (the pesticide may kill the predator or parasitoid). Using biological intervention effectively requires good observations and a sound understanding of the biology of the pest and beneficial.
There are three types of biological control agents.
- Predators catch and consume pest prey. Major groups of predators include the insect orders Hemiptera, Neuroptera, Diptera, Coleoptera, and Hymenoptera, the Arachnida, and vertebrates such as snakes, birds, and fish. Predators are often fairly generalist, preying on a wide range of prey according to abundance and ease of capture.
- Parasitoids lay eggs on or in a pest host, which the resulting larvae consume and ultimately kill. Most parasitoids are far smaller than their prey, and therefore mass rearing and release of parasitoids can be relatively convenient. Parasitoids are found only in the insect order Hymenoptera and the Tachinid family of flies (Diptera). Parasitoids usually exhibit high host specificity.
- Pathogens infect pests with fatal or debilitating diseases and include fungi, nematodes, bacteria, viruses, and other microbes. Fungi, particularly Deuteromycetes, can infect pests externally under favourable conditions, but other pathogens must be ingested to be effective as control agents. Pathogens are very specific to their hosts. Pathogens are often referred to as biopesticides because they can be applied in similar ways to chemical interventions.
Categories of biological interventions are listed below.
- Natural Biocontrol. Any method that indirectly conserves or increases the effectiveness of existing beneficials in the ecosystem can be considered as natural biocontrol. This definition can be expanded to include any method which encourages biodiversity, but usually this category encompasses measures that encourage specific beneficials such as hoverflies (which need flowers to complete their development).
In order to promote and conserve beneficials, IPM workers need to know something about their biology. Birds and bats can be encouraged by erecting bird and bat houses, but how does one go about promoting beneficial fungi? Increasing the biodiversity of the ecosystem will increase natural biocontrol, as will devoting a larger percentage of the total area to non-crop or perennial plantings, including hedges. Flowering herbs and aromatic plants tend to attract beneficial insects, while compost and organic matter will improve the soil habitat for natural biocontrol.
- Augmentation (of beneficials). The release of additional beneficials into an agro-ecosystem in order to help the existing beneficial population control a pest problem is called augmentation.
Augmentation is a short-term solution, and biocontrol agents that are released in an augmentation release will disperse quickly if the ecosystem cannot provide them with food and/or shelter. Conservation and creation of habitat for the released beneficial species should accompany augmentation in order to make the releases more sustainable.
- Inundative Release. Releasing large numbers of beneficials into an agro-ecosystem can overcome a pest problem.
Inundation is particularly effective against pests that only experience a single generation per season or against pests that occur in rare outbreaks. For example, Trichogramma (a parasitoid) is mass-released against cotton bollworm in parts of the United States, where only a single generation of bollworm threatens the cotton crop.
Inundation is most often used with pathogens, used to control an outbreak or build-up of pests. Pathogens are generally small or micro-organisms, and raising millions or billions of them to release at the same time can be feasible.
- Seasonal Inoculative Release. Releasing a small number of beneficials into an agro-ecosystem early in the season in order to give the beneficials a ‘head-start’ on pest populations, is a particular technique to establish beneficial populations in areas where they previously existed in small numbers or did not exist at all.
- Classical Biological Control. Classical biological control is introduction of a non-native organism into an ecosystem or region in order to manage either a native pest or more often, an introduced one.
Where a pest organism is introduced into a new environment, the indigenous natural enemies that had controlled its population in the country of origin, are not present in the country into which they are introduced. There are many documented instances of pests that are kept in check and hence seemed innocuous in their native lands, becoming huge pests in their new countries. An important example in the Asia-Pacific region is the introduction and spread of the apple snail, which has had a devastating effect on rice cultivation in several countries. In Brazil and Paraguay, where the snail is native, natural enemies maintain the population of snails at less than 1 or 2 per square meter of rice paddy. In Asian rice paddies, however, densities of 300 or 400 per square meter have been recorded due to the absence of things that eat them or infect them.
The obvious reaction to a problem like this is to determine what natural enemies exist in a pest’s native region and introduce them to the new area where the pest is a problem. While this is a good idea in theory, often the factors which allow particular natural enemies of a pest to control it in the country where it originates are overlooked or misunderstood. Thus classical biocontrol has seen mixed success, with some introduced control agents doing their job, some being ineffectual, and still others causing different problems. According to a study in 1988, of 563 attempts to establish classical biocontrol against insect pests, only 40% were successful. 31% of these releases against weed pests (126 releases) were deemed successful by the same study (Waage, J.K and Greathead, D.J. 1988. Biological control. Phil. Trans. R. Soc. Lond. (B), 318:111-26).
Classical biocontrol is practiced mainly by governments these days, and usually as a last, hopefully carefully-researched resort. Therefore, classical biocontrol is interesting to most IPM workers, but practically they would need to draw attention of the competent official authorities to any new pest problems that might benefit from the introduction of beneficials from other countries. It would then be the role of the official agencies to commission researchers to seek beneficials, and then licence their release.
- Herbivores. Managing livestock, insects, and other organisms so that they consume and control weed populations can be a useful IPM tactic.
The major natural enemies of weeds are herbivores that eat plants. Unlike the major natural enemies of insect pests, many herbivores have been domesticated by humans, and are much easier to ‘release’ than predators, parasitoids and pathogens. Cattle, ducks, geese, goats, pigs, chickens, quails, and other vertebrate herbivores are well-known as farm animals but rarely considered for weed control. Properly managed and understood, domesticated herbivores can effectively manage weeds in crops, orchards, forests, and gardens.
Ducks are sometimes used as combined weeders and mollusc removers in rice-based farming systems. Chickens confined to a small area will not only remove weeds, but will also dig up and eat weed seeds, so can clear weedy ground. Cattle have selective preferences for certain forages, and can be used to manage crops in otherwise fallow periods. Sheep are used in forestry to prevent undergrowth from choking out young tree seedlings.
3.2.3 Cultural Interventions
Cultural interventions use the way a crop is grown to manage pests. They tend to be kind to the environment. While many cultural controls are considered ‘traditional’ or modified versions of traditional practices, new methods have been introduced and shown to be effective in cropping systems around the world. Many cultural controls are largely preventative although all methods, whether preventative methods or interventions, are listed here.
Cultural controls can affect pest populations in three ways. Firstly, they can make the crop plant or ecosystem unacceptable to the pest, and the pest will avoid the crop. Secondly, they can displace the crop plant in time or space, causing it to be unavailable to the pest during the period when it normally feeds. Thirdly, they can make the agro-ecosystem a dangerous place for the pest by increasing beneficial populations.
Cultural interventions have been part of agriculture since humans started farming around 10,000 years ago. Partly because of this long development time, and partly because of the diversity of crop husbandry practices used around the world, there are quite a few different strategies.
Some cultural interventions are listed below.
- Crop Rotation. A crop rotation can be used to interfere with the pests of one crop by growing another in the same place afterwards.
Crop rotations are particularly effective at controlling soil-borne diseases, as well as soil-dwelling organisms that do not disperse well. They work best when the crops in the rotation have few shared pests. A common rotation in temperate areas is maize, followed by wheat, and then red clover. Of the 50 or more serious pests that attack these crops, only 3 are important pests of all three crops. Crops with few pests, such as garlic, are often grown in a rotation to break pest cycles.
Some crops directly reduce pest problems for the succeeding crop. African marigolds release an anti-nematodal compound called thiopene, which can reduce nematode populations for subsequent horticultural crops. Cucurbits, irish potato and sweet potato are often included in a rotation because they tend to choke out persistent weeds or because their cultivations disturb weed reproduction.
- Multiple Cropping. A quick clean-up crop, or a complementary crop grown in the same season can reduce pest problems.
Many farmers grow a cereal crop immediately before higher-value crops such as tobacco, cotton, or vegetables. Multi-cropping a monocotyledonous crop with a dicotyledonous crop effectively disrupts soil borne pests. It also improves weed management, as the persistent monocot weeds of cereals can be easily identified in a dicot crop and vice versa.
- Over-seeding and Undersowing. Helpful companion crops can be sown into the main crop to control certain pests.
Over-seeding with IPM principles in mind can be very successful. Brassicas are often under-sown with clover to reduce cabbage root fly infestations. In Georgia, USA, farmers who planted cotton into strip-killed crimson clover reduced their use of insecticides and nitrogen fertilizer.
Orchards and plantations are often under-sown with annuals, herbs, and other plants with the aim of reducing pest problems. Flowering herbs are commonly used as cover crops in grapes and apple orchards. Traditional shade-grown coffee is inter-planted with native shade trees, increasing biodiversity and reducing pest problems.
- Border Crops. Crops can be sown at the periphery of the main crop to specifically prevent pests from entering a crop field or harbour beneficials, and can be an effective way of controlling pests.
Some cereal farmers in the United Kingdom have stopped spraying the outside edges of their fields. They do this to allow partridges to nest undisturbed in the field edges – the partridge is a valuable game bird. A side benefit of not spraying the field edge is that it is home to a high population density of beneficials. While no formal calculations have been made, it is possible that the pest control provided by this unsprayed perimeter compensates for the slightly reduced yields there. (Sotherton, N.W., Boatman, N.D. and Rands, M.R.W. (1989) The ‘conservation headland’ experiment in cereal ecosystems. Entomologist, 108, 135-43.)
An experiment in California found that some border crops are better than others. Lambsquarters borders (various species of goosefoot, or pig-weed ) were found to be effective at reducing pest problems in cauliflower while mustard and radish increased problems, so clearly the choice of border plant is an important consideration.
- Trap Crops. A trap crop is a sacrificial crop that is more attractive than the main crop to particular pests.
Trap crops have to be more attractive than the main crop. Often, a trap crop is the same species as the main crop, but is a different cultivar or grown in a different way. For example, a small sowing of maize preceding the main crop will effectively trap aerial pests. This is because the earlier sown, taller plants are more attractive to the pests.
Trap crops of different species can be employed. Eggplants are more attractive to many potato pests than potatoes are, and small, sacrificial sowings can be used to protect potato crops from various insect pests. Sesame is favoured over cotton by the cotton bollworm (Heliothis spp) and has been investigated as a potential trap crop for cotton in Texas (Laster, M.L. and R.E. Furr. 1972. Heliothis populations in cotton-sesame interplantings. Journal of Economic Entomology. Vol. 65, No. 5. p. 1524-1525.)
- Use only Disease-free Plants and Seeds. Seed-borne diseases and diseases that are introduced with vegetative planting materials can be eliminated or reduced by using disease-free sources because many pest problems are borne on planting stock or seeds.
Many countries have certification programs for seed and planting stock. Certified seed is often more expensive than locally-available seed but the extra cost should be balanced against the potential risks of using uncertified seed. Vegetatively-propagated crops such as banana are often grown in tissue culture to obtain disease-free strains, but like other planting materials, bananas can be sterilized by the farmer himself.
INIBAP (see http://bananas.bioversityinternational.org/) has outlined a method for preventing banana disease by using a hot water bath on planting stock. Insect pests can be removed from seed stock by dry heating in an oven. Sunlight, carbon dioxide, steam, and bleach solutions are other low-tech ways to reduce transmission of pest problems that are spread through infected planting stock.
- Altered Planting and Harvest Dates. Planting and harvesting out of synchrony with pest populations can reduce losses due to pest damage.
Sowing may be delayed until after the pest has emerged and dispersed, or the crop can be sown earlier so that it will reach a resistant growth stage by the time the pest emerges. For example, the hessian fly is a major pest of wheat.
By delaying sowing of wheat, the peak of activity of the hessian fly can be avoided and the crop protected. Early planting of sugar cane allows it to reach a size where it is not susceptible to the emergence of borers, before the pest numbers peak.
Altering the sowing date alters the phenology (how seasons affect life cycle of the plant), and may expose the crop to other pests at later growth stages. For example, delayed sowing of dry-season crops may expose them to fungal damage if they mature after the start of the rainy season, e.g., tomatoes.
- Mulches. A mulch is a layer of material applied to the surface of an area of soil.
Mulch can also prevent soil-borne disease spores from splashing onto plants during irrigation or rainfall. It can also provide habitat for a diversity of beneficials. For example, spiders are abundant in straw or hay mulch, and the use of such mulch in vegetables can result in significant insect pest reduction.
Living mulches, such as clover or flowering herbs, are discussed on the over-seeding page. These also increase biodiversity. Plastic mulches are widely used in horticulture, mainly for weed control or to exclude pests and to heat cold soils. They can also be used to control some soil borne diseases.
- Efficient Harvest and Storage. An efficient harvest can break pest cycles and result in clean product. Properly managing storage pests maintains value of the stored crop and prevents it from becoming a pest vector for future crops. Proper sanitation measures should be followed, and crop materials that can harbour pests until the next season should be removed.
Crops should be properly handled before storage. Drying, curing, and other primary processing activities should be planned and carried out with IPM in mind. If a crop is going to be machine-dried, for example, part of the drying cycle could be at a high enough temperature to kill any potential storage pests.
Managing pests in stored crops is a challenge for most IPM practitioners. Pests can rapidly multiply and damage large portions of a stored crop but they are often hard to detect. Stored crop is often neglected until it is time to eat or sell it – this is too late to apply appropriate interventions!
Stored crop IPM follows the same procedures as field IPM. First, store the crop in a way that prevents pest problems in the first place. Second, monitor for pests, identify any found, determine whether they will cause economic damage, and intervene if necessary.
3.2.4 Physical Interventions
Physical interventions alter or exploit a physical characteristic of the environment in order to manipulate pest populations. Different temperatures, humidity levels, and even atmospheres can be used to manage pests, as can mechanical intervention such as tillage and shredding. In situations where the farmer has a large degree of control over the physical environment, such as greenhouses, physical interventions can be the most important methods of IPM. Even in field situations, physical manipulations such as compaction, flooding, or mulching can adversely affect potential pests.
There are probably hundreds of physical interventions used by the world’s farmers. Several categories of physical interventions are listed here.
- Direct. Physical interventions that involve the direct physical removal or destruction of the pest. Examples include weeding, destruction of crop residues, pruning, and vacuuming.
- Traps: Traps are often used for monitoring pest populations, but trapping can also be used for control. Many kinds of traps exists. Banding with sticky bands or bands impregnated with repellent can exclude pests from tree crops. Light, colour, pheromones, fermentation, and sound can all be used to lure pests into traps. Trap crops are discussed elsewhere, but plant parts from the main crop can also be used for trapping and destroying pests.
- Shaking: Shaking crop plants can dislodge pests to groundsheets where they can be collected and destroyed.
- Handpicking and weeding: Hand picking and destruction of pests is widely practiced wherever labour and time are available.
- Pruning: Pruning to remove egg masses of pest insects can be an effective way of controlling orchard and ornamental pests. Pruning also opens up the canopy to create conditions that promote health of the crop plant.
- Tillage. Physical manipulations of the soil that help to manage pests. Compaction, soil shaping, and no-till cropping are included in this category.
- Barriers: Barriers act to physically prevent pests from feeding or laying eggs on a crop. Several types of barriers are described here.
- Screens: Screens are commonly used in greenhouses to allow circulation of air but prevent entry of pests. Screenhouses are similar to greenhouses, except that their outer covering consists of screening that is appropriately sized for the target insect. Screens can also be used in fields to protect valuable plants.
- Use of greenhouses and other structures: Built structures that can be closed off from the outside environment can very effectively exclude pest organisms. Greenhouses provide protection not only from extreme weather and temperatures, but also from dispersing pests. Many greenhouses can be sealed for fumigation, which is often required if pests establish a population inside.
- Row covers and mulches: Row covers and mulches are placed on the ground around the stems of the crop plants. They are effective against soil-born pests such as cabbage maggots, flea beetles and other insects.
- Trenching: Some insect pests are unable to escape from trenches, and lining a crop field with a trench can provide substantial protection. For example, Colorado potato beetles are trapped by a ‘V’-shaped trench with a sharp slope, resulting in substantial reductions in adults and egg deposition within potato crops.
- Bags: Valuable fruits are sometimes covered with individual bags. This method is time-consuming and labour-intensive, but can lead to premium prices for the resulting unblemished fruits. Entire bunches of bananas can also be protected by using large bags.
- Packaging: Many materials have been developed that physically exclude pests from stored crop products. These include metal foils, plastics such as cellophane and polypropylene, and paper.
- Fences: A well-built fence can protect crops from many pests as long as it is properly installed and maintained. In particular, mammals and low-flying insects can be excluded by good fencing. Floating row covers.
- Nets: Nets placed over fruit trees or other crops physically stop pests such as birds from gaining access to the crop.
- Temperature.The use of heat can be used to manage pests. Examples include soil solarization, flaming, burning, etc.
- Air. Manipulation of the atmosphere to manage a pest. This method is usually associated with storage of harvested product in high-nitrogen or high-carbon dioxide environments.
3.2.5 Genetic Interventions
Genetic interventions manipulate or exploit the characteristics of either the crop itself or the pests that attack them, or even the beneficial organisms that help to protect the crop. They do this by altering the underlying genes, chromosomes, or reproductive systems.
The major genetic intervention used in agriculture today, breeding for host-plant resistance, has had a profound effect on IPM for most major crops. Induced sterility, although potentially widely applicable, has seen limited use since its initial success with screw-worm fly (Cochliomyia hominovorax) in the early 1950’s.
As the world population exceeds 7 billion, the rate of improvement of crop characteristics using conventional breeding faces increasing challenges in feeding people. Alongside this increasing challenge there have been pressures from legislators and the public to reduce reliance on pesticides. This has encouraged agrochemical companies to diversify their efforts away from pesticide-based pest management technologies to develop genetically modified (GM) crops. In fact, for as long as farmers have been saving seed, they have been using breeding techniques to “genetically modify” crops to improve quality and yield in the old sense of the term. Nowadays, modern biotechnology allows plants breeders to select genes that produce beneficial traits and to transfer them from one species to another. Plant biotechnology is far more precise and selective than crossbreeding in producing desired agronomic traits, so more radical characteristics can be incorporated over a short development period.
Due to recent successes, plant biotechnology has been adopted by farmers worldwide and in 2011 biotech crops were grown by 16.7 million farmers on 160 million hectares in 29 countries. Many believe that GM crops have contributed significantly to an improvement of farming techniques This use of GM has in many cases reduced the use of chemical pesticides by making plants resistant to pests and diseases.
Thus, genetic interventions are attractive to IPM workers because they are perceived as being specific, controllable, and limited only by the science. Genetic interventions can be packaged, propagated and delivered in a form that is readily acceptable and easy to use – seeds or other planting materials. They are self-perpetuating, do not pollute the environment with spray drift or run-off, and are affordable. Only in very rare cases have negative outcomes been observed and this is impressive, bearing in mind all the studies that have been carried out. The success of the Green Revolution was dependent on host-plant resistance bred into key crops, and many advocates of genetic engineering envision a second Green Revolution based on the latest molecular advances.
The development and application of genetic engineering and improvements in gene mapping are expected to have an increasing impact on IPM. Already, many crops have been bred for increased host-plant resistance using genetic material from other species. Other crops have been altered to allow other interventions, such as herbicide application, to be performed more easily.
Unfortunately, there is not yet general agreement about the safety, benefits and risks of genetic engineering in IPM. Critics of genetic interventions in IPM point out that dependence on ‘silver bullets’ for pest management is a reversion to the routine use of pesticides – a so-called calendar spraying mentality that caused many of the pest management problems that exist today. A crop that has a systemic pesticide engineered into its genome is a clever invention, but growing it in a way that induces pest resistance (such as large-scale monocultures) would be incompatible with rational IPM. Nevertheless, perhaps we need to remember that the letter ‘I’ in the acronym, stands for intergrated. GM technologies such as herbicide resistance and engineered systemic pesticide within the physiology of crop plants can work well alongside more traditional techniques such as crop rotations and biological control.
Some genetic interventions are listed below.
- Host-plant Resistance. A wide range of breeding programs have developed crops with resistance to one or more major pests.
Crop plants can resist pests in three main ways.
- Non-preference. The crop plants are avoided by the pest because they do not ‘like’ it.
- Antibiosis. Plants cause a reduction in the biological performance of the pest.
- Tolerance. Plants can tolerate the pest and still provide an adequate yield.
The mechanisms of resistance are diverse. Crops resist pests through colour, palatability, hairiness, waxy coatings, gross morphology, gumminess, necrosis, hardness, phenological shifts, toxin production, nutrition, integration with biological control, and compensation. Each mechanism of resistance may be classified under one of the three main types above.
Many resistant varieties have lost their resistance through mass plantings and adaptation of pest populations. Resistance management plans are designed to reduce or prevent the development of resistance, by planting resistant varieties in a particular way. For example, many maize growers will grow a strip of an older, susceptible variety around a new resistant variety so that the pest has something preferable to eat and will maintain its current resistance status.
- Sterilization. Releasing large numbers of sterile male insects into an agro-ecosystem can quickly reduce the local pest population.
- Genetic displacement. Replacement of a wild pest population with a population that has been bred to be less of a pest problem.
- Genetic improvement of beneficials. Breeding and release of improved beneficials into the ecosystem.
Regulatory and legislative approaches to IPM operate at an organizational level that is larger than a field or farm. They manage pests on a regional or national level. Many types of pests, such as migratory pests or those that have been accidentally introduced, can be controlled effectively through regulatory interventions. Isolated cropping regions enjoy comparative advantages over other regions due to successful quarantine and eradication programs against major crop pests.
Regional or national programs to manage pests rely on cooperation by (and enforcement of) individual farmers, and in many countries effective regulatory interventions are difficult or impossible to achieve. As international trade of crops and crop products increases, so does the potential that new pests will be introduced into previously pest-free areas. Sometimes, regulatory interventions have little to do with practical pest management, but are used to protect domestic producers from competition with cheaper imports.
Some regulatory interventions are listed below. You may click on each method to view supplementary information about that method.
- Quarantine. Preventing the introduction of known or unknown crop pests by excluding infected materials and vectors from a particular region.
- Eradication. Regional campaigns that aim to completely eliminate a particular pest.
- Control districts. Areas where particular plants or other organisms are prohibited due to their status as alternate hosts for major crops.
- Crop-free periods. Attempts to eliminate pests that move from old crops to younger crops by cutting off their food supply for a designated period.
- Planting time restrictions. By enforcing a planting time for a particular crop, pests that emerge on predictable environmental cues can be managed.
- Enforced growing of particular cultivars. In areas where pest problems are endemic, enforced growing of resistant cultivars can reduce populations permanently.
- Compulsory sanitation measures. Strict sanitation measures, such as burning of residues or burying diseased animals can prevent the spread of persistent pests.
- Regional diversification. By promoting and coordinating crop diversification on a regional scale, area managers can reduce the pest problems that are associated with a dominant monocrop.
- Isolation. Growing valuable crops in an area that is physically isolated from important pests can, along with quarantine, result in sustainable, efficient production.
Legislation can be a tool to manage the development of resistance to a pesticide, as happened in Australia where different cotton-growing regions used a different pesticide regime that rotated from year to year. Equally, legislation can be a very effective tool in the promotion of IPM on a large scale. One of the most famous IPM programs in the world, the Indonesian Rice IPM program, was based on legislation.
Implementing IPM at a national or international level is the role of governments, larger NGOs, and IARCs. The lobbying, promotion, and reform required to change legislation, institutions, and popular perceptions related to IPM is well beyond the scope of this course! Adoption of IPM on a large-scale is infrequently achieved, although there have been some success stories, such as the implementation of rice IPM in Indonesia during the 1980’s, described below.
Indonesian Rice IPM
Indonesia was at one point the world’s largest importer of rice, but Green Revolution varieties led to self-sufficiency by 1984. Due to regular spraying and widespread planting of pest-resistant varieties, pest problems began to increase in severity. For example, 350 000 tons (worth US$100 million) was lost to brown planthoppers (Nilapavarta lugens) in the 1976-77 season despite heavy spraying. By 1986 the Indonesian government was subsidizing rice pesticides by US$100 million per year.
In 1986, a Presidential decree banned 57 broad-spectrum pesticides for use on rice, and subsidies for the remaining narrow-spectrum pesticides on the market were gradually reduced until 1989, when they were withdrawn. In infected areas, only brown planthopper resistant varieties were allowed to be grown, and IPM was massively implemented through intensive farmer training, reorganization of research institutes, and widespread publicity. Indonesian IPM in rice is considered by many to be the most successful implementation program ever conducted.