Module 2: IPM Foundation Principles and Concepts
In this module we will introduce some of the fundamental principles associated with IPM.
The first principle is that the basic management unit of IPM is the agro-ecosystem and that any management action that does not consider the system as a whole may produce unexpected and even undesirable effects.
The second principle is that any pest population can exist at some tolerable level, and this concept can be applied to the development of economic injury levels and action thresholds. Strategies to achieve high levels of control using chemical pesticides can result in the development of serious agro-biological problems. Under some situations, the pest situation may become more acute if pesticides are over-used.
The third principle is that IPM focuses on maximizing the effectiveness of natural control factors to regulate pest populations. Only when this strategy fails are chemical pesticides and other tactics used.
This picture shows a parasitoid laying eggs in the eggs of a pest, so that what emerges is parasitoids, rather than the pest. Such natural control can be important in reducing pest numbers.
The following lessons are found here:
- Lesson 2.1: Agroecology
- Lesson 2.2 Economic Principles
- Lesson 2.3 Agrobiological Effects of Chemical Pesticides
In addition, you will also find the following resoures/simulations as part of this module (see Resources in IPM Home) here:
- Crop production simulator
- Economic injury simulator
- History of IPM
- Agro-ecology tutorial
- Sampling tutorial
- Information on record keeping
Perhaps the most fundamental concept underlying the development and application of IPM is agro-ecology. Agro-ecology involves the application of ecological science to the management of sustainable agriculture, with an agro-ecosystem being a farming system being managed as an ecosystem. The goal of Agro-ecology is to develop and manage sustainable agro-ecosystems. It is a holistic approach to agriculture and agricultural development that links ecology, socioeconomics and culture to sustain agricultural production, farming communities, and environmental health.
IPM is a highly complex technology and knowledge of the ecological basis of the pest problem is essential in order to develop and implement tactics and programs that attempt to alter the crop environment to reduce pest problems. Understanding Agro-ecology allows a practitioner to manipulate the numerous factors in an agro-ecosystem to make the environment unfavourable for a pest while maintaining a favourable environment for the crop. Selecting and balancing non-chemical controls and pesticides to use in combination is a difficult task, but the rewards are worth the effort.
When developing strategies for an IPM program it is important to have an understanding of and reliable information on the ecological basis of the pest problem. This allows IPM practitioners to manipulate factors in the agro-ecosystem to make the environment unfavorable for weeds, insects, and plant pathogens while producing an optimal crop yield.
2.1.1: Features of Agro-ecosystems
The pest situation differs greatly between natural and agricultural ecosystems. While it is very uncommon for natural ecosystems to have serious pest problems, frequent and devastating pest outbreaks are relatively common in agro-ecosystems. Several unique features of agro-ecosystems that contribute to relatively frequent pest outbreaks are discussed in this lesson.
Unique features of agro-ecosystems (after Pedigo, 1996)
- Agro-ecosystems experience frequent major disturbances.
- Agro-ecosystems are dominated by domesticated plants.
- Agro-ecosystems have low species diversity.
- Agro-ecosystems feature a main crop species that is planted, grows and is harvested at the same time.
- Agro-ecosystems receive nutrients from external sources, therefore the main crop is lush and nutrient-rich.
2.1.2: Agro-ecosystems experience frequent major disturbances
Stable ecosystems such as forests exhibit something called temporal continuity. This means that these ecosystems remain relatively undisturbed over long periods of time. For example, a mature forest contains a relatively stable number of mature trees, understory plants, herbivores that feed on these plants, and predators that prey on the herbivores. Minor local disturbances, such as a mature tree dying and falling over, have little effect on the overall stability of the forest.
After a major disturbance, such as a forest fire, ecosystems exhibit dramatic changes. Trees destroyed by fire allow light onto the forest floor and provide a surge of nutrients from their ashes. Weedy annuals grow profusely, herbivore populations increase, and predator populations grow to cull the abundant herbivores. Slowly trees and other perennials grow back, blocking light for the understory plants and stabilizing the herbivore and predator populations. Eventually, the ecosystem returns to a stable state, with a balance of trees, understory plants, herbivores, and predators.
Most agro-ecosystems experience major disturbances on a regular basis. Farmers over the centuries have learned to stimulate lush, rapid growth of weedy annuals by mimicking the conditions that occur after major disturbances in natural ecosystems. By burning existing vegetation or tilling the land, farmers release nutrients and provide space and light for their favoured plants – crops – to grow.
Unfortunately, disturbed agro-ecosystems also appeal to opportunistic pests. Because recently disturbed ecosystems are inherently unstable, agro-ecosystems tend to have more pest outbreaks than undisturbed ecosystems in the same climactic zone.
For a simplified representation of how the climax succession of a forest is reached, see http://www.youtube.com/watch?v=k03vxRYsJ4Y.
2.1.2: Agro-ecosystems are dominated by domestic (cultivated) plants
In natural ecosystems, plant biomass is made up of a large number of species, each occupying a specific ecological niche. In agro-ecosystems, domesticated plants make up most of the biomass, and many niches are left unfilled. Often, as in a field of rice or corn, the plants are of one cultivar of one species, and have very low levels of genetic diversity.
The species cultivated by farmers have been selected and adapted from their wild relatives by humans. The characteristics we require, such as the ability to provide food, fibre, and fuel, are not necessarily characters that will improve a plants adaptability to natural environments. This can be demonstrated by the fact that few domesticated plants are found in undisturbed natural environments, and those that are, have often reverted to their ‘wild’ or ‘weedy’ forms.
On the other hand, domesticated plants are well adapted to the agro-ecosystem environment. In agro-ecosystems, farmers try to provide optimum conditions for their crops through tillage, pest management, watering, fertilization, and other methods. These conditions suit only a few wild plants, many of which have become ‘domesticated’ themselves by evolving into weeds alongside the development of agriculture. Thus, a farmer’s main job is to provide optimum environmental conditions for domesticated crops to grow in, while preventing weeds and other pests from taking advantage of those conditions to the detriment of the crop.
2.1.4: Agro-ecosystems have low species diversity
Most agro-ecosystems are dominated by a single crop species, and undergo frequent major disturbances. For this reason plant diversity, particularly perennial diversity, is low. Species that cannot recover and re-establish after ploughing, burning, or other agricultural activities, are unrepresented in agro-ecosystems.
Because plants are the habitat and food of smaller organisms, low plant diversity generally results in a low diversity of insects, spiders, other invertebrates, and microorganisms.
Soil content and structure in agro-ecosystems often tends towards uniformity due to tillage and the application of fertilizers, manures, and other amendments. This uniformity often leads to reduced biodiversity in the soil. In particular, soils where organic matter content has been reduced due to excess tillage, erosion, or heavy cropping, tend to have low biodiversity.
Ecosystem stability is correlated with ecosystem biodiversity. Agro-ecosystems tend to be unstable relative to natural ecosystems because of their lower overall biodiversity.
An article explaining the relationship between biodiversity and system stability can be found at http://www.nature.com/scitable/knowledge/library/biodiversity-and-ecosystem-stability-17059965.
2.1.5: Agro-ecosystems feature a main crop species that is planted, grows and is harvested at the same time
In most agro-ecosystems, crops are planted, grow, and are harvested at the same time. At any point in time, most of the plants in a field will be the same age and/or be at the same stage of development. This phenomenon is also known as phenological synchrony.
Phenological synchrony in crop plants simplifies management tasks for farmers, and provides them with an abundant food source at harvest time. It also presents a potentially abundant food source to pests throughout the growth of the crop.
Many wild plant populations exhibit phenological synchrony. For example, many bamboo species flower synchronously after many years of vegetative growth. This mass flowering and subsequent mass production of seeds floods the environment with potential offspring (seeds) that are too numerous to be eaten by rats or other organisms. This ensures that a few seeds will survive the inevitable onslaught of herbivores.
Producing a large number of seeds so that only a few will survive is not a viable option for farmers. Farmers must have a strategy to protect their crop from herbivores and other pests that take advantage of phenological synchrony in crops.
Phenology and climatic conditions can be used to help predict pest activity within an IPM programme, see http://www.entomology.umn.edu/cues/Web/049DegreeDays.pdf.
2.1.6: Agro-ecosystems receive nutrients from external sources, so the crop is often lush and nutrient-rich
Farmers provide nutrients in amounts that increase yield or at least prevent nutrient deficiencies. These nutrients can be applied in the form of chemical fertilizers, manures, green manures, and composts. Crops benefit immensely from optimally fertilized soils, showing rapid growth and good plant health. At harvest, many of the added nutrients are removed from the system in the form of seeds, fruits, leaves, and other agricultural products.
In natural ecosystems, nutrients tend to be scarce and most wild plants are nutrient-limited. Nutrients are recycled rather than imported, and large-scale harvesting of nutrients does not occur.
Conditions of nutrient abundance can benefit plants other than the crop in an agro-ecosystem. Weeds benefit from the application of fertilizers when crops are unable to immediately use available applied nutrients.
Excess or unbalanced levels of nutrients can also promote lush, soft growth in crop plants. This is particularly true of nitrogen fertilizers. Crop plants that exhibit lush growth and are macronutrient-rich can be an attractive target for herbivorous pests. Lush growth is also prone to diseases.
2.1.7: Agro-ecosystems experience frequent pest outbreaks
Pest outbreaks do occur in natural ecosystems, but they are more common in agro-ecosystems. Pest outbreaks occur because:
- Agro-ecosystems are relatively unstable and are frequently disturbed.
- There is an abundance of nutrient-rich food available.
- The food is usually available in large quantities over a short period of time.
- The number and diversity of beneficial organisms that can control the pests is relatively small.
Understanding the ecological factors that contribute to pest outbreaks in agro-ecosystems is important when implementing an IPM program. Many of the preventative methods of IPM aim to reduce the influence of the above factors on pest outbreaks through careful management of the ecosystem within the crop environment.
Hom Mali (Jasmine) rice production in Northeast Thailand is a major commercial export crop. From an agroecological perspective, this crop production system differs from the monsoon forest that once covered much of North-eastern Thailand in the following ways.
- As an annual, it lacks temporal continuity, especially when compared with the long-lived forest ecosystem it replaced.
- Most of the biomass in the Hom Mali rice paddies is domesticated rice, although a large number of non-domesticated plants and animals live in and around flooded paddies. There are many trees and hedges within the agroecosystem, although these are declining in numbers.
- Rice paddies in Northeast Thailand have surprisingly high diversity compared to many commercial crop production systems. It is likely that the natural monsoon forest had more, but it would be difficult to say how much more.
- Hom Mali rice is planted, grows, and is harvested synchronously. Within a field, most individual are phenologically synchronized, but over a region, there is some variation in planting dates and growth stages.
- Fertilizers are added to Hom Mali rice paddies, but in moderate amounts compared to rice production in the rest of the region. One reason is that poor fertility actually improves the taste of the rice. At harvest time, large amounts of nutrients are removed as paddy rice, and often the straw is burned.
Pest outbreaks are more frequent than in a natural monsoon forest, but not as frequent as in more heavily fertilized rice crops.
Recommendations for making this crop production system more like a natural ecosystem are:
- Intercrop a perennial cash crop, such as papaya.
- Allow more trees to grow in and around the paddy fields.
- More trees and intercrops may increase biodiversity.
- Coordinate offset planting dates on district or regional basis.
- Return straw to the field as compost.
- Increase habitat for beneficial organisms such as predators and parasitoids that control the major pests.
As agricultural professionals, we tend to talk about the importance of achieving high and sustainable yields in agriculture, but we know that farmers rarely make high yields or sustainability their top priority. We must understand that a farmer, like any other businessperson, knows that survival depends on profit and return on investment. This is equally true of both subsistence and commodity (market driven) farmers.
While a high yield seems like a good thing, it often doesn’t make sense economically. This is because a lot of money must be spent for crop protection, fertilizer, and labour in order to keep the crop in top condition. Although the farmer could get a lot of money at market time, the high expense of obtaining a high yield could result in low profits or even a loss. Moreover, prices may be lowered if each farmer produces top yields, leading to a glut in which supply is greater than demand.
To see how different levels of inputs result in different yields, revenues, costs, and profits, experiment with the Crop Production Simulation.
The implication of this profit-driven farming approach is that to promote widespread adoption of IPM we first need to develop pest control strategies that are cost-effective. Legislation may force farmers to adopt IPM, as with the EU Sustainable Use Directive that will oblige all farmers to adopt such methods. The SUD can be downloaded in many languages at http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32009L0128:en:NOT. However usually we need to convince farmers that IPM pays by translating ecological benefits into economic ones. A pest control measure makes economic sense only if the profit with the new pest control technology is greater than the profit with the old pest control technology.
Most farmers apply a pest control strategy focused on prevention of an anticipated crop loss. They try to predict the probability of loss depending on observation, experience and information. For example, a farmer observing the presence of a particular pest knows that lack of action will probably result in crop and profit loss if nothing is done. Unfortunately, a farmer’s (not to mention an agricultural scientist’s) predictive capability is often limited. This can result in over-reaction or failure to react in time or with sufficient force.
Further complicating the farmer’s IPM and control strategy decision-making process is the fact that IPM does not rely on a single action – i.e. to control or not to control. The farmer must determine which control tools to use, when and how often to use them, the degree (intensity) to which it is used, how much it costs, how effective it will be, and the economic cost:benefit ratio of the control measures.
Agricultural professionals recognize this problem and are working very hard to come up with better decision tools that provide accurate estimates of crop loss and yield functions in response to pests and pest populations. Their goal is to give farmers a better way to assess and quantitatively measure risk and reduce the uncertainty which is a part of decision making. A key tool that farmers can use for economic decision making is concept of Economic Injury Levels and their associated Economic Thresholds.
2.2.1: Economic Injury Levels (EILs)
In response to knowledge of farmer economic behaviour, early IPM researchers concentrated on developing decision-making tools that included the cost of pest control practices in management decision-making. Initial efforts were focused on production agriculture and cost:benefit analyses.
IPM researchers realized that complete control of pests was not necessary (or even desirable from an agro-ecological perspective) to ensure profitability. Almost all crops can tolerate a certain amount of pest damage without affecting yields or crop health. Work therefore focused on determining the relationship between damage levels and crop yield, and establishing what were allowable levels of damage or pest population densities.
EILs need to be determined. As pest populations increase they cause an increasing amount of crop loss. Using an intervention would prevent this crop loss, but there is also a cost to using the intervention. At some point the value of the crop lost will be greater than the expense of intervening to protect the crop. This point is the EIL. If a farmer intervenes at pest population levels below the EIL, the farmer loses money due to inefficient use of the intervention method. If the farmer waits until the pest population has increased past the EIL then he has lost more crop than the cost of intervention. In theory, intervening just before the EIL always maximizes profit.
The “Economic Injury Level” or EIL concept was first proposed by Stern et al. in 1959 as “The lowest population density of a pest that will cause economic damage; or the amount of pest injury which will justify the cost of control.”
Today, Stern’s original definition remains essentially intact with its key elements of economic damage, economic injury level, and economic threshold. Although the EIL concept was founded on economic considerations, it has been expanded to embrace concerns about environmental, social, and resource concerns, and sustainability (see Pedigo and Higley, 1992). This has led to the coining of additional terms for injury level such as Aesthetic-injury Level which is “Analogous to the EIL, except that aesthetic rather than economic considerations motivate the pest management decisions.”
The following graph may help to clarify the concept. It shows a pest population increasing over time until it reaches a point (the EIL) where returns (benefits) to an investment in control are greater than the costs of the control measure.
A key point to remember about EILs is that the concept is flexible and may vary from area to area, crop variety to crop variety, and even between two adjacent fields, depending on numerous factors including crop growth stage and specific agronomic practices.
2.2.2: Calculating EILs
Gain thresholds are a simple way to determine the relationship between the costs of an intervention and the value of the harvested crop. Gain thresholds are simply the cost per area of an intervention divided by the value per unit of harvested crop. This can be expressed as:
gain threshold = intervention cost per area/value per unit of crop
For example, if the cost of treating a field with a pesticide is $75 per hectare and the value of the crop is $3 per kilogram then the gain threshold would be:
gain threshold = $75 per hectare/$3 per kilogram = 25 kilograms per hectare
This means that the increase in yield, or gain, has to be 25 kilograms per hectare for this pesticide to be economic.
Theoretically, the EIL occurs where the cost of damage is equal to the cost of control. This has been expressed by Pedigo (1986) using the following equation:
V x I x P x D = C/K
V = value per unit of produce per area
I = injury units per pest per area
P = number of pest per area
D = damage per injury unit
C = intervention cost per area
K = proportion of damage controlled
As the density of pests is the usual way that farmers measure the EIL, we can express this equation as:
P = C/VxIxDxK or EIL = C/VxIxDxK
EILs are usually determined empirically, by experimentally growing crops under different pest population levels or by using production data from farmers. Generally, researchers determine the following:
- estimate the units damaged per pest per area
- determine the gain threshold
- calculate EIL as EIL = gain threshold/loss per pest
So from our previous example, if the gain threshold is 25 kilograms per hectare and we determine the damage per pest to be 0.015 kilograms per pest per hectare, then the EIL would be:
EIL = 25 kilograms per hectare/0.015 kilograms per pest per hectare = 1667, or approximately 1700 pests per hectare. So if we were to sample this field and found more than 1700 pests per hectare, we should use an appropriate intervention. If there are fewer than 1700 pests per hectare, then we can save money by not intervening as the gain threshold is not high enough to make intervention economically feasible.
Play the Economic Injury Level Simulator.
2.2.3: Economic Threshold (ET) or Action Threshold (AT)
It should be obvious from the previous discussion that the EIL represents the economic breakeven point. It therefore follows that a farmer will need to maintain pest populations below the EIL or take some action before a pest population reaches the EIL and causes economic damage. The point at which the farmer can reliably predict that the population will reach the EIL is known as the Economic or Action Threshold and always represents a pest density or level of pest damage lower than the EIL. It is usually calculated to be about 80% of the EIL.
The action threshold can be defined as “The pest density at which control measures should be implemented to prevent it from reaching the Economic-injury Level (point where economic loss occurs.”
2.2.4: Limitations of the Concepts
Although a powerful tool, you should realize that many factors have limited both the design of new economic thresholds and the development of existing ones. Some of the major limitations include:
- Lack of a thorough mathematical definition of the ET.
- Lack of valid EILs due to difficulties of making precise estimates of crop loss and yield functions in response to pest population.
- Inability to make cost effective and accurate population estimates.
- Inability to predict critical ET variables such as market values and population trends.
- Lack of a simple means to incorporate externalities, especially environmental costs into EILs.
- Subjective nature of EILs. Depending on an individuals circumstances and viewpoint, the EIL could vary widely even for the same pest on the same crop.
Pedigo’s article on Economic Thresholds and Economic Injury Levels can be viewed at http://ipmworld.umn.edu/chapters/pedigo.htm.
A PDF file on economic decision-making can be downloaded at http://extension.usu.edu/files/publications/publication/economic-injury-level96.pdf.
IPM: Opportunities and Challenges for the Private Sector can be viewed at http://ipmworld.umn.edu/chapters/hutchins.htm.
After going through Module 1 of this course, you are already aware of the diverse and sometimes divisive opinions about the role of chemical pesticides in agriculture. People are concerned about the presence of these products in their food and the environment, and their potential for harm. There are also some fundamental agro-biological problems associated with their use that have a huge influence on how and why IPM programs are implemented. There are three main agro-biological problems that can arise when chemical pest control products are used. There are pesticide resistance, pest resurgence, and pest replacement.
2.3.1: Pesticide Resistance
Pesticide resistance develops in an insect population because of natural genetic variation in a population’s susceptibility to pesticides. In a pest population that has not been previously exposed to a new pesticide, most individuals are susceptible, but a small percentage of individuals are resistant. When the pesticide is applied to the pest population, many of the susceptible individuals die, while most of the resistant individuals survive. This survival means that the proportion of resistant individuals in the population increases. As resistance is usually controlled by genetics, resistance can be inherited by the next generation. That next population of pests will have a higher proportion of resistant individuals than the original population. If the same pesticide is applied again, it is less effective because more of the individuals are resistant.
Each time the pesticide is applied to the pest population, susceptible individuals are killed disproportionately, and the percentage of resistant individuals increases. If the farmer increases the dosage or frequency of spraying, moderately resistant individuals may be killed, but this results in an even more resistant survivor population. Eventually, after repeated and intensified applications of the same pesticide to the same pest population, the pesticide becomes ineffective.
Pesticide resistance was noted as early as 1914 (Melander). 504 species of insects were resistant to one pesticide or another by 1993, with 17 species resistant to all major classes of pesticide. Chemical pesticide resistance has also been documented in hundreds of species of fungi and plant pathogens, around 300 species of weeds, and several species of rodent. There is a database of arthropod resistance to pesticides at http://www.pesticideresistance.com/.
Pesticide resistance is a serious issue for IPM practitioners even though repeated sprayings are not generally practiced in IPM. If other non-IPM-practicing farmers in the region spray a pesticide repeatedly and the local pest population develops resistance, then this pesticide becomes useless to IPM farmers as well. This reduces the intervention options available to an IPM farmer.
Information on pesticide resistance can be found at the British Columbia website, see http://www.agf.gov.bc.ca/pesticides/a_4.htm.
2.3.2: Strategies to Reduce the Development of Pesticide Resistance
From your experience with the Pesticide Resistance Simulator we hope you have a good understanding of how pesticide resistance occurs and how quickly it can develop in an insect population. Even under ideal IPM conditions, resistance to pesticides can develop. Several strategies have been suggested to prolong the useful product life of pesticides. These are listed below.
- Low kill strategy. Using a low dosage of pesticide or not spraying part of the crop will allow some susceptible individuals to survive. These susceptible individuals will breed and contribute genetic susceptibility to the next generation, maintaining susceptibility in the pest population. A high kill strategy eliminates most or all susceptible individuals, leading quickly to resistance.
- Switching life-stage target. Resistance develops more rapidly if juvenile forms are targeted as opposed to adults. Targeting both juvenile and adult forms develops resistance even faster. Applying pesticides to the adult stage will increase the useful product life.
- Limited spray windows. By spraying for only a certain period in the season, only one generation is exposed to the pesticide and resistance development is delayed. Pesticide rejuvenation. When the resistance mechanism to a particular pesticide is known, it may be possible to add a compound to the pesticide which blocks the resistance mechanism. This extends the usefulness of the pesticide, but selects for stronger resistance.
- Diversification of control methods. Using IPM principles is one of the best ways to prevent the development of pesticide resistance. When pests are exposed to multiple controls, the pest population is less likely to develop resistance to any particular one. In particular, where cultural, physical, or biological controls are used in combination with judicious pesticide application, the effective life of the pesticide can be extended a great deal.
2.3.3: Pest Resurgence and Pest Replacement
Pest resurgence and pest replacement are also relatively common agricultural problems associated with the use of chemical pesticides. Pest resurgence occurs when a pest population that has been suppressed using pest control measures rebounds to a population level higher than before it was controlled. Pest replacement occurs when a minor or secondary pest becomes an important pest due to control measures used on the target pest population. Both of these are a result of the effect of chemical pest control products on key biological processes. The main causes of these problems are reduction in natural enemy populations, hormoligosis, and removal of competitors. Homoligosis (explained further in 2.3.5 below) occurs when several sub-lethal stresses occur that stimulate organisms and populations in a stressful environment. There may be increased development rate of insects or mites, or increased reproduction, following exposure to insecticides.
2.3.4: Reduction in Natural Enemy Populations
Reducing the population of a pest’s natural enemies can cause pest resurgence or pest replacement. Pest resurgence can occur when a broad-spectrum control measure reduces both pest and natural enemy populations. The pest population often rebounds faster than the natural enemy population, resulting in a resurgence of pest levels above the original equilibrium level. If control measures are repeated, this new equilibrium between a reduced natural enemy population and an enhanced pest population will be maintained.
Another effect of reducing natural enemies is that a minor pest can become a major pest – pest replacement. When a control measure is applied, the natural enemies of a minor pest are reduced along with the target pest population. The minor pest may or may not be directly affected by the control measure, but once the natural enemies of the minor pest are reduced or eliminated, the population of the minor pest increases to harmful levels.
In summary, pest control measures can adversely affect natural enemy populations in the following ways:
- The control measure kills natural enemies directly. Natural enemies are often more susceptible to pest control than pests.
- The control measure suppresses the pest population to a point where the natural enemies starve.
- Affected pests are consumed by natural enemies and secondary toxin accumulation kills the natural enemies.
- The natural enemies are repelled by the controlled measure and emigrate.
Many pests show increased vigour when exposed to sub-lethal doses of pesticide – a phenomenon known as hormoligosis. Hormoligosis is an important cause of both pest replacement and pest resurgence and can be explained by the following mechanisms. If a chemical has a direct favourable influence on the physiology and/or behaviour of the target pest species, pest resurgence may result. If suppression of the target pest is accompanied by hormoligosis in the minor pest, then pest replacement can occur. Hormoligosis results when:
- The control measure causes a change in the physiology of the pests (e.g. increased metabolism).
- The control measure causes a shift in the environment of one or more pests (e.g. pesticides cause a nutritional change in a plant food source).
- The control measure causes a shift in the behaviour of one or more pests (e.g. pesticides cause pests to engage in reproductive behaviours).
2.3.6: Removal of Competitors
If two species of insect are competing for the same resources, the selective removal of one of them will benefit the other species. If a pesticide disproportionately affects a pest’s competitors, when the pest population rebounds it will resurge to fill the niche left by it’s competitor. If a target and minor pest compete for the same resources then suppressing the target pest will benefit the minor pest and pest replacement can occur.
2.3.7: Preventing/Avoiding Pest Replacement and Pest Resurgence
Several strategies can be employed to prevent pest resurgence and replacement. These include.
- Avoiding hormoligosis.
- Avoiding natural enemy destruction.
- Avoiding competitor destruction.
- Use physiologically selective control measures.
- Use ecologically selective control measures.
- Inoculative release of natural enemies.
- Diversification of control methods.
Using IPM principles is one of the best ways to prevent pest resurgence. When target pests are exposed to multiple controls, minor pest populations are less likely to explode. If competition with the target pest is the cause of resurgence, then specific strategies to manage minor pests must be employed. In particular, where cultural, physical, or biological controls are used in combination with judicious pesticide sprays, pest replacement can be avoided.