Perspectives in Ecological Theory and
Integrated Pest Management
Bogor Agricultural University
Here are a few terms that will frequently be used in this module:
Ecology is the study of the interrelationships of organisms and their surrounding environment. In other words, ecology is the study of how organisms interact with their abiotic and biotic environment.
Abiotic components are the non-living components of the environment such as space and weather. Pesticides could also be considered an abiotic factor.
Biotic components of the environment are living organisms. Biotic components of the environment include primary producers (e.g., rice plants), herbivores (e.g., rice insect pests), natural enemies of rice pests (e.g. parasitoids, predators, pathogens) and competitors (individuals of the same or different species competing for resources).
Habitat is the environment in which the individual organism lives.
A species is a group of reproductively isolated organisms. It is the basic unit of taxonomic classification of organisms, designated in scientific nomenclature by a unique Latin binomial with a genus and specific epithet. For example the scientific name of the rice yellow stem borer is Scirpophaga incertulas.
Population is a group of individuals of the same species occupying a distinct space.
Community is the species that occur together in space and time.
Ecological niche is all the components of the habitat with which an organism or population interacts.
Pest is an organism that interferes with the availability, quality or value of a managed resource. In an agronomical sense a pest is any organism or microbe with the potential to lower the value of a crop by reducing crop yield, quality or reproductive ability (e.g. storage pests that reduce seed viability).
Integrated Pest Management (IPM)
IPM is an ecosystem-based pest management strategy that focuses on long-term prevention of pests and their damage through a combination of techniques such as biological control, habitat manipulation, modification of cultural practices, and use of resistant cultivars. Pesticides are used only when needed as determined by established guidelines.
Economic Injury Level (EIL)
EIL is the lowest population density of a pest that will cause economic damage to a crop. Sometimes referred to as the �damage threshold�
Economic Threshold Level (ETL)
ETL is the pest population level at which a control action, such as pesticide application, should be considered in order to prevent economic loss. ETL is synonymous with �action threshold� or �treatment threshold�. At the ETL, the economic return of the control effort is greater than the cost of control.
Ecology and Pest Management
IPM introduced ecological thinking into crop protection. Ecology gives context to pest management; otherwise there would be a hodgepodge of treatments put together for pest control. No longer do we think of “pest control” simply to reduce crop damage, rather we are concerned with the prevention of outbreaks of pests. Why do pest outbreaks occur? Pest outbreaks usually occur if the normal equilibrium of populations is disturbed by natural disasters (e.g., drought, floods, fire), crop management (e.g., removal of natural enemies by insecticides, over-use of nitrogen); or massive pest immigrations.
Some of the ways we can improve pest management include:
- Reducing pest levels � e.g., using insecticides, cultural practices
- Improving natural enemy levels � e.g., establishing refuges
- Improving resistance of the plant to pest attacks (e.g. nutrient management and breeding for host plant resistance to pest)
It is important to know what limits pest population growth. Abundance of resources and natural enemies influence pest abundance. In the context of management it is also important to know the most vulnerable stage of the pest population.
Ecological Principles And Applications
Understanding the interactions among pests, natural enemies, host plants, other organisms and the environment improves pest management decisions. Understanding the ecological factors that allow pests to adapt and thrive in a particular ecosystem will help to identify weak links in the pests’ life cycle and factors that can be manipulated to manage them.
“The law of constant final yield” (Kiraet et al. 1953) states that yield is constant over a wide range of densities for many wild plants. Does this law apply to cultivated crops like rice? Because of the application of fertilizers, the availability of essential minerals are not necessarily a limiting factor to yield as density increases. But what happens to the pests (e.g., insects) feeding on the rice as we provide nutrients? The maximum population size of a species that can be supported indefinitely in an environment is called the “carrying capacity”. By adding fertilizer to the soil of the rice field we increase carrying capacity (K) of the rice environment. This also increases K of the rice field for insects, pathogens and weeds. Increasing K of rice pests also increases K of their natural enemies. If the EIL is greater than the carrying capacity, then the species is self-limiting and cannot be considered a pest
Some species have K>EIL, but have a typical abundance below EIL due to natural enemies. Such species are potential pests that may become actual pest if natural enemies are removed, so that and the rate of population increase of the pest exceeds the rate of mortality. The initial population size (Nt), births (B), deaths (D), immigration (I) and emigration (E) determine the present pest population. Births and immigration increase pest population while deaths and emigration reduce it. The present population of a pest can be described by the equation:
Nn = Nt + B – D + I – E.
Importance Of Scale
The first level of organization in an ecosystem is the individual followed by population, community, and ecosystem (Figure 1). There are three levels of IPM implementation:
Level I: Population level integration – Control of a single pest species in an individual crop.
Level II: Community level integration – Management of pests (insects, weeds or pathogens) for a single crop.
Level III: Ecosystem level integration – Management of all pests within entire cropping systems.
3.1. Ecology of the Individual
The fundamental unit of ecology is the individual organism (Schowalter 1996). Understanding the factors effecting individual behavior enables us to manipulate pest behavior to our advantage. For example, by understanding the chemical communication system of insects, we can build pheromone traps that attract key pests. By understanding how pests find, exploit, and allocate resources we can either interfere with the process, or use the process to trap pests.
3.2. Population Ecology
Population ecology is the study of the variables that determine the abundance and distribution of a population in time and space. The genetic makeup of the population together with the local environment and ecological factors determine its success – how well it survives and how fast it grows.
3.2.1. Population density
The number of individuals of a species in a defined area is a measure of population density. Population size increases and decreases over time. There are factors such as resource availability, competition, parasitism, predation, climate, etc. that usually operate in an ecosystem to keep populations within certain boundaries.
Figure 1. Levels of organization in an ecosystem
(Source – Flint, ML and Gouveia, P. 2001)
3.2.2. Age distribution
Birth and death rates, and immigration and emigration determine age distribution (proportion of individuals in each age group) of a population. Fecundity (rate at which females produce eggs), fertility (rate at which females produce zygotes), and sex ratio (proportion of male and female in the population) affect birth rate. Typically, expanding populations have a large percentage of young individuals while declining populations have a large percentage of old individuals, and stable populations have a relatively even distribution among age groups. Many of the pests in the managed systems are short-lived, with a life cycle well synchronized with the culture of the crop.
3.2.3. r– and k– species
Depending on their reproductive strategies, species can be characterized as r or k species. Here r is the instantaneous rate of population increase while k is the carrying capacity. The r-species possess characteristics of high biotic potential, rapid development, early reproduction, single period of reproduction per individual, short life cycle, and small body size. On the other hand k-species possess characteristics of low biotic potential, slow development, delayed reproduction, multiple periods of reproduction per individual, long life cycle and larger body size. Populations of r-species usually remain below the carrying capacity and are regulated by the density-independent (affect populations regardless of population density) factors; while populations of k-species are usually maintained near the carrying capacity and regulated by density dependent (have a different effect when populations are high than when they are low) factors. In disrupted habitats r-species are more common while k-species are common in stable habitats. Many of our agricultural pests are r-species. Most organisms however actually have attributes that fit both r and k species.
Dispersal is movement of individuals or their offspring into or out of an area. Dispersal allows individuals to colonize new areas of crop fields. Dispersal, along with birth and death rates, regulates population size, and plays an important role in evolution through mixing of genes between populations. Dispersal is accomplished through immigration (movement into a population), emigration (movement out of a population) or migration (frequent movement into or out of a population area).
3.2.5. Population growth
Population growth occurs when birth rates exceed death rates or immigration exceeds emigration. Population size may be regulated by physical factors (weather, water and nutrient availability) and by biological factors (food availability, predators, parasitoids, competitors, diseases). Factors that affect population density can be density-dependent or density-independent. Competition for resources, parasitism, predation and diseases are example of density-dependent factors while flood, drought, fire and other climatic conditions and most pest control actions are examples of density-independent factors. Density-dependent factors are important in regulating populations and in keeping populations at equilibrium.
Population growth can be explained by using the demographic equation
Nn = Nt + B – D + I – E.
A population may grow exponentially or logistically (Figure 2). The exponential or geometric population growth curve is described by the formula rN = dN/dt; where N is present population size, t is time, r is a constant called the instantaneous rate of population increase. The logistic growth curve dN/dt = rN (K-N)/K where, N, t, r are the same as in the exponential growth model and K is the carrying capacity, or the maximum number of individuals the environment can support. However, populations cannot grow exponentially for ever. When a population is growing in a limited space, the density gradually rises until interaction reduces the rate of increase ultimately leading to a reduction in population growth. This is logistic growth and the growth curve is sigmoid or S-shaped. The S-curve differs from the geometric curve in two ways: (i) it has an upper asymptote and (ii) it approaches this asymptote smoothly, not abruptly.
Figure 2. Exponential (geometric) and logistic (S-shaped) growth curves
(Source – Krebs 1972)
3.3. Community Ecology
Community ecology is the study of co-existing, interdependent populations (of different species – Figure 1). In many cases relatively few species exert a major controlling influence on the entire community. Interactions among populations can be complex and may be sensitive to disturbance or fluctuations. Control of one component of the community can have a positive or negative impact on other organisms in the community. The major interactions within the community are competition, parasitism, predation, mutualism and commensalism.
Competition between organisms is for limited supplies of essential resources, such as food in the case of animals, and water, nutrients and light in the case of plants. There are two types of competition: interspecific and intraspecific. The competition between individuals of the same species is intraspecific competition. Intraspecific competition can be of contest type (some individuals survive at the expense of others) or scramble (all individuals obtain insufficient resources) type. On the other hand, the competition between individuals of different species is interspecific competition. In such a case, one species is likely to be superior to the other in a given habitat.
Predation is the consumption of one organism by another where the consumed organism (prey) was alive when the predator first attacked it. Predatory behavior is widespread among insects, spiders and mites. More than 40 families of insects, all 60 families of spiders, and 27 families of mites are predators.
Parasitism is a relationship between two species in which the host is harmed, but not killed immediately, and the species feeding on it (parasite) is benefited. A parasite is an organism that obtains its organic nutrients from one or very few host individuals without causing immediate death. A parasitoid is an insect that parasitizes and kills other insects. Parasitoids are parasitic only in their immature stages, killing the host before emerging as a mature larva or adult. However, parasitoids are often referred to as insect parasites. Members of 43 families of the order Hymenoptera and some members of 12 families of the order Diptera are parasitoids of arthropods. Parasitoids that insert their eggs into a host�s body are called endoparasitoids, and those that lay their eggs externally and whose larva develops externally are called ectoparasitoids. Parasitoids of non-parasitoid hosts are primary parasitoids and parasitoids that attack other species of parasitoid are hyperparasitoids.
Mutualism is the relationship between two species that have developed a positive, reciprocal dependency, and both populations benefit from this association. Through the relationship, both the species strengthen their chances for survival, fitness, or growth. Mutualism is widespread.. The relationship between certain species of ants and mealybugs provides an example of mutualism. Mealybugs produce a waste material, known as honeydew, consisting of highly concentrated plant sugars. Ants harvest this material and in the process protect mealybugs from natural enemies. Mealybugs are an important pest of pineapple. Commercial pineapple plantations in many parts of the world find it advantageous to control ants, so that natural enemies are able to keep mealybug populations in check (Jahn and Beardsley 1996, 2000; Rohrbach et al. 1988).
Commensalism is the association between two species in which one species benefits from the association, and the other is unaffected. An example is algae growing on a turtle’s shell that benefits from the substrate provided but it does not apparently harm or benefit the turtle.
3.4. Ecosystem Ecology
An ecosystem is the community of organisms in an area and their non-living environment (Figure 1). The concept of communities interacting with their physical environment such as dead matter, minerals, water; and energy is the basis of ecology. It can be defined in terms of energy and matter fluxes and can be described at various scales such as crop ecology, landscape ecology or global ecology.
3.4.1. Food chain and food web
Energy flow drives the ecosystem, determining limits of the food supply and the production of all biological resources. Light energy from the sun is captured by green plants and converted to chemical energy. Energy is stored in plants as carbohydrates and used by the plant to support all functions such as vegetative growth, fruit maturation and respiration. Other organisms use and convert this chemical energy to various forms through food chains (Figure 3). A food chain is a succession of organisms in a community that constitutes a feeding sequence in which food energy is transferred from one organism to the next as each consumes a lower number and in turn is preyed upon by a higher number (Figure 4). At the bottom of the chain is a photosynthesizing plant, usually followed by an herbivore, a successions of carnivores, and finally decomposers. At each step, some of the chemical energy is assimilated and used by the organism and the rest is released in respiration and waste products. The Food web is the complex interrelated food chains in a community (Figure 5). Trophic structure is the series of links in a food web that describe the transfer of energy from one nutritional level to the next. The goal of crop production is to maximize ecosystem energy into a harvestable product; utilization of plant energy by pests is undesirable as it takes away from crop production.
Figure 3. In a typical biogeochemical cycle, the minerals or inorganic elements required for the growth and development of living organisms circulate from the non-living to the living and back to the non-living components of the ecosystem (Source Flint, M.L and Gouveia, P. 2001)
Figure 4. Links of food chain (source – Flint, M.L. and Gouveia P. 2001)
Agroecosystems or agricultural ecosystems are predominantly monocultures. In a monoculture, the age and genotype of crop plants are relatively uniform and species diversity is limited. Complex food webs are changed into simple, short food chains. The physical diversity of the system is much lower than natural ecosystems. The uniformity of monoculture systems encourages pest outbreaks.
Figure 5. A schematic diagram of a food web in alfalfa. Each arrow represents a transfer of food, or energy from one organism to another. The web becomes more complex as more species are introduced into the system. (Source – Flint, M.L. and Gouveia, P. 2001).
4. Summary And Conclusion
Understanding how organisms interact with their environment is the basis of successful pest management decision-making. If the economic injury level is greater than the carrying capacity then the species is self-limiting and cannot be considered a pest. Potential pests become actual pests if natural enemies are removed (e.g. by insecticide application) and if the rate of pest population increase exceeds the rate of pest mortality.
5. Selected References
Begon M, Harper JL, Townsend CR. 1990. Ecology: Individuals, Populations and Communities (2nd ed). Blackwell Science, London, 1068p.
Department of Entomology, College of Agricultural Science, Penn State. http://www.ento.psu.edu/home/courses/379A/EcologyPresn/.htm
Flint ML, Gouveia P. 2001. IPM in Practice: Principles and Methods of Integrated Pest Management. University of California, 296p.
Huffaker CB, Cutierrez AP. 1999. Ecological Entomology. John Wiley & Sons, New York, 756p.
Jahn GC and Beardsley JW. 1996. Effect of Pheidole megacephala (Hymenoptera: Formicidae) on the survival and dispersal of Dysmicoccus neobrevipes (Homoptera: Pseudococcidae). J. Econ Entomol. 89, 1124-1129.
Jahn GC and Beardsley JW. 2000. Interactions of ants (Hymenoptera: Formicidae) and mealybugs (Homoptera: Psedococcidae) on pineapple. Proc. Hawaiian Entomol. 34, 181-185.
Krebs, CJ. 1978. Ecology: the Experimental Analysis of Distribution and Abundance (2nd ed.). Harper & Row, New York, 678p.
NRC 1996. Ecologically Based Pest Management: New Solutions for a New Century. National Research Council, Washington DC, 144p.
Rohrbach, K. G., J. W. Beardsley, T. L. German, N. Reimer and W. G. Sanford 1988. Mealybug wilt, mealybugs, and ants on pineapple. Plant Disease 72, 558-565.
Schowalter TD. 1996. Insect Ecology: An Ecosystem Approach. Academic press, San Diego, 483p.
Speight MR, Hunter MD, Watt AD, 1999. Ecology of Insects: Concepts and Applications. Blackwell Science, London, 350p.