Across millennia, humans have used a variety of approaches in attempts to maintain pest insects at tolerable levels. The character of these approaches has evolved over time. Since the late 1960s, an approach termed integrated pest management (IPM) has ascended to dominance internationally. In essence, IPM is a decision-based process involving coordinated use of multiple tactics for optimizing the control of all classes of pests (arthropods, microbial pathogens, vertebrates, weeds) in an ecologically and economically sound manner.
HOW INSECT PESTS ORIGINATE
The manner by which an insect has become a pest influences the strategy used for managing it within an IPM framework. Some species have reached the status of pest because human thresholds for tolerating them have decreased as economic well-being has increased. Other species have become pests as a consequence of the availability of abundant resources for their survival and reproduction; resource concentration in time and space is conducive to rapid buildup of insect populations. Still other species have become pests in regions of their origin when they expanded their host range from comparatively unimportant native hosts to economically important introduced hosts. Finally, some species have become pests almost instantly upon being transported by humans to new locales having favorable resources but devoid of effective natural enemies.
CLASSIFICATION OF INSECT PESTS
For purposes of devising an appropriate IPM strategy, insect pests may be classified as key, secondary, or induced. Key pests are those whose populations, if unmanaged, repeatedly exceed tolerable levels; these are the principal focus of IPM endeavors. Secondary pests are those whose populations occasionally reach intolerable levels; their potential threat is recognized by the very act of devising IPM strategies. Induced pests are those whose populations rarely exceed tolerable levels under natural conditions, but if they become resistant to pesticides or other single-tactic control measures that harm their natural enemies, they can reach outbreak proportions. Usually induced pests return to nonpest status under a true IPM approach.
EVOLUTION OF CONCEPT OF IPM
Through trial and error across centuries, humans gradually came to use tactics such as cultural control, host resistance, and biological control in efforts to protect themselves, livestock, crops, and forests against pests. By the latter half of the 19th century, some pest control strategies that blended these tactics could rightfully be considered to be the precursors of modern IPM strategies. During the 20th century, efforts to maintain pests at tolerable levels became more formalized as they became more intensive; these efforts can be considered as having progressed along the following four pathways.
“Pest control” was the terminology used during the first half of the 20th century to describe the set of actions taken to avoid, attenuate, or delay the impact of pests. Early in the century, inorganic and botanical insecticides gained increasing prominence as a control tactic against pest insects. By the mid-century, use of organosynthetic insecticides supplanted virtually all other tactics in becoming the dominant approach to insect pest control, especially in developed countries.
“Integrated control” surfaced about 1950 in California as a concept of pest control aimed at combining and integrating biological and chemical control. The emergence and eventual popularity of this concept had roots in problems associated with overuse of organosynthetic insecticides, particularly a surge of key, secondary, and induced insecticide-resistant pests and environmental harm caused by insecticides. The publication of Silent Spring by Rachael Carson in 1962 was especially important in spurring widespread interest in the principles of integrated control.
“Pest management” represents a shortened version of “protective population management,” a concept coined in 1964 by Australian ecologists to emphasize direct human interference in maintaining pests at tolerable levels, as opposed to reliance on unmanaged natural abiotic and biotic factors acting on pest populations. The concept of pest management embraced a broader range of pest control tactics than did the concept of integrated control. Until the late 1960s, both these concepts flourished simultaneously in accenting the need to move beyond the use of insecticides as a sole pest control tactic.
“Integrated pest management” originated in 1968 as a contraction of “integrated pest population management,” an expression used first in 1967 by R. F. Smith and R. van den Bosch. Soon afterward, the abbreviation “IPM” came into use worldwide for signifying a desirable and holistic approach to controlling pests. Numerous definitions have been put forward for IPM, but one that captures the broad and essential elements of many others is the following: IPM is a decision support system for the selection and use of pest control tactics, singly or harmoniously coordinated into a management strategy, based on cost-benefit analyses that take into account the interests of and impacts on producers, society, and the environment. No single definition, however, is likely to encompass all facets of IPM for all time. IPM has been and likely will continue to be an evolving concept. For example, a compendium of IPM definitions, available on the World Wide Web (http://ipmnet. org/IPMdefinitions/home.html), listed 67 definitions in the mid-2002.
ECOLOGICAL FOUNDATION OF IPM
Ecology is the study of relationships among organisms and their environment. Consideration of these relationships usually begins with focus on individuals or populations of a single species and subsequently broadens to include communities of organisms and, eventually, ecosystems.
Conceptually, the foundation of IPM is ecological. Ideal IPM programs are those that fully embrace ecosystem structure and processes in time and space. In reality, ecological complexity increases dramatically with each step from population to community to ecosystem. Such complexity challenges the realization of ideal IPM programs.
One approach that has been taken toward recognition of the ecological foundation of IPM, especially in developing countries, accentuates the pest-suppressing properties inherent in natural ecosystems as primary building blocks for the construct of human-designed ecosystems. After system construction based on ecological principles, the intent is to minimize human intervention to the greatest extent possible while still maintaining pest populations within tolerable levels.
Another approach, common in developed countries, takes as its starting point an existing ecosystem constructed by humans and aims at reducing negative impact in a succession of steps used in the management of pests. Such an approach usually commences with a focus on the ecology of a single-species population and may expand to consideration of structures and processes associated with communities and ecosystems.
ADVANCING LEVELS OF INTEGRATION IN IPM IMPLEMENTATION
A hierarchical structure results from the ecological foundation of IPM that lends itself to viewing IPM as progressing in scope through a series of ecologically rooted steps. These steps are characterized by ascending levels of complexity and spatial scale: from focus on a single-species population in a restricted locale to focus on a community of pest and other organisms in a larger area to focus on a whole ecosystem. Coincident with this ecocentric hierarchy is another step-wise hierarchy conceived of as a vehicle for measuring progress in achieving the goals of IPM. This hierarchy comprises a succession of levels from single-tactic (almost invariably based on pesticide use) to multitactic measures of pest and habitat management. The steps further involve ascending from focus on a single pest species in a single class of pests (e.g., insects) to multiple pest species across all classes of pests (insects, microbial pathogens, vertebrates, and weeds). These distinctive hierarchies can be blended in the form of a continuum of advancing levels of integration in IPM implementation. Three more or less distinctive levels along the continuum are described in the following sections (Fig. 1). Degree of success in integration at each level is shaped not only by ecological processes but also by government policy, regulatory legislation, social relations, economic forces, and cultural background that may enhance or constrain progress.
FIRST-LEVEL IPM
In the most basic and also the most widely practiced form of IPM, emphasis is on monitoring development and/or abundance of a single pest species at a single locale (e.g., a household, cow barn, greenhouse, cropped field, or woodlot) and using thresholds for deciding whether to take action. Application of a pesticide is by far the most common form of action taken under the first-level IPM. Integration occurs when abundance of natural enemies of the pest in question also is considered in the decision-making process and when selection among candidate pesticides involves explicit attention to minimizing harm to these and other beneficial organisms. This form of IPM has been characterized by some as “integrated pesticide management.”
Monitoring Pest Development
Because the developmental rate of an arthropod is regulated largely by temperature, the monitoring of developmental rate for pest management purposes usually takes the form of measuring accumulation of heat units above a threshold temperature at which development begins. At temperatures above these fostering the maximal developmental rate, development may decrease. Such decrease has not been investigated for most pest arthropods and has not yet played a significant role in making pest management decisions.
The simplest and most prevalent approach to measuring accumulation of heat units above developmental threshold temperature involves the use of degree-days (DD). For a specific date, the number of accumulated DD equals the average temperature of that date minus the developmental threshold temperature of the arthropod. Several procedures have been devised to estimate average daily temperature. The most common one, albeit somewhat crude, consists simply of averaging
FIGURE 1 Levels of IPM integration: main targets, ecological scales, and levels of ecological complexity.
the maximum and the minimum ambient temperature of the day. To illustrate the DD approach, if the high and low temperature for a given day were 30°C and 20°C, respectively, with a developmental threshold temperature of 10°C, then 15 DD would have accumulated on that day.
Pest development as monitored by DD accumulation may benefit decision making under first-level IPM in several ways, particularly for optimal timing of management activities. For example, ability to predict when a majority of pest adults is about to emerge from pupae is useful for optimal timing of deployment of traps for monitoring adults. Knowledge about when oviposition is likely to begin and peak can facilitate optimal timing of pesticide application against newly hatched larvae, which often is the stage most vulnerable to pesticide treatment. Sometimes this determination is made in conjunction with date of first capture of adults by traps, known as a “biofix” point for initiation of DD accumulation. The ability to forecast when a majority of larvae or nymphs is at a particular growth stage can aid in optimal timing of sampling their abundance and the abundance of their natural enemies.
Monitoring Pest Abundance
Ideally, an IPM practitioner would have available a precise count of the number of individuals of an insect pest species present in an area of concern; realistically, obtaining information on absolute densities of pests is prohibitively costly. Therefore, most practitioners rely on imprecise estimates of pest population density obtained by using one or more population sampling techniques. The intent is to capture a more or less consistent, if unknown, proportion of the pest population. Choice of appropriate sampling technique varies considerably according to pest species, its developmental stage, and the crop plant or other habitat where the pest occurs.
For sampling comparatively mobile individuals such as adults, traps using odor and/or visual stimuli are common tools. Odor stimuli usually consist of synthetic equivalents of either attractive sex odors (sex or aggregating pheromones) or attractive food or host odors (kairomones). Visual stimuli normally rely on synthetic mimics of visually attractive sites where feeding, mating, or egg laying occurs.
For sampling less mobile individuals such as larvae, common techniques include visual searching of the target area accompanied by direct counts of detected pests, use of a sweep net (especially effective for sampling individuals on foliage of nonwoody plants), and use of a loose or framed cloth placed beneath vegetation that is shaken or tapped to dislodge pests. Sampling immobile individuals such as eggs or pupae usually is done by visual inspection.
To obtain an acceptably accurate and cost-effective estimate of the size of a pest population by means of one of these techniques, careful attention must be given to the program under which sampling is conducted. Effective sampling programs take into account the daily activity pattern of the target species as well as its characteristic spatial distribution (uniform, random, or clumped). Historically, most programs have incorporated sampling at several or numerous sites in a target area to acquire sufficient representation of the size of a pest population; then researchers have counted the sampled individuals of the target pest. New programs developed for some pests simplify these procedures. Sequential sampling is an approach that optimizes the number of sampling sites needed for classifying a pest population as below or above a density requiring action. Binomial sampling is an approach that classifies an individual species as either present or absent at a sampling site, thereby precluding the need to count all members of that species taken in a sample. Both these simplifying approaches require substantial species-specific background information for their development and use.
The emerging technologies of global positioning systems (GPS) and geographical information systems (GIS) offer unsurpassed capability of aiding in the mapping of site-specific variation in characteristics of areas under consideration for sampling. A GPS uses triangulation of signals from a constellation of satellites to identify the precise location (within a meter) of an area on the earth’s surface. A GIS is a computer program for the mapping and spatial analysis of georeferenced information. GIS capabilities include assemblage, storage, manipulation, retrieval, and graphic display of information about attributes of precise locations identified through GPS. Such information can be exceptionally useful in forming associations between characteristics of a specific locale (e.g., terrain, soil, extent of vegetative growth, microclimate) and density of a population (Fig. 2). For pests, sampling can be directed toward specific sites in which densities are suspected to be the highest.
Deciding Whether to Take Action
Several approaches have been developed for deciding whether an insect pest population has or has not reached a level requiring intervention, such as an insecticide application. For agricultural purposes, the approach used most often centers on the concept of “economic injury level” (EIL), formalized in 1959 by V H. Stern, R. F. Smith, R. van den Bosch, and K. S. Hagen and defined by them as the “lowest pest population density that will cause economic damage.” These entomologists also proposed a related concept, which they termed the “economic threshold” (ET), defined as the “pest density at which control measures should be applied to prevent an increasing pest population from reaching the economic injury level” ( Fig. 3 ).
The decision-making concepts of EIL and ET have been fundamental to the development and implementation of first-level IPM, particularly for insect management in agriculture (but less so for
FIGURE 2 Dispersal of Neoseiulus fallacis for biological control of spider mites in a strawberry field, 8-15 weeks following release of 100 adult females at each of 15 sites: squares, release sites; crosses, sample points. This distribution is due to ambulatory foliar movement and aerial dispersal (dominant winds from south and southwest). Data represented using GIS (GRASS v. 4.1).
FIGURE 3 Graphs depicting theoretical population fluctuations of two insect pest species.
disease, vertebrate, and weed management). They have been especially useful when insect pest populations are expected to increase over time within a crop, can be sampled reliably, can be related in a predictable way to reduction in crop yield or quality, and can be controlled readily by taking immediate action (e.g., application of insecticide) to prevent further damage. They are less valuable when human comfort or aesthetics, rather than economic damage, is paramount. Even for agriculture, the concepts of EIL and ET cannot be applied rigidly because of inherent unpredictability of such factors as future weather (which can markedly affect rate of pest population growth and degree of crop susceptibility to a pest) and future value of the crop in the marketplace. Also, a type of action that may require considerable time before reducing pest density, such as application of a biocontrol technique, is likely to be less appropriate than an insecticide application within an EIL/ET framework.
A refinement of the concept of EIL, put forward by L. P. Pedigo and L. G. Higley, introduces the element of environmental quality into the decision-making process. Negative effects of insecticides on natural enemies of pests and on other organisms in the environment are treated as costs in addition to monetary costs associated with insecticide application. Quantifying environmental costs has proven to be challenging and subject to much debate, but progress under this refined concept of EIL nonetheless has been made.
In 1984, W. L. Sterling advanced the concept of “inaction level,” which is the density of natural enemies sufficient to maintain a pest below the EIL. M. P. Hoffmann and collaborators developed a sampling program for eggs of tomato fruitworm, Helicoverpa zea, that
permits ready identification and quantification of parasitized eggs in addition to healthy eggs. If the level of egg parasitism is determined to be too low to prevent larval numbers from exceeding the EIL, the lowest adequate rate of a “soft” pesticide (one having least impact on parasitoids and other nontarget organisms) is recommended. This approach is an example of application of the inaction level concept and represents a high degree of first-level IPM implementation.
Area-wide IPM is an expansion of first-level IPM that may represent a significant transitional step toward second and third levels of IPM. Under area-wide IPM, the key pest of a crop is targeted for management by means of the most effective noninsecticidal approach. For example, the codling moth, Cydia pomonella, is managed in apple and pear orchards by tactics such as the pheromone mating disruption technique or the sterile insect release method that impair normal reproduction. Such tactics are implemented over areas large enough to preclude recolonization by fertile females from adjacent areas. By reducing the impact of broad-spectrum insecticides, natural enemies are preserved and are usually capable of regulating most secondary pests in the crop. As area-wide IPM programs expand to incorporate multiple pest interactions, they become natural springboards to higher level integration in IPM systems.
SECOND-LEVEL IPM
Second-level IPM is intermediate between basic and advanced. It is receiving increased research attention, but inherent complexities have greatly limited its effectiveness. Emphasis is on management of key pests of all classes and their associated natural enemies comprising a community (e.g., a village of dwellings, an entire farm, a wooded area surrounding a village). Emphasis also is on substituting a variety of comparatively environmentally benign management tactics (e.g., cultural management, host resistance, biological control, behavioral control), to the greatest extent possible, for the therapeutic practice of pesticide application. Decision makers must determine how best to integrate these tactics to achieve long-term suppression of pests within a cost-benefit framework.
Cultural management is purposeful manipulation of the environment to reduce pest abundance. It is most effective when directed at the most vulnerable life stage of a pest. Four forms of habitat or environmental manipulation aimed at controlling pests have been practiced for centuries in agriculture: crop rotation, timing of planting or harvest to minimize pest damage, sanitation or elimination of noneconomic resources available for pest reproduction, and poly-culture, or the interplanting of different crops to diffuse resource concentration. More recent practices include the planting or encouragement of selected types of noneconomic vegetation in the vicinity of crops or as cover crops to serve as harborage for natural enemies of pest insects. Analogues of these practices have been developed for managing insect pests in nonagricultural situations.
Host resistance is any inherited characteristic of a host that lessens effects of an attacking pest. For centuries, humans may have unknowingly or intentionally selected for cultivars of plants or breeds of animals that are best able to withstand pests. Modern breeding programs, however, often have placed more emphasis on increasing yields than on protection against pests. Entomologically, resistant traits are preadaptive characteristics of a host that reduce its detecta-bility, acceptance, or nutritional value, or enhance its toxicity to a pest insect. Molecular genetics techniques that facilitate introduction of specific pest resistance genes into cultivars or breeds possessing desirable commercial or aesthetic traits are beginning to replace traditional resistance breeding approaches.
In biological control, parasitoids, predators, or pathogens are deployed as natural enemies in the reduction of pest populations. Of the myriad insect species that could become pests, most do not because they are suppressed effectively by naturally occurring populations of biological control agents. Natural levels of biocontrol, however, often are insufficient for IPM purposes. Biological control then takes the form of importing absent natural enemies from other locales (termed importation or classical biocontrol), augmenting existing natural enemies by rearing and then releasing substantial numbers into the target community (termed augmentation), or tailoring management tactics to reduce negative effects on existing natural enemy populations (termed conservation). The latter is the most widely practiced form of biocontrol.
Behavioral control is manipulation of the behavior of pest individuals to prevent them from causing harm or unpleasantness. Because of the expense and technological challenges associated with its use, behavioral control usually is directed only at key pests. Behavioral control may involve use of natural or synthetic chemical or physical stimuli to lure pests to sites where they are killed, or use of such stimuli to disrupt the ability of pests to find or use a potential resource. An ideal form of behavioral control might involve joint use of disruptive and attractive stimuli to achieve maximum effect, but this form is not yet widespread in practice.
Vineyards in parts of Europe and North America represent one of the few areas in which pest management is practiced effectively under the second-level IPM concept. Besides using essential elements of first-level IPM for insect pests, certain practitioners of vineyard IPM in these locations blend host plant resistance with cultural, biological, and behavioral controls for suppression of key pest insects and also use a suite of cultural controls for managing key disease and weed pests. This approach has resulted in marked reduction in pesticide use and greater stability of relationships among organisms comprising vineyard communities.
THIRD-LEVEL IPM
Although many IPM practitioners aspire to implement third-level IPM, the most advanced form remains largely in an embryonic state of development. Emphasis is on using principles and practices of second-level IPM in harmony with all other elements that affect long-term productivity or well-being of an ecosystem. Such elements include sound horticultural or husbandry practices (for agriculture), sound forest management (for silviculture), and sound community health practices (for villages or subunits of cities). Third-level IPM features attention to environmental and societal costs and benefits in the making of pest management decisions. The focal ecosystem may be an entity no larger than a community, as considered under second-level IPM, or it could be an entity as extensive as an ecological region. For crops, third-level IPM is roughly synonymous with the concept of integrated crop management. It is not, however, synonymous with organic agriculture, which disallows some materials and practices acceptable under third-level IPM.
Spearheaded by P. E. Kenmore, an approach has been developed for growing rice in developing parts of Asia that reflects many of the tenets of third-level IPM. This approach accentuates societal contribution to the IPM decision-making process. It involves weekly gatherings of small groups of rice farmers, accompanied by experienced pest management personnel, who jointly conduct agroecosystem observations, engage in data analysis, and consider local ecosystem structure, environmental health, and a range of immediate and long-term tactics before making pest management decisions. This process has resulted in dramatic increases in awareness by entire communities of an advanced form of IPM and often dramatic decreases in use of pesticides on rice. It stands in contrast to modes of decision making and levels of popular awareness characteristic of less advanced forms of IPM implementation in many developed countries, where it is commonplace for farmers to make IPM decisions either acting alone or at most interacting with a private consultant, government extension representative, or employee of a pesticide distributor.
IPM AND SUSTAINABLE DEVELOPMENT
In a broad sense, sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. Sustainable development is rooted in the concept of ecosystem integrity and permeates all facets of human endeavor, whether economic, social, or cultural.
The scope of concerns and practices of third-level IPM as described here corresponds closely to that of sustainable development. Each emphasizes preservation of processes associated with natural ecosystems, long-term well-being of humans as members of communities, economic viability, and deployment of exogenous resources only after careful consideration. For agriculture, concepts underlying third-level IPM can be equated with concepts underlying sustainable agriculture. For both, one can expect concepts to evolve further over time.
