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CHAPTER 7 Control by Parasites, Predators, and Competitors One of the oldest and most successful methods of controlling insect and related pests is by using their natural enemiesâparasites, predators, and disease organisms-to attack and destroy them. Disease organisms and their use are considered in Chapter 8. Parasites and predators have received much attention, and they are discussed at length in this chapter, together with insect competitor displacements. The use of competitors to suppress pest populations is a recent idea. The deliberate use of natural enemies for pest control is widely known as biological control, or biocontrol. Man has known for many centuries that insects attacking crops were in turn attacked by many kinds of natural enemies that, at times and in certain places, exerted a high degree of control over the pests. It was not until about 100 years ago, however, that deliberate attempts were initiated to use these enemies in control activities, either by introducing new ones to the environment of a pest or by increasing the effectiveness of species already present. The first demonstrations of effective biological control resulted from a recognition of the fact that in various parts of the world many agricultural pests are aliensâimmigrants that have spread from other areas and become established in new homes, either fortuitously or by the movement of agri- cultural and forest products through man's commercial activities. The first noteworthy demonstration of biological control occurred in California in 1888. Entomologists about that time discovered that many pests that be- came established in new environments often did so unaccompanied by the full spectrum of natural enemies that attacked and often controlled their populations in the countries of origin. The idea was conceived of bringing 100
CONTROL BY PARASITES, PREDATORS, AND COMPETITORS 101 these enemies from the native homes of the pests and releasing them in the new environments to attack the immigrant pests. For many years, this move- ment of natural enemies dominated biological control activities, and even today it receives a major share of the effort devoted to the biocontrol method. Although the work in California toward the end of the nineteenth century was not, strictly speaking, the first involving the movement of natural enemies, it is generally agreed that it established the method as a valid and useful device for pest control. In 1887 the developing citrus industry in California was seriously threatened by the cottony-cushion scale, Icerya purchasi Maskell, and many growers were forced to abandon their plantations. No sprays or other chemical treatments known at that time would control the scale. C. V. Riley, a prominent entomologist, suggested that the original home of the scale was Australia or New Zealand and that natural enemies of the scale should be introduced to California from these countries. The idea found immediate favor, and in 1888 Albert Koebele, an entomologist, was sent to Australia for this purpose. He soon found a small lady beetle known as vedalia,/?ocfo//<z cardinalis (Mulsant), attacking the scale in the Adelaide area, and in November 1888 the first shipment of the beetle reached California. The beetles in this and later shipments were liberated immediately on scale-infested trees and they soon completely cleaned the trees of the scale. Within a year, a startling and highly effective degree of control had been obtained throughout the citrus-growing area of the state at a total cost of less than $2,000. To this day, the vedalia con- tinues to provide completely satisfactory control of the cottony-cushion scale in California, and the savings it has brought to the citrus industry of the state have been incalculable. Since 1888, projects in biological control have been conducted all over the world, and many of them have been successful. Effective programs of biological control have been established in many areas, from Japan to West Africa and from Canada to Australia. More than 110 different pests in more than 60 countries throughout the world have been controlled by the use of parasites and predators. Canada and California have been especially active in this field and have recorded 15 and 18 successes, respectively. Success is directly related to the research effort expended on biological control, and the number of pests effectively controlled may increase dramatically as more and more countries turn from excessive reliance on pesticides to the promise offered by the biocontrol method. Considerable emphasis has been given to biological control at several centers in the United States and Canada, and the United States Department of Agri- culture has played an important role in its development. In addition, several international organizations have been established specifically to conduct re-
102 INSECT-PEST MANAGEMENT AND CONTROL search on biological control and to facilitate the movement of beneficial species between countries. The largest of these is the Commonwealth Institute of Biological Control, established in 1927 to work on biological control in the British Commonwealth. The Institute has major laboratories in Switzerland, Trinidad, India, and Pakistan and maintains small field stations in a number of other countries. It is equipped and prepared to conduct biocontrol research and exploration for natural enemies throughout much of the world at the request of any country willing to provide financial support for its activities. Another international organization for biological control is the Organisa- tion Internationale de Lutte Biologique, formed by a group of Western European nations to foster biological control and to stimulate and facilitate the movement of natural enemies across their borders. Many other countries have devoted considerable effort to biological control; especially noteworthy are Australia, Russia, Japan, and Israel. ADVANTAGES AND KINDS Biological control, as defined here, has a number of distinct advantages not offered by most other approaches to pest control now available. Three advantages are permanence, safety, and economy. Biological control is relatively permanent once it is established. The natural enemies on which it depends are self-perpetuating, barring natural catastrophes or the unwise interference of man, and they continually adjust to changes in the population size of the pests they attack. There are a few examples of insect hosts having developed resistance to their parasites, which interfered with successful biological control. It may be that any host-parasite or predator- prey interaction involves a continuing adaptive race between the antagonists, with the fed-upon species moving constantly toward greater protection from attack and the attacker compensating by becoming more effective. In most cases for which information is available this race seems continuously unre- solved, and effective biological control has a high degree of permanence. Biological controls have no side effects such as toxicity or environmental pollution, and they are not hazardous to use. In the current concern over the quality of the environment in which we live and over the possible consequences of continued long-term exposure to nontoxic levels of pesticides, the ad- vantages of biological control are being weighed more heavily when decisions are to be made on the best strategies for pest control. However, biological controls should not be thought of merely as useful and safe alternatives to chemical controls; they should be thought of as offering distinct advantages and having unique features in situations where they are applicable. There are three main kinds of traditional biological control, each of which is discussed in detail in this chapter. These are:
CONTROL BY PARASITES, PREDATORS, AND COMPETITORS 103 1. Introduction of exotic species of parasites and predators. This entails the search for natural enemies in foreign countries, their introduction to areas where the pest is causing damage, their rearing, and their release. The princi- ple underlying this approach has already been mentioned: many pests were accidentally introduced to new areas without their normal complement of natural enemies. These enemies may be found in the native home of the pest, or in the home of a close relative, and reassociated with the pest. Most of the successes achieved with biological control have involved introduction, and much contemporary research and activity centers on this approach. Two other approaches, however, have increasingly been added to this preoccupa- tion, and this complementary trend in interest will undoubtedly continue and perhaps even accelerate. 2. Conservation of parasites and predators. This emphasizes the impor- tance of making full use of natural enemies that attack a given pest in a particular location, regardless of whether they are introduced or native. The best opportunity for accomplishing optimal use of parasites and predators is by changing their environment in ways that will increase their effectiveness and thus reduce survival of the pest. In its simplest form, this has involved the adjustment of pesticide programs to prevent harm to the beneficial species. In a more complex form it may entail the alteration of single or multiple environmental factors that restrict the effectiveness of beneficial species by reducing or regulating their abundance at points below the optimal. Such manipulations can only stem from very broadly based and intensive research at the systems level in pest-management programs. 3. Augmentation of parasites and predators. This approach, sometimes called inundation, involves the mass-rearing and periodic release of large numbers of a natural enemy of proven value. Releases are made over small areas, with the objective of temporarily raising the abundance of natural enemies to a high level at times when the pest is most vulnerable to them. ECOLOGICAL BACKGROUND In the 1950's, biological control was a clear-cut entity, readily separable from other forms of insect-pest control, and easily defined. It was the sup- pression of the numbers of pests by exploiting the biological attributes of certain organisms, principally parasites, predators, or competitors that are antagonistic to the welfare of pest species. Such organisms were usually called "natural enemies," not a particularly appropriate name, because enmity was not involved in the relationship and many of the organisms were unnaturally introduced or propagated in the environment. At that time, two other broad classes of control were recognized: cultural and chemical control, each readily distinguished from biological control.
104 INSECT-PEST MANAGEMENT AND CONTROL However, a host of new ideas for controlling pests has since been intro- duced, few of which clearly fall into any of the older categories. Instead, they tend to fall between them and could logically be included in two or more of them. Thus, there is a tendency for the old boundaries to merge and blur, and clear-cut distinctions have become lost. This tendency has been en- couraged by a trend to use various controls in combination, so that one type supplements or reinforces another type to produce maximal suppression of the pest, with minimal deleterious side effects. The old categories, however, still serve a useful purpose. The concepts behind them, while changing, are still valid in principle. The names applied to them designate activities whose outlines still have enough clarity to make them useful; thus, biological control is still a useful term to designate activities in which parasites, predators, and competitors are used to suppress pest populations. The basis of biological control is intimately involved with the theories of larithmics (Greek loos, population, + arithmos, numbers). In an effort to understand the basis of their art, biological control workers have contributed extensively to larithmics, and it is now difficult to know which ideas were derived from which source. Basic to the theory of biological control is the concept of community homeostasis: all living things constitute components of self-regulating com- munities and as such are subjected to naturally occurring regulative processes that tend to maintain a degree of numerical balance between all elements of the community. According to this concept, the biological activities of each organism impinge on, and interact with, the activities of some of the other organisms that share the same general area. Thus, a web of action and inter- action is generated that both amplifies and restricts the activities of each organism, the sum of all activities forming a coherent, integrated unitâthe community. The community is thus a fairly abstract idea; its boundaries can rarely be delimited, and its characteristics can seldom be clearly defined. Nevertheless, that the community is an entity is clearly shown by the facts that competitive and antagonistic organisms do live and persist on the same site and that each depends on at least one other kind of organism for the essentials of life and for many subtle benefits. This is discussed in Chapter 3. Predators (including parasites) have long been recognized as important elements in the dynamics of arthropod communities. It was recognition of the high degree of interdependence between predators and their prey that gave rise to the idea of density dependence. Observations of natural popula- tions revealed that as a prey population increased in numbers an increasing proportion of them were killed by predators, and as it declined in numbers a decreasing proportion were killed by predators. The mechanism was thought to work as follows:
CONTROL BY PARASITES, PREDATORS, AND COMPETITORS 105 An increasing prey population provides a surplus of easily found food for predators, and the surplus permits the predator population to expand. As the predator population expands, an increasing proportion of prey is killed by predators before the prey reach maturity, the process progressively decreas- ing the rate at which the prey population can reproduce. Eventually, such a large proportion of prey is killed that the survivors are too few to produce as many offspring as were produced by the previous generation, and the prey population declines, usually precipitously. Fewer prey provide less food for the predators, and prey become more difficult for predators to find. Thus, the predator population, faced with a shortage of food, declines even more precipitously. Relieved of excessive predation, the prey population is again free to expand until once more checked by a revived predator population. This mechanism constitutes a less than perfect, but nevertheless effective, regulating system that maintains populations of both prey and predators within certain fairly narrow limits of abundance and scarcity. A system dominated by forces such as these agrees with a cybernetic system in the process of "hunting." The force initiating change in the system is the inherent capacity of the organism to increase in numbers by reproduc- tion; the counter-force generated within the system is predation of indi- viduals before they achieve reproductive maturity. The system "hunts" be- cause the degree of predation can neither be exactly proportional to the change in the prey population nor exactly synchronous with the change. This can be demonstrated simply. As long as the proportion of reproductive females in a prey population surviving each generation exceeds the inverse of the mean number of offspring each can produce, the population will increase; if the proportion is less, the population will decrease. Thus, if each female can produce X offspring, the number of offspring in generation n + 1 will exceed the number in generation n when more than l/X of the individuals born into generation n survive to reproduce; if less than l/X survive, the number of offspring in n + 1 will be less than in n. Consider a prey population in which less than l/X of the individuals have escaped predation in generation n. At the conclusion of that generation there must be a predator population capable of destroying at least (X - 1)1 X of the prey individuals. If the predator population was able to overcome the prey population it must have a reproductive potential approximately equal to or greater than X. At the end of generation n, both predator and prey popula- tions reproduce. The new prey generation will be X(<\/X) = <1 times the previous generation, while the new predator generation will be at least X[(X- 1)1 X], or X- 1 times the previous generation. At the start of genera- tion n + 1, the number of prey is less than at the start of generation n, but
106 INSECT-PEST MANAGEMENT AND CONTROL the number of predators is severalfold that of generation n. Thus, there are enough predators in generation n + 1 to kill all the remaining prey; the only prey that escape are a few not found by any predator. Most of the predators do not find enough food in generation n + 1 and die before maturity. There- fore, both populations decline precipitously to very low levels. Because at least one prey is needed to support each predator, and because at low population densities a decreasing proportion of the prey is found by predators, the number of surviving predators must be substantially less than the number of surviving prey. Thus, the prey population is relieved of ex- cessive predation and starts to increase again. This, in turn, provides more food for predators, and the predator population can also start to rise. But the efficiency with which predators can find prey remains fairly low until prey density increases to certain levels, after which predators gain efficiency and, in the presence of a surplus of easily found food, multiply rapidly at the ex- pense of the prey population until they finally overwhelm the prey again. Therefore, predation, the corrective force of this system, is never propor- tional to the changes induced by the reproductive capacity of the prey popu- lation, nor is it ever fully synchronized with these changes. In the early stages of a cycle, when the prey population starts to increase, the response by the predator population is weak and delayed. In the final stages the re- sponse is overpowering and is maintained beyond the appropriate period. The system is therefore inherently unstable and cannot be called upon to produce community stability. It could, however, and probably does, consti- tute a principal device by which the observed fluctuations of natural popula- tions are generated and sustained and by which such fluctuations are con- tained within certain limits. This model, of course, is a gross oversimplification of the processes occurring in natural communities. If this were the only mechanism influ- encing populations, fluctuations would follow periodic cycles of abundance and scarcity. Such regular cycles are no more a feature of natural populations than is stability of numbers. A vast number of factors other than abundance of and searching by predators affect the survival of prey, and a similar num- ber of factors other than number of prey influence the survival of predators. Weather, for instance, can be considered a population-determining device; it includes a great number of everchanging elements that strongly influence populations of all kinds of organisms. It can affect populations either directly, by influencing the physiology of the organisms themselves, or indirectly, by affecting the quality and quantity of foods and shelters. By either route, weather may cause populations to expand or collapse. However, neither favorable nor unfavorable weather is generated or modified by any feature of the populations acted upon; weather acts impartially on all populations, regardless of their abundance or stage of growth. Such elements cannot con-
CONTROL BY PARASITES, PREDATORS, AND COMPETITORS 107 tribute to regulation; rather, they are agents that modify the workings of the homeostatic system, sometimes amplifying it, sometimes short-circuiting it, always introducing unpredictable variations into its workings. A simple, two-element relationship postulated by the predator-prey model is not likely to exist in any natural community. Most prey populations are attacked by several kinds of predators, and each kind attacks in a different manner, with a different intensity, and at a different time, and each responds differently to changes in prey numbers as well as to variations in weather. Moreover, only a few kinds of predators are completely dependent on a single kind of prey and therefore may not respond directly to changes in abundance of any one kind of prey as required by the predator-prey model. The quantity, quality, and distribution of foods, on which the survival of all prey populations is dependent, are governed by weather, soil composition, and the number of other organisms feeding on the same commodity. Predators of predators can influence the success of the latter. These are but a few of the multitude of factors that can affect the numbers of both predators and prey quite inde- pendently of any direct interaction between them. The regulation of natural communities is obviously a much more complex phenomenon than is indicated by our simple model. Nevertheless, most popu- lation ecologists consider that some variation or elaboration of this theme forms the basis of community stability. Two forms of population-determining forces are at work: nonreactive forces, usually of extra-community origins, which act independently of the affected populations; and reactive forces, usually generated by living agents within the community, which are evoked by changes in the affected population itself, and which tend to counter such changes, although imperfectly. Nonreactive forces are mainly meteorological, geological, and, to some extent, man-made forcesâman-made because man, although an organic entity himself, must be considered an element apart from organic communities when he demands special concessions from community mechanisms. Reactive forces are mainly the so-called "natural enemies," that is, parasitic, predacious, and competitive organisms. Combined, the nonreac- tive and reactive forces provide the motive and opportunity for evolutionary adaptation. Either of the forces may determine the level of any given popula- tion at any given time; only the reactive forces, however, can maintain a popu- lation between finite levels of abundance, i.e., only they have homeostatic properties. APPLICABILITY OF BIOCONTROL When genuine pest situations are found to exist, the most appropriate strategies for controlling them must be determined. Biocontrols are not
108 INSECT-PEST MANAGEMENT AND CONTROL suitable to every pest situation; they may be suitable only as a part of a broader strategy that includes other devices. Before biocontrol procedures are initiated, they should be thoroughly justified on the basis of biological and ecological information. Definite and specific flaws in the regulative structure of the community should be recognized, and a reasonable possibility of allevi- ating or correcting these flaws by practical biological manipulations should be established. Some features of pest communities that may influence the feasibility of biocontrols are: gaps existing in the natural-enemy complex, which might be filled by introducing new natural enemies; native natural enemies that possess attributes of good control agents but are inhibited by some lack or maladjust- ment of the community; ineffective native natural enemies that might be replaced by more effective foreign ones; the degree of pest suppression needed to achieve control (economic threshold); the suitability, costs, and degree of protection achievable by other kinds of controls; and the probability of unde- sirable side effects that may accompany certain controls, such as pollution, toxic residues, damage to other organisms, resistant strains of pests; and preservation of esthetic values. The recognition and resolution of these matters require a careful analysis of the pest community. Until a substantial body of community information is available, no rational strategy can be evolved. COMMUNITY STUDIES The amount of information necessary to launch a biocontrol program varies with the urgency and complexity of the problem. Some situations require immediate action, and sometimes experienced workers can take such action on an intuitive basis. But a carefully planned and executed investigation of the pest situation is usually essential for the development of a rational program. Essential information may consist of only a chronological life history of the pest and its major natural enemies, plus a gross description of the physical environment. However, a long and complex analysis of community relation- ships may be needed to determine the factors regulating the pest, to estimate the stages of the pest most vulnerable to attack, and to determine the kinds of agents most likely to succeed in suppressing the pest. Biocontrol programs usually begin with a minimal investigation. If actions taken on that basis fail, the investigation becomes more and more detailed- leading to new and presumably more sophisticated actions. The kind of infor- mation required for a minimal investigation is much the same as that required for the initial stages of an in-depth investigation. In starting a project, there-
CONTROL BY PARASITES, PREDATORS, AND COMPETITORS 109 fore, it is wise to conduct the first steps so that the information gathered is usable for an in-depth investigation if such should become necessary. Virtually every pest situation is unique; therefore, the research methods applied to investigate each situation must be unique and must be devised by the researcher as he develops his program. In developing a program, the re- searcher should have specific questions i