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Cost Effectiveness

Clausen's 3-Host Generation / 3-Year Rule

Biological Control From Naturally Occurring Organisms

Single Larval Parasitoid Importations

Estimation of the Costs of Classical Biological Control

Single Pupal Parasitoid

Justification of Need for Biological Control

Pest Groups

Biological Control & Pesticide Use

Natural Enemy Groups

Analysis of Successes


Island Theory


Multiple as Opposed to "The Best" Species

[Please refer also to Selected Reviews 

           &  Detailed Research ]


Cost Effectiveness

          Abundant empirical evidence shows that biological control, as practiced by professionals is among the most cost effective methods of pest control. Because of its highly positive social and economic benefits, biological control should be among the first pest control tactics to be explored.

          Biological control workers must not be indiscriminate in introducing exotic organisms, however. Biological control is a serious endeavor for professionals: it cannot become a panacea for enthusiasts having little of the formal training and understanding of the basis of this discipline. In pest control the rights of society and the environment are increasingly in conflict with private profit. Classical biological control and other forms of natural control, plus other environmentally and economically sound methods must fill the gap. Biological control has the best pest control record and remains a considerable untapped future resource (A. Gutierrez, pers. commun.).

          It is difficult to make an analysis of costs and benefits for biological control because the definition "biological control" has been given various meanings (Caltagirone & Huffaker 1980, NAS 1987, Garcia et al. 1988, Gutierrez et al. 1996). Perhaps it is appropriate to distinguish classical and naturally occurring biological control from other methods such as the use of pesticides derived from biological organisms (e.g., Bacillus thuringiensis toxins, ryania, pyrethrum, etc.), the use of sterile males, etc.). Gutierrez et al. (1991) consider periodic colonization of natural enemies (inundative and inoculative) as an extension of biological control. It is a mistake to call biological control any procedure of pest control that involves the use or manipulation of a biological organism or its products as was done by Reichelderfer (1979, 1981, 1985). Reichelderfer's contribution has been to show how economic theory applies to an analysis of the economic benefits of augmentative releases of biological control agents, and in this sense the arguments are similar to those for estimating the benefits of using pesticides or any other control method.

          In this discussion of economic gains, the discipline of biological control as an applied activity, concerns itself with the introduction and conservation of natural enemies that become, or are essential components of self-generating systems in which the interacting populations (principally predator/prey or parasitoid/host) are regulated. In biological control of pests the manipulated organisms include predators, parasitoids, pathogens and competitors. No judgments are made concerning the value of other procedures, except to note those which encourage environmentally safe and economically sound approaches. Biological control of pests has been implemented worldwide, in environments that are climatically, economically and technologically diverse (Clausen 1978). The net benefits derived from this tactic as a whole are difficult to quantify with any degree of accuracy. However, the considerable number of cases that were successful, and continue to be so, and the fact that no environmental damage has been detected in the great majority of them make this tactic a very desirable one. Nevertheless, the classical biological control approach (introduction of exotic natural enemies) has been challenged on the basis of possible negative effect on native organisms. For example, Howarth (1983) proposed that in Hawaii the introduction of some natural enemies has adversely affected the native fauna, and that to restore the ecological situation by removal of these organisms is nearly impossible. This points to the vexing aspect of possible environmental risk in using exotic biological control agents (Legner 1985, 1986). It has been accepted that these organisms, when introduced according to restrictions established by regulatory agencies (Animal and Plant Health Inspection Service in the United States) are considered to pose no environmental hazard. Routinely, risk is recognized when considering candidate natural enemies to control weeds. A comprehensive discussion on this aspect of biological control is given by Turner (1985), and Legner (1986a,b).

          The biological impact of exotic biological control agents on target pests is difficult to assess and few cases have been thoroughly documented (Luck et al. 1988), making economic analysis difficult. Even more demanding would be to include in the equation the monetary value of the side effects as referred to by Howarth (1983) and the positive ones (e.g., the benefit that society derives from the reduction in or the elimination of the use of objectionable pesticides) as a result of the introduction of an effective natural enemy.

Biological Control From Naturally Occurring Organisms

          The economic benefits of naturally occurring biological control have been repeatedly demonstrated in those cases where secondary pests became unmanageable as a result of overuse of chemical pesticides to control primary pests. DeBach (1974) clearly showed the effect of DDT in the disruptions of pests in many crops. The rice brown plant hopper, Nilaparvata lugens, in southeastern Asia continued to be a pest as a result of it overcoming the new varieties' resistance and the use of pesticides to control it.

          Host plant resistance may be overcome by natural selection of new biotypes of phytophages in the field in less than seven years (Gould 1986). Kenmore (1980) and Kenmore et al. (1986) showed that the rice brown planthopper is a product of the green revolution wherein the increased prophylactic use of pesticide destroyed its natural enemies and caused the secondary outbreak of this pest. Recognition of this problem recently led to the banning of many pesticides in rice in Indonesia (Gutierrez et al. 1996). This prohibition has resulted in no losses in rice yields. Most of the pests in cotton in the San Joaquin Valley of California (Burrows et al. 1982, Ehler et al. 1973, 1974; Eveleens et al. 1973, Falcon et al. 1971), the Cañete and other valleys in Peru (Lamas 1980), Australia (Room et al. 1981), Mexico (Adkisson 1972), Sudan (von Arx et al. 1983) and other areas are pesticide induced. This often causes these pests to become more important than the original target pests. These examples substantiate the benefits of naturally occurring natural enemies in controlling pests. Furthermore, these benefits are largely free of cost, unless special procedures are required to either augment or reintroduce them (Gutierrez et al. 1996).

Estimation of Benefits & Costs of Classical Biological Control

          The costs of a classical biological control project (C) may be calculated easily. One simply sums the cost of the base line research, the cost of foreign exploration, shipping, quarantine processing, mass rearing, field releases and post release evaluation. The last cost must be evaluated judiciously as pursuing academic interests may push these costs beyond those required by the practical problem at hand. Harris (1979) proposed that costs be measured in scientist years (SY), with one SY being the administrative and technical support costs for one scientist for one year. For example, the U. S. Department of Agriculture estimated that one SY in biological control cost $80,000 in 1976 (Andrés 1977).

          DeBach (1974) gave a rough estimate of the cost of importing natural enemies at the University of California. He commented that he had imported several natural enemies into various countries with resulting impressive practical successes where the cost had been less than $100 per species. In other cases the cost may run much higher, but he believed not more than a few thousand dollars per entomophagous species at most. These tentative costs suggest that some classical biological control projects may be very inexpensive, but others may cost more because of the biological and other complexities encountered. Also, the efficiency of the organization involved may cause costs to vary considerably, and the cost of the biological control efforts on a per organization, per country, or worldwide basis must include the cost of fruitless efforts. Like any other tactic, biological control must record not only its successes but also failures (Ehler & Andrés 1983). A monetary loss due to a failure may still provide a scientific gain in knowledge which is usually unmeasurable. Such knowledge may be applied positively in future efforts with a consequent savings of cost.

          Once establishment and dispersal in the new environment is obtained in classical biological control, no further costs for this natural enemy are incurred unless new biotypes are introduced. Other uses of natural enemies may involve repeated releases of natural enemies in the field or glasshouse. These costs are analogous to the cost of pesticide applications. The release of Aphytis in California orange orchards (DeBach et al. 1950), Pediobius foveolatus against Mexican bean beetle on soybean (Reichelderfer 1979), Trichogramma spp. in many crops worldwide (Hassan 1982, Li 1982, Pak 1988), Encarsia formosa against whiteflies in glasshouses (Hussey 1970, 1985, Stenseth 1985a), phytoseiid mite predators in strawberries (Huffaker & Kennett 1953), almonds (Hoy et al. 1982, 1984), and glasshouses (Stenseth 1985b) are examples in which costs of manipulation of natural enemies are incurred periodically. The use of sterile males in campaigns against screwworm, Mediterranean fruit fly or pink bollworm was aimed at eradication rather than regulation of the pest. Under these circumstances it is assumed that much higher costs can be tolerated.

          The environmental costs of biological control derived from the possible suppression or eradication of native species by introduced exotic natural enemies (Howarth 1983, Turner 1985) could be included in a benefit/cost analysis if some monetary value could be placed on them. More often than not such factors cannot be accurately priced in much the same way that increased cancer risks due to the use of some pesticides cannot be priced.

          Biological Control Benefit Computation is a more difficult task. One of the most successful, and historically the first, case of biological control in California was the control of the cottony cushion scale, Icerya purchasi, by the imported natural enemies Rodolia cardinalis and Cryptochaetum iceryae. In 1889-1889, when these natural enemies were imported to California at the cost of a few hundred dollars, the young citrus industry was at the verge of collapse because of the scale. One year later shipments of oranges from Los Angeles County had increased three-fold (Doutt 1964). What figures should we use to determine the benefits of such a program? Obviously the benefits continue to accrue to the present. In 1889 there was no other effective way to control the scale even though it is possible that some of the modern chemical pesticides could control it today. So is the yearly benefit the full net value of the citrus crop (assuming the uncontrolled pest could destroy all of the crop and many of the trees as well), or the total cost of using an effective pesticide? Should we include the benefits of introducing these natural enemies from California to 26 other countries, in 23 of which the scale was completely controlled? Whichever method is chosen, the benefits of this project are vast but undocumented.

          Much more difficult are those cases were partial noneconomic control occurs: the natural enemy becomes established, regulates the population of the target species to a lower level, but not low enough as to have economic significance. It is conceivable that in cases like these the natural enemies may make it easier to implement a more effective, complementary control tactic (e.g., IPM). The effects of biological interactions are complex and they are often influenced by other factors including weather, and the beneficial effects of the natural enemy may not be obvious. When the results of biological control are clear-cut, increased production and increased land values may be only part of the equation, as enhanced environmental and health effects may also occur but may go undocumented. The basis for a comparison between the situation prior and after establishment of biological control must further consider the changing real value of money over time, changing markets for the commodity involved, and the dynamics of land use. Enhanced yield may be due to reduced pest injury, but also to reduction in diseases the pest may vector.

          Benefits which are easiest to estimate are those to the agricultural sector. Because of the permanent nature of biological control, the net benefits (II) [i.e., benefit (B) - costs (C)] corrected for the present value of money using the discount rate (1 + @)-1 accrue over t years (i = 1,...,t). Note that @ is the interest rate of price of money.


II = Z (Bi - Ci) / (1 + @)i


[ Z = summation sign]

          Gross revenue (B) to the grower equals P (Y-DN(1-E)) with P being price, Y the maximum possible yield, D the damage rate per pest N, and E the efficacy of the biological control. In reality, D is a function of N (i.e., D(N(1-E))), but for simplicity we assume that D is a constant. In fact, the benefit of biological control for the ith year is Bi = PDNiE, and in the extreme may equal PY.

          DeBach (1971, 1974), van den Bosch et al. (1982) and Clausen (1978) summarized several classical biological control projects worldwide. A few of them are reviewed also in Gutierrez et al. 1996), who note their benefit/cost ratios (B/C). This ratio is however dimensionless and tells nothing about the total gain, rather it is a useful measure of the rate of return per dollar invested. Some projects, such as control of the Klamath weed and the Citrophilus mealybug have B/C ratios in the thousands, while the ratios for most of the others are in the hundreds. These estimates are, at best, rough approximations for the reasons stated previously. But even if they overestimate the benefit by 50% the B/C ratios will overwhelmingly favor the use of classical biological control. In fact the estimates of benefits are conservative and the errors are in the opposite direction.

          There are many other examples of the benefits of biological control. Tassan et al. (1982) showed that the introduced natural enemies of two scale pests of ice plant, an ornamental used in California to landscape freeways, potentially saved the California Department of Transportation ca. $20 million dollars in replanting costs (on 2,428 ha.). Chemical control at a cost of $185/ha., or $450,000 annually, did not prove satisfactory. Therefore, if suitable biological control agents did not exist the minimum long term benefit would appear to be the replacement cost. The total cost of the project was $190,000 for a one year B/C ratio of 105. This was certainly a cost effective biological control project.

          The control of cassava mealybug by the introduced parasitoid Epidinocarsis lopezi over parts of the vast cassava belt in Africa was a monumental undertaking. Successful control of the mealybug enabled the continued cultivation of this basic staple by subsistence growers, thus potentially helping to reduce hunger for 200 million inhabitants in an area of Africa larger than the United States and Europe combined. What monetary value could be assigned to this biological control success? How is the reduction or prevention of human misery priced? This project has been characterized as the most expensive biological control project ever ($16 million to 1991) by some of its critics, but all things being relative, the costs of this program since its inception in 1982 are less than those of the failed attempt to eradicate pink bollworm from the southwestern United States, or roughly about the cost of a fighter plane bought by many of these countries. The per capita cost of the project amounts to eight cents per person affected in the region, which contrasted to average yield increases in the Savannah zones of west Africa of 2.5 metric tons per cultivated hectare would appear to be a good return on the investment (Neuenschwander et al. 1991). Finally, the project has been diligent in documenting nearly all phases of the work (Herren et al. 1987, Gutierrez et al. 1988a,b,c; Neuenschwander et al. 1991), and satisfying as much as possible the concerns of Howarth (1983).

          There are also recent cases of successful biological control where the benefits are just as impressive but an economic analysis has not been conducted. The control of three Palearctic cereal aphids over the wheat growing regions of South America reduced the pesticide load on the environment causing direct enhancement of yields. New wheat varieties were being developed at the time, but their yield potential had not been stabilized. Thus it is not possible to assess the maximum contribution of the biological control effort. But if as a result of the establishment of natural enemies there was a saving of one application of pesticide per annum the total savings in Argentina, Brazil and Uruguay on 8,996,000 ha. of wheat alone (FAO 1987) would be substantial, especially if it is contrasted with the cost of the biological control component, which has been estimated at less than $300,000 (Gutierrez et al. 1996).

          Gutierrez et al. (1991) compare the economic benefits of several successful classical biological control projects and compare them with the use of inundative releases of natural enemies in soybean for control of Mexican bean beetle and for greenhouse pests, and the well known sterile male eradication program. The release of resistant predatory mites in almonds gave a B/C ratio of 100 (Headley & Hoy 1987), and the screwworm eradication project is estimated to have given a ratio of 10. Although impressive, these B/C ratios on the average are still not as high as those achieved using classical biological control which is self sustaining.

          In augmentative release and especially eradication programs, the cost of starting and maintaining them may be very high. In some cases a particular pest may be understood to be of such damaging nature and effective natural control may not be available that the high costs of eradication may be deemed necessary. However, eradication programs are not without risks. For example, an economic analysis of the proposed eradication of the boll weevil from the southern United States predicted that the eradication of the pest would cause the displacement of cotton from the area (Taylor & Lacewell 1977). In this scenario increased cotton production due to eradication of the pest would cause prices to fall forcing production to move to the west where it is more efficient. In the case of the ill fated pink bollworm eradication effort in the desert regions of southern California, early termination of the crop was available as an alternative, but it is not favored by growers because they did not pay for the full cost of the eradication program or the environmental costs of high pesticide use, and yields were lower. Only resistance to insecticides in pesticide induced pests made them reconsider alternatives such as short season cotton varieties and conservation of natural control agents.

Justification of Need for Biological Control

          The question is then why do we feel the need to make economic justifications for biological control? Why hasn't biological control been more widely supported worldwide? Economists would call this a market failure, because the users of pesticides do not pay for long term consequences of pesticide use and hence may ignore environmentally safer alternatives (Regev 1984). But there are also problems of perception, for as Day (1981) assessed in his review of the acceptance of biological control as an alternative for control of alfalfa weevil in the northeastern United States: "At first, the general opinion was that biological insect control was outmoded, because it had not been effective in the east in decades, and it was not likely to be competitive with synthetic insecticides or the newer synthetic chemicals such as pheromones, chemosterilants, attractants and hormones." Thus, biological control was not appreciated as competitive with newer technologies and it was not considered modern. The recent over selling of bioengineering solutions for crop protection can also be added to the list of reasons why classical biological control is not currently strongly supported.

          Often the damage of important pests may not be obvious to funding agencies, or grower groups may not be sufficiently organized to provide the funding. For example, a related weevil species, the Egyptian alfalfa weevil in California is a very serious pest not only in alfalfa, but more important in pasture lands where it depletes the nitrogen fixing plants. In 1974 feeding damage resulted in $2.40 - $9.59 reduction in fat lamb production (or $5.00 reduction in beef production) and $1.00 - $1.50 reduction in fixed nitrogen per acre per year, in addition to spraying costs of $2.50/acre/year plus materials (Gutierrez et al. 1996). These losses averaged over the vast expanse of grazing land in California and other western states make an enormous sum. Despite the economic significance of this pest, funding for a project has proved elusive, thereby greatly hindering biological control efforts. In contrast, funding for the biological control of the ice plant scales in California was rapid because damage was readily visible along the freeways, and the California Department of Transportation, which funded the project, had ready access to funds from fuel taxes.

          The technologically advanced countries the advocates of biological control, compared to those promoting predominantly the use of chemical pesticides, are much fewer in number, generally have sparser resources and have a more difficult educational task. It requires great educational skills, financial resources and personal dedication to effectively convey the necessary information in order to enable growers to make educated decisions about pest control. The processes of biological control are not visible to the majority of agriculturists, and with rare exception its benefits become part of the complicated biology that is absorbed in the business of crop production, and is quickly forgotten by old and new clients alike. On rare occasions the biological and economic success was so dramatic, as occurred with Klamath weed in California, that the generations four decades later is aware of the history of the control. The problem is also increasing in developing countries as modern agrotechnology displaces traditional methods, and they too become dependent on pesticides for the control of pests. To combat this problem the United Nations sponsored project on rice in southeastern Asia headed by P. E. Kenmore has set as its goal the training of millions of rice farmers on how to recognize the organisms responsible for the natural control of rice pests. Thus, perceptions of the seriousness of a pest control problem often determine whether an environmentally sound alternative is selected.

Biological Control & Pesticide Use

          In a free market economy individual growers make their own pest control decisions, and purveyors of alternatives such as pesticides have the right to market them in accordance with state laws. Under such a system, the perceptions of the problem by growers and the marketing skills of those proposing alternative solutions often dictate how well biological control is adopted in the field.

          In evaluating the effectiveness of chemical control or augmentative release of natural enemies, economists traditionally look at the balance of revenues (B(x)) = the value of the increase in yield attributable to using x units of the control measure (e.g., pesticide or augmentation) minus the out-of-pocket cost (C(x)) of causing x units of the control measure. Only infrequently are the social costs (S(x)) associated with the control measure included. For augmentative releases of natural enemies and biological control, S(x) is usually zero. The benefit function is usually assumed to be concave from below and the cost per unit of x constant. The net benefit (II) function should be:

          II = B(x) - C(x)

The optimal solution to this function occurs when dB/dx = dC/dx, hence the optimal quantity of x to use is x1 when S(x) is excluded, but is x2 when included? If the cost per unit of x used increases with x, costs rise rapidly and less pesticide (x3) is optimal. Unfortunately, the social or external costs of pesticides in terms of pollution, health and environmental effects are seldom included in the grower's calculations because there is no economic incentive to do so. In contrast, augmentative releases of natural enemies also engender ongoing costs, but they are environmentally safe and may be more economical than pesticide use. Prime examples of the successful use of this method are the highly satisfactory control of pests in sugarcane in Latin America (Bennett 1969), and in citrus orchards in the Filmore District of southwestern California (van den Bosch et al. 1982).

Conservation of natural enemies for control of pests such as Lygus bugs on cotton in the San Joaquin Valley in California and in other crops elsewhere (DeBach 1974) often yields superior economic benefits than does insecticidal control (Falcon et al. 1971). In such cases the ill advised use of chemical pesticides (x) may induce damage resulting in additional pest control costs and, at times, lower yields (Gutierrez et al. 1979). With naturally occurring biological control and economically viable classical biological control (BC), the costs of other pest control tactics and social costs often become zero, and the whole of society obtains the maximum benefits: the natural and biological controls supplant other methods of control and may solve the problem permanently. In such cases biological control should be favored as the equation for profit becomes,

          B(BC) - C(BC) > B(x) - C(x) > B(x) - C(x) - S(x).

          Even with the presence of effective natural control, growers may still visualize a high positive risk of pest outbreak and may apply cheap pesticides as insurance against risk of pests such as Lygus in cotton, but in paying the premium they may become stuck in a treadmill of pesticide use as described by van den Bosch (1978). DeBach (1974) named pesticides "ecological narcotics" because of their effect of suppressing problems temporarily, but causing addiction to their continued use. Regev (1984) also referred to the addiction to pesticides, and concluded that generally the root of the problem is that pesticides are preferred because the social costs are not paid by the users.

          Two ideas appear in an analysis of the reliance of growers on pesticides: one is a measure of the mean and variance of profits and the other is the perception of risk (Gutierrez et al. 1996). If there is effective natural control (e.g., San Joaquin Valley cotton), growers who do not wish to take risks still consider the distribution of profits with and without pesticides. Obviously if such growers think that despite the same average profit, the variation in profit is lowest using pesticides they will undoubtedly choose to control pests by using them. If growers are more informed about all the issues, they may still judge the distribution more favorable using pesticides (2B) because they have no incentive to assume responsibility for social costs. The decision might not be so certain in the latter cases, if increases in pesticide costs cause a significant shift in the perception of risk involved in the various control alternatives. A desirable outcome might be that natural controls are increasingly preferred. If resistance occurs, growers soon learn that preserving natural enemies in the field is an option to bankruptcy. In cases of complete biological control, the mean profits may be greatly increased because pesticides would no longer be required, yields would be near maximum and the variance of yield narrowed.

          It is therefore important how a grower understands risk which determines how much he will be willing to pay for pest control to minimize that risk. Adding the social cost of pesticide use to the cost of pesticides narrows the gap between unrealistically perceived risk and the real risk to profits. Taxing pesticide users to fund biological control efforts would be a socially responsible way to fund permanent solutions for pest problems (Gutierrez et al. 1996).

Analysis of Successes

          The most thorough resume of biological control efforts and successes may be found in Clausen (1978). Another publication by the University of California Press that discusses in great detail some of the outstanding contributions to pest control employing the biological control method:  Bellows, T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA.

          The so-called Island Theory seems to be borne out in thee results, because a substantial portion of the more striking successes in biological control have occurred on such islands as Hawaii, Fiji and Mauritius, and ecological islands such as portions of California. One reason is that biological control work began early in such places, and a disproportionate amount of research and importation was undertaken there in comparison to continents (excepting California). However, the present record shows that about 60% of all the complete successes have occurred on continents; thus, the island theory is no longer fully acceptable.

          Parasitoids have been argued to be better than predators as biological control agents. Because a predaceous larva consumes many host individuals during its lifetime and a parasitoid but one host, it might appear that a predator is inherently more destructive and thus makes a better biological control agent. However, analysis of the 139 species of entomophagous insects imported and established in the United States as of 1967 showed that 113 were parasitoids and 26 predators. This ratio has remained similar into the 1990's. Roughly twice as many successes in biological control have resulted from parasitoid introduction in the United States. However, about four times as many on the world scene.

          The apparent superiority of parasitoids is the subject of contemporary debate and research. This may only reflect the fact that parasitoids have received the greatest amount of attention in terms of the number of species introduced and the number subjected to field analyses.

Multiple as Opposed to "The Best" Species.

          The question has arisen whether multiple importation of different natural enemy species attacking a given host and the resulting Interspecific competition among them produces a greater or lesser total host mortality than would be the importation of the so-called "best" species allowed to act alone. Analysis of past successes suggests that multiple species importation, whether made simultaneously or sequentially, have nearly always resulted in enhanced biological control.

          Multiple introductions provide a series of natural enemies that can attack a sequence of host stages in any one habitat. Here environmental changes may adversely affect one natural enemy yet favor another, so that the latter natural enemy may tend to compensate for the reduced efficiency of the former.

          Howard and Fiske made these points the basis of their so-called sequence theory of multiple importations. When several natural enemy species are established on a common host, they are more likely to parasitize that host over a greater geographic range than a single species of natural enemy. Multiple introductions increase the chances of obtaining a species of natural enemy that can use alternate hosts to overcome difficulties associated with seasonal fluctuation in pest abundance. Multiple importations favor the chance of establishing a truly superior species of natural enemy.

                  It is well known that wild parasitoid populations exhibit seasonal and geographical differences in behavior and morphology.  Therefore, collections meant for importation should optimally include isolates from diverse areas and different times of the year.  Differences include aggressiveness, heat and cold tolerance, uniparentalism, gregarious versus solitary development, the number of eggs deposited into a single host, larval cannibalism intensity and parasitoid size.  Detailed studies on Muscidifurax uniraptor, M. raptor and M. raptorellus demonstrate the great amount of diversity that can be found within one genus (fly-par.htm).

Clausen's 3-Host Generation / 3-Year Rule

          A good exception to the Clausen rule is provided by the mymarid egg parasitoid, Patasson nitens imported from Australia into South Africa in 1926. Complete biological control of the eucalyptus weevil was achieved within the required three years in southern and southeastern parts of the country. However, in the northeastern highlands where conditions were less favorable to both host and parasitoid, several additional years were required for the parasitoid to bring about substantial control of the eucalyptus weevil. This example also nullifies the generalization that egg parasitoids alone would not prove capable of biological control.

Single Larval Parasitoid Importations

          A good example of a single larval parasitoid working successful biological control is the tachinid, Ptychomyia remota, introduced into Fiji from Malaya in 1925, which resulted in the complete control of the coconut moth. This also illustrates a case where an area other than the native home of a pest produced a useful biological control agent, since Ptychomyia's natural host in Malaya was a related, but innocuous species of native moth.

Single Pupal Parasitoid

          The imported cabbage worm controlled in New Zealand by Pteromalus puparum introduced from North America in 1933 is a notable example.  Periodic liberations of Muscidifurax zaraptor to control muscid flies breeding in decomposing wastes is sustained by several commercial insectaries worldwide.

Other Generalizations

          Such generalizations as biological control being more likely to succeed against pests of perennial rather than short-lived annuals, against sessile or nonmotile pests, or against alien rather than native pests, must also be qualified. As with any generalization, there are exceptions to the rule. Analyses of the results of past efforts can provide useful guidelines.

          It will probably continue to hold that the number of successes attained in biological control in any one country is directly proportional to the amount of research and importation work carried out there. Hawaii, California, the rest of the United States, New Zealand and Australia, as well as the former Commonwealth Institute of Biological Control, currently lead in the number of cases of successful biological control of insect pests and weeds brought about by imported natural enemies. This reflects the proportionately greater amount of biological control programs instituted by each of those countries where early impetus was provided by the proportionately greater losses that those countries have suffered from introduced pests.

          There are of course many other countries reporting successful cases of biological control. Many of these are represented by only one or two successes that resulted largely from trans-shipments of biological control agents of proven value following their initial successful employment in other countries. Four insect pests that have been controlled in this manner in various countries are:

          A. Cottony-cushion scale controlled by the Rodolia (Vedalia) beetle in 55 countries following its initial success in California.

          B. Woolly apple aphid controlled by Aphelinus mali in 42 of 51 countries into which it was introduced following its initial success in New Zealand.

          C. White peach scale controlled by Prospaltella berlesei in 5 countries following its initial success in Italy.

          D. Citrus blackfly controlled by Eretomocerus serius in 9 countries following its initial success in Cuba.

Pest Groups

          Further analysis reveals that 55% of the 107 pest species brought under some measure of biological control through 1960 belong to the Homoptera, nearly 40% of which are scale insects. 20% of the pests are Lepidoptera; 17% are Coleoptera, while 8% belong to other taxa.

Natural Enemy Groups

          Because a majority of successes have involved coccids, it follows that a large proportion of the natural enemies involved in biological control success have been natural enemies of scale insects:

          Hymenoptera-- Encyrtidae & Aphelinidae

          Coleoptera-- Coccinellidae

          This grouping will probably change as more emphasis is given to nonhomopterous pests.

          For weed control, Homoptera-Hemiptera, Thysanoptera, Coleoptera, Lepidoptera, Diptera and Hymenoptera.

          It is suggested that biological weed control has registered a proportionately greater measure of success than biological control of insect pests. Only during the last few years has the method been used against weeds other than those infesting relatively stable, undisturbed rangelands. Weeds engage in intense competition for space, water and nutrients with other plants, and the competitive advantage of these other plants may be strongly favored by further additional insect injury to the weeds. Plant injury by weed-feeding insects may be attended and intensified by the action of plant pathogens. The work has been necessarily restricted to promising prospective biological control agents.

          Unlike insect hosts, plants do not always die from the attack of a single insect. The greater numbers of natural enemies that are thus generated at low host densities makes for a greater searching effectiveness on the part of biological weed control agents.


Exercise 5.1-- What evidence supports the contention that biological control is among the most cost effective methods of pest control?

Exercise 5.2-- Explain how naturally occurring biological control organisms have been shown toe be important in maintaining pest insects at relatively noneconomic levels.

Exercise 5.3-- How have the benefits and costs of classical biological control been evaluated?

Expercise 5.4-- Explain the Island Theory in biological control.

Expercise 5.5-- Why are parasitoids thought to be better biological control agents than predators?

Expercise 5.6-- Discuss the Multiple versus The Best species opinions for biological control introductions.

Expercise 5.7-- What is Clausen's 3-Host Generation/3-Year Rule?

Expercise 5.8-- Give examples of classical biological control involving (1) a single larval parasitoid (2) a single pupal parasitoid.

Expercise 5.9-- Give four examples of transhipments of biological control agents of proven value following their initial successful deployment in other countries.

Expercise 5.10-- Summarize biological control successes according to (1) pest groups (2) natural enemy groups.




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