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                                      ECONOMIC GAINS FROM BIOLOGICAL CONTROL

 

 

I.  Abundant empirical evidence shows that biological control, as practiced by professionals is among the most cost effective methods of pest control. 

 

A.  Because of its highly positive social and economic benefits, biological control should be among the first pest control tactics to be explored. 

 

B.  Biological control workers must not be without caution in introducing exotic organisms, which mitigates against granting too wide a license for such introductions.  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. 

 

C.  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 (Gutierrez et al. 1999).

 

D.  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. 1999).  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. (1999) consider periodic colonization of natural enemies (inundative and inoculative) as an extension of biological control.  It is confusing 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. 

 

E.  In the present 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 merits 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 1986a,b).  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).

 

F.  The biological impact of exotic biological control agents on target pests is difficult to assess and few cases have been rigorously documented (Luck et al. 1988), which makes economic analysis correspondingly 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.

                                                                      

II.  Biological Control From Naturally Occurring Organisms

 

A.  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. 

 

B.  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. 

 

C.  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. 1999).  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. 1999).

                                                                      

III.  Estimation of the Benefits and Costs of Classical Biological Control

 

A.  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).

 

B.  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. 

 

C.  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.

 

D.  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.

 

E.  Biological Control Benefit Computation.

 

  1.  This 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.

 

  2.  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.

 

  3.  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.

 

 

                            t

II  =  Z (Bi - Ci) / (1 + @)i

                          1=1

[ 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.

 

  4.  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. 1999), 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.

 

                   5.  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.

 

  6.  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 1999) 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).

 

  7.  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. 1999).

 

 

  8.  Gutierrez et al. (1999) 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.

 

  9.  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 perceived 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.

 

F.  Justification of Need For Biological Control.

 

 

  1.  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 perceived 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.

 

  2.  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. 1999).  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.

 

  3.  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 formers 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. 

 

G.  Biological Control Versus Pesticide Use.

 

                  1.  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. 

 

  2.  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).

 

  3.  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).

 

Despite effective natural control, growers may still perceive 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.

 

  4.  Two concepts 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. 1999).  If there is effective natural control (e.g., San Joaquin Valley cotton), growers who do not wish to take risks still perceive 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. 

 

  5.  Thus it is important how a grower perceives 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. 1999).

 

 

REFERENCES:

 

Adkisson, P. L.  1972.  The integrated control of insect pests of cotton.  Proc. Tall Timbers Conf. Ecol. Anim. Control Habitat Mngmt., Tallahassee, Florida 4:  175-88.

 

Andrés, L. A.  1977.  The economics of biological control of weeds.  Aquatic Botany.  3:  111-23.

 

Bennett, F. D.  1969.  TAchinid flies as biological control agents for sugarcane moth borers, p. 117-18.  In:  J. R. Williams, J. R. Metcalfe, R. W. Mungomery & R. Mathes (eds.), Pests of Sugar Cane.   Elsevier Publ., New York.  568 p.

 

Burrows, T. M., V. Sevacherian, H. Browning & J. Baritelle.  1982.  History and cost of the pink bollworm (Lepidoptera: Gelechiidae) in the Imperial Valley.  Bull. Ent. Soc. Amer. 28:  286-90.

 

Caltagirone, L. E. & C. B. Huffaker.  1980.  Benefits and risks of using predators and parasites for controlling pests.  Ecol. Bull. (Stolkholm) 31:  103-09.

 

Clausen, C. P. (ed.). 1978.  Introduced Parasites and Predators of Arthropod Pests:  A World Review.  U. S. Dept. of Agriculture, Agric. Handbk. No. 480., Washington, D.C.  545 p.

 

Cullen, J. M.  1985.  Bringing the cost benefit analysis of biological control of Chondrilla juncea up to date, p. 142-5.  In:  E. S. DelFosse (ed.), Proc. 6th Internal. Symp. Biol. Contr. Weeds, 19-25 Aug, 1984.  Vancouver, Canada.  Agric. Canada.

 

Day, W. H.  1981.  Biological control of alfalfa weevil in northeastern United States, p. 361-74.  In:  G. C. Papavizas (ed.), Biological Control in Crop Production.  BARC Symp. No. 5, Allenheld, Osmun, Totowa, New Jersey.  461 p.

 

Dean, H. A., M. F. Schuster, J. C. Bolling & P. T. Riherd.  1979.  Complete biological control of Antonina graminis in Texas with Neodusmetia sangwani (a classic example).  Bull. Ent. Soc. Amer. 25(4):  262-67.

 

DeBach, P.  1971.  The use of imported natural enemies in insect pest management.  Proc. Tall Timbers Conf. Ecol. Anim. Control Habitat Mngmnt., Tallahassee, Florida 3:  211-32.

 

DeBach, P.  1974.  Biological Control by Natural Enemies.  Cambridge Univ. Press, London.  323 p.

 

DeBach, P., E. J. Dietrick, C. A. Fleschner & T. W. Fisher.  1950.  Periodic colonization of Aphytis for control of the California red scale.  Preliminary tests, 1949.  J. Econ. Ent. 43:  783-802.

 

Doutt, R. L.  1964.  The historical development of biological control, p. 21-42.  In:  P. DeBach (ed.), Biological Control of Insect Pests & Weeds.  Reinhold Publ, New York.  844 p.

 

Ehler, L. E. & L. A. Andrés.  1983.  Biological control:  exotic natural enemies to control exotic pests, p. 295-418.  In:  C. L. Wilson & C. L. Graham (eds.), Ecotic Plant Pests and North American Agriculture.  Academic Press, New York.  522 p.

 

Ehler, L. E. & R. van den Bosch.  1974.  An analysis of the natural biological control of Trichoplusia ni (Lepidoptera: Noctuidae) on cotton in California.  Canad. Ent. 106:  1067-73.

 

Ehler, L. E., K. G. Eveleens & R. van den Bosch.  1973.  An evaluation of some natural enemies of cabbage looper in cotton in California.  Environ. Ent. 2:  1009-15.

 

Eveleens, K. G., R. van den Bosch & L. E. Ehler.  1973.  Secondary outbreak induction of beet armyworm by experimental insecticide application in cotton in California.  Environ. Ent. 2:  497-503.

 

Falcon, L. A., R. van den Bosch, J. Gallagher & A. Davidson.  1971.  Investigation on the pest status of Lygus hesperus in cotton in central California.  J. Econ. Ent. 64:  56-61.

 

FAO.  1987.  Production Yearbook 1986.  United Nations, FAO, Rome. Vol. 40.  306 p.

 

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Gould, F.  1986.  Simulation models for predicting durability of insect-resistant germ plasm:  a deterministic diploid, two-locus model. Environ. Ent. 15:  1-10.

 

Gutierrez, A. P., Y. Wang & U. Regev.  1979.  An optimization model for Lygus hesperus (Heteroptera: Miridae) damage in cotton:  The economic threshold revisited.  Canad. Ent. 111:  41-54.

 

Gutierrez, A. P., P. Neuenschwander, F. Schulthess, J. U. Baumgaertner, B. Wermelinger, B. Loehr & C. K. Ellis.  1988a.  Analysis of biological control of cassava pests in Africa. II.  Cassava mealybug Penococcus manihoti.  J. Appl. Ecol. 25:  921-40.

 

Gutierrez, A. P., J. S. Yaninek, B. Wermelinger, H. R. Herren & C. K. Ellis.  1988c.  Analysis of the biological control of cassava pests in Africa.  III.  Cassava green mite Mononychellus tanajoa.  J. Appl. Ecol. 25:  941-50.

 

Gutierrez, A. P., B. Wermelinger, F. Shulthess, J. U. Baumgaertner, H. R. Herren, C. K. Ellis & J. S. Yaninek.  1988b.  Analysis of biological control of cassava pests in Africa. I.  Simulation of carbon, nitrogen and water dynamics in cassava.  J. Appl. Ecol. 25:  901-20.

 

Gutierrez, A. P., L. E. Caltagirone & W. Meikle.  1999.  Economics of biological control.  In:  Principles and Application of Biological Control.  Academic Press, San Diego CA.  1046 p.

 

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