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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. 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. 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). 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. Exercises: 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)
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