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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). Analysis of Successes The most thorough resume of biological control efforts
and successes may be found in Clausen (1978). Another publication will be
released later in 1991 by the University of California Press that discusses
in great detail some of the outstanding contributions to pest control
employing the biological control method. 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. 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. 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) pest groups (2) natural enemy
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