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[ Please
refer also to Selected Reviews & Detailed Research ] Introduction The biological control of pests with imported natural enemies
involves the addition of new biotic mortality factors to the pest's
ecosystem. This practice is often carefully scrutinized by regulatory
agencies, which strive to eliminate the establishment of potentially harmful
organisms. Biological control researchers increasing seek more effective
guidelines for judging a natural enemy's capabilities before importation in
order to accelerate biological control success rates and to reduce project
costs (Coulson 1981). The manner by which biological control is achieved
varies considerably among projects and the various countries utilizing the
technique; and there is a continuing debate on proper procedures for
selection of natural enemies and regulation of their importation (Legner
& Bellows 1999). 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). The primary
goal of federal, state or university importation programs is the same, i.e.,
the collection, safe transport, and quarantine processing, leading ultimately
to the colonization in the field of candidate biological control agents.
However, there are differences in the methods which are, or can be, used by
each entity. Perhaps the main factor in the United States is that the U. S.
Department of Agriculture (APHIS) either on its own initiative or in
concurrence with overriding dicta (as from the Environmental Protection
Agency) issues regulations regarding the importation and quarantine handling
of biological agents which the USDA (ARS), individual states and universities
are expected to follow. Of mutual
concern to the explorer/collector/shipper and government regulatory agencies
and quarantine personnel are the identification of target species and their
hosts, permits to import the material collected, packaging and labeling,
method of shipment, clearance at the port of entry by customs and
agricultural inspectors, and the quarantine facility itself. Most of the
technical and biological considerations relative to acquiring and shipping
biological agents remain much the same as those described for entomophagous
arthropods and /or weed feeders by Bartlett and van den Bosch (1964), Boldt
and Drea (1980), Coulson and Soper (1989), Klingman and Coulson (1983), and
for phytophagous (weed feeders) organisms by Schroeder and Goeden (1986). In
actual operation USDA (ARS) sponsored quarantine laboratories receive
shipments which usually originate from a USDA laboratory abroad where the
material has been screened for contaminants before being shipped to a primary
USDA quarantine facility in the United States, such as the laboratory at
Newark, Delaware, where further screening for unwanted organisms may occur
before the biological agent is forwarded to requestors in the field who may
or may not work out of a secondary quarantine facility where the biological
agent can be propagated or released directly into the field. State
departments of agriculture or universities usually send out members of their
staff as explorer/collectors, who typically do not have access to laboratory
facilities while in the field. As a consequence shipments sent to their
quarantine laboratories may contain more than one targeted pest species and
more than one natural enemy of each of these. They must then be segregated in
quarantine and studied through one generation (for newly introduced species)
before they can be released. Unsolicited extraneous material inadvertently
included may warrant further study in quarantine. If so, specific
arrangements must be made with APHIS PPQ regarding the handling of such material.
USDA collectors when abroad can utilize all available U.S. governmental
facilities (embassies, agricultural attaches, commissary, vehicles,
communication facilities, etc.) to expedite their missions. Thus far U.S.
state and university collectors abroad have only rarely been able to avail
themselves of similar federal cooperation even though their missions were
financed by public funds and their efforts would potentially accrue to the
benefit of agricultural crop production on a regional if not national scale
in the U.S. International
geo-political and socio-economic unrest may impact heavily on the success of
failure of foreign exploration missions. Terrorism in its broadest sense has
become a major deterrent to the search for biological agents in many areas of
the world. Colleagues in such areas or intermediary organizations (i.e.,
charging a fee for service), such as the Commonwealth Institute For
Biological Control, Silwood, UK, may be able to supply the desired beneficial
organisms, but experience has shown that biological control workers who know
what they need and who physically participate in the collecting process tend
to make a better showing in terms of successful introductions (Legner &
Bellows 1999).. A highly
important consideration is that during the last 25 years the number of
students trained in biological control and population ecology entomology
worldwide has been on the increase. The hope is that this expanding pool of
"applied ecologists" portends improved international cooperation
regarding greater use of the biological method of pest control. However, it
is anticipated that further legal constraints on biological control of pests
are, or will be, imposed by new and/or pending technical regulations
ostensibly aimed at protecting endangered species or the environment. These
regulations could severely hamper or preclude importation and field use of
new candidate natural enemies. Purpose
and Need The purpose
for exploration is to search for, import and colonize natural enemies of our
pests from areas where the pest is indigenous, or at least present in low
numbers because its natural enemies keep it in check. The need for
exploration is to protect our environment from needless or questionable use
of chemical pesticides, especially those with long half lives and/or broad
spectrum toxicity which can adversely affect non-target species and
beneficial organisms and ultimately the food chain within a wide range of biologically
diverse species. The basic
goal is to import species of strains presumed to be pre-adapted to areas
targeted for colonization of beneficial organisms. One tries for large
founder numbers in order to keep the gene pool as large as possible. Although
traditionally used for homopterous pests of perennial crops (DeBach 1964), it
is increasingly considered for non-homopterous and annual pests in
agricultural, urban and glasshouse environments. Extra agricultural uses in
medical, forest and household entomology are expanding. Environmental
concerns and laws, public opinion and resistance of arthropod and weed pests
to chemical pesticides are increasingly forcing a consideration and implementation
of non-chemical solutions of pest problems. Classical biological control is a
powerful and proven tool. The increasing threat that federally mandated
regulations may neutralize the importation and colonization of new natural
enemies by greatly slowing the process far beyond sound biological protocols
which have served applied biological control and society for well over 100
years. Planning
and Preparation Funding.--There is a need for a long term stable commitment so that
chronic pest problems can be pursued. Ongoing search missions usually require
a minimum of 12 to 18 months of preparation, and the explorer must have
assurance that the funds will become available when the trip finally is
activated. Commodity
groups plus state funding can provide support for "brush fire"
needs, but coming up quickly with a qualified explorer or well planned
biological control campaign may be difficult to arrange on short notice.
Experiment station staff might use sabbatical leaves for extended studies
abroad, academic responsibilities permitting. Or, a full time person might be
hired to carry on broad biology field studies abroad, as is currently
practiced by the USDA, ARS in biological weed control. The Explorer/Collector.--The long tradition of classical biological control in the
University of California (beginning in 1923) has included exploration by its
academic and nonacademic staff. Initially this effort served mainly the
California citrus and subtropical fruit industries. In the late 1940's and
through the 1950's other crop systems began incorporating biological control
into their pest management programs. In California early notable biological
control successes of certain pests occurred in alfalfa (spotted alfalfa
aphid, alfalfa caterpillar, pea aphid), walnuts (walnut aphid), olives (Parlatoria
scale), and native pasture (Klamath Weed). Ongoing research with natural
enemies has resulted in partial to complete economic control of other pests
on natural those and other crops. The results are now obvious: in crop
systems which are managed from an ecological perspective, natural enemies
must be accorded high level consideration in the planning and implementation
of pest control practices...they cannot be ignored! Diplomacy by the explorer must be a primary concern. Explorers also must be aware that the
public they encounter in the field will invariably be ignorant of their
goals. In some cases, especially
among primitive societies, hostility
may be shown if the search and collection activities appear strange. When gifts such as fruit are presented to
the explorer, they must always be received graciously and with a minimum of
scrutiny. An example would be if one
is presented with oranges from a resident’s backyard, even though the fruit
may be crawling with insect life this should be ignored in the presence of
the bearer. Choosing the Explorer/Collector.--The explorer must have a broad knowledge of the target pest
and its known natural enemies, including their range, host plants, biology,
and taxonomy. The explorer must also be willing to travel, often under
adverse circumstances. Typically designated to perform this service are
academic or professional grade staff from state or federal agriculture
experiment station (usually scientists already working on the target pest or
its close relatives). Foreign
Collaborators Because of
their own time commitments and responsibilities, it is usually not productive
to ask or expect foreign colleagues to search for and ship to the home
quarantine facility the desired natural enemies. However the cooperation of
colleagues or contacts in the area being contemplated for search is highly
important for maximizing the often compressed time frame within which an
explorer may be obliged to perform assignments. Countries of the British Commonwealth have been especially
helpful in locating beneficial species due to their maintenance of permanent
research laboratories staffed with highly competent scientists. Problems
Encountered by Explorers From
Universities Because of
personal promotion constraints which require research, publication and
teaching, university academic appointees often cannot afford the time
required to thoroughly search for and study target natural enemies in the
field, either foreign or domestic (Legner & Bellows 1999).. Universities
also often set maximum limits for lodging expenditures, which although
usually based on the U. S. Government format, are frequently outdated and too
low, such that the explorer is faced with incurring personal expenses even
though granting agencies have provided ample funds. For expedient processing
of large field plant and insect samples, air conditioned spacious
accommodations with adequate lighting are essential even though such may
exceed the limits posed by institutions for reimbursement. A straight per
diem allowance without reimbursement by receipt tends to overcome the higher
charges which are equalized by lower cost accommodations en route or by some
nights spent in travel vehicles, tents, etc. A per diem allowance gives the
explorer flexibility to judge, which is the most effective way to direct the
project. However, a punitive attitude
of incrimination sometimes has developed by “clerks” in some prestigious
institutions in dealing with a scientists expenses (e.g., the University of
California). Choosing
Targets and Procedure Two schools
of thought are (1) narrowly targeted natural enemies based on available
information and (2) shotgun approach if information base is weak. The
procedure involves a literature search, taxonomy and the study of museum
material, or possibly voucher specimens from earlier trips dealing with the
same pest, correspondence with collaborators abroad in order to determine the
best season to search. The latter include referrals from colleagues, local
agricultural extension people, botanists, botanic gardens, nature preserves, especially
in the search area, and a letter of introduction from the host country's
Consulate to institutions for the temporary use of their facilities. Permits are
then obtained which involve technical and bureaucratic requirements. Such
permits are required for importation to the home country (in the US the
agency is APHIS issuing PPQ 526 stickers), and often in the host country
where regulations govern the export of living or dead (museum) materials.
Individual states or provinces may impose further restrictions. Arrangements
must be made with the receiving quarantine laboratory. The explorer should
have a valid passport, necessary visas, immunization inoculations, letters of
authorization from the home institute, USDA and proper officials in the
country of search, showing names of cooperating institutions and/or
individuals collaborators. Field notes
should be taken of the names of contacts, villages, farms, host plants, other
pests noted, and possible sources of beneficial organisms for other crops,
etc. Maximizing
Success Potential and Reducing Risks A potential
for maximizing biological control successes is the placement of the various
natural enemy groups into different risk categories before proceeding with
data based decisions for introduction. Although the full impact of natural
enemies which have never been studied is impossible to accurately predict
before establishment (Coppel & Mertins 1977, DeBach 1964, 1974; Ehler
1979, Miller 1983), and therefore involves empirical judgment, there is
nevertheless a strong desire that the process proceed with an educated
empiricism (Coppel & Mertins 1977, Ehler & Hall 1982, Legner 1986b). Whatever the
theory behind biological control by natural enemies, it is still one of the
most awesome weapons in our arsenal of pest management techniques. Because of
its relative permanency, mistakes cannot be readily corrected. Imported
organisms, once established, are not easily extirpated; and in some instances
their elimination is impossible altogether, regardless of the amount of
effort and funding. There is, therefore, some risk involved in any biological
control approach. But risks are a companion to life itself and any pest
management strategy involves some degree of risk, with alternatives to
importation of natural enemies undoubtedly being more formidable (Legner 1986b, Pimentel et
al. 1984). Trying to eliminate too much risk through government regulation is
not advisable as it can have the paradoxical effect of making life more
dangerous as well as more expensive and less convenient (see Huber 1983). The broad
nature of biological control including manipulation of vertebrates, arthropods
and pathogens, allows categorization of two major risk groups according to
(1) risks to the environment and health of humans and domestic animals, and
(2) risks of making wrong choices which may preclude or adversely affect
biological control at a later date. Environmental
risks are of especial concern in the biological control of weeds, where both
arthropods and pathogens are candidates for importation. The use of
vertebrates for biological control of terrestrial pests is loaded with
potential risk and is rarely practiced. An apparent desirable example is the
common myna bird, Acritotheres
tristis L., importation from
India to tropical areas for insect control. However, mongoose, Herpestes auropunctatus birmanicus
(Hodgson), importation from India to tropical islands for rat, Rattus spp., control and giant
toad, Bufo marinus (L.), from America to
tropical islands and Australia for insect control have produced undesirable
side effects, either through numerical abundance or in the latter example by
predation of beneficial dung beetles (Macqueen 1975). Environmental
risks are minimized when vertebrates such as fish are imported to restricted
aquatic ecosystems for the biological control of noxious aquatic weeds,
mosquito habitats or mosquitoes and chironomids. Such fish can be studied
under natural conditions, but in isolation for adverse effects before being
widely disseminated. Yet, there are still possibilities that undesirable
unforseen behavioral and adaptive traits, such as spawn-feeding on other
desirable fish species, or an extension of subtropical species into temperate
climates (e.g., Gambusia
spp.) may be expressed once populations are allowed to establish broadly
(Legner & Sjogren 1984). The risk of making wrong choices of
parasitic and predatory arthropods, especially host specific ones, does not
pose obvious environmental threats, as the outcome of the establishment of an
innocuous natural enemy is the pest density remaining at status quo; although
there is some theoretical debate on that issue (Ehler 1982, Turnbull 1967).
However, wrong choices could possibly preclude the achievement of maximum
biological control (Force 1970, 1974; Legner 1986a), and add to
the list of failures, so that careful decisions are desirable (Franz 1973a,
1973b; Hughes 1973). The need for choosing the best biological control
candidates has generated considerable discussion and controversy over the
past 2 1/2 decades, and continues to stir controversy (Ehler 1976, 1982;
Huffaker et al. 1971, Legner 1986b, Turnbull &
Chant 1961, Pimentel et al. 1984, van den Bosch 1968, van Lenteren 1980, Watt
1965, Zwolfer 1971, Zwolfer et al. 1976). However, the manner in which the
best biological control candidates are chosen is not clearly delineated for
most groups of organisms (Coppel & Mertins 1977, DeBach 1974, Pimentel et
al. 1984); albeit there is a common desire for prejudgment, if for no other
reason than to expedite a biological control success. Securing
Natural Enemies in the Native
Range Some of the
most dramatic successes in biological control, where the target pest's
population density is permanently reduced to below the economic threshold,
involved the introduction of one or two species of natural enemy (DeBach
1964, 1974; Clausen 1978, Franz 1961a, 1961b; Franz & Krieg 1982, Hagen
& Franz 1973, Luck 1982, van den Bosch 1971), in both stable and unstable
habitats (Hall et al. 1980, Ehler & Miller 1978). The usual procedure
when a pest species invades a new area is to seek natural enemies in its
native home. The first and most widely known biological control success, the
cottony-cushion scale, Icerya
purchasi Maskell controlled
by Cryptochaetum iceryae (Williston) and Rodolia cardinalis (Mulsant), followed that pattern (Quezada &
DeBach 1973). The scale invaded California and the natural enemies were found
in its native range, southern Australia. It seemed logical to follow the same
format with subsequent biological control efforts; and indeed this is still
considered one of the first approaches for a newly-invaded pest. However,
using this approach solely restricts the number and kinds of successes that
can be realized. Searching
Outside the Native Range In many parts
of the world, especially Europe, Africa and much of Asia, there are numerous
native pests whose natural enemies are incapable of maintaining density
levels below the economic threshold under prevailing agricultural management.
What may be done other than costly and hazardous cultural and chemical
control? Pimentel (1963) and Hokkanen & Pimentel (1984) pointed out the
best approach for successful biological control. In many instances, the
natural enemies which caused significant drops in population densities of
organisms had never experienced evolutionary contact with their hosts. When
we consider other cases than those exemplified by the host/parasitoid/predator
relationship in cottony-cushion scale, we find examples of great reductions
of a population density by organisms that originated in places other than the
native home of the host. Such cases may include the devastation of desirable
native species by accidentally invaded organisms. Well known examples to
illustrate this phenomenon are the American elm, Ulmus americana
L., destroyed by the fungus, Ceratoystis
ulmi (Boisman) C. Moreau, of
eastern hemispheric origin and vectored primarily by the European beetle Scolytus multistriatus (Marsham); the American chestnut, Castanea dentata (Marsham) Borkhauser, practically eliminated by a
fungus, Endothia parasitica (Murriu) Anderson
and Anderson, of Asian origin; and Asiatic citrus destroyed by Icerya purchasi from Australia before biological control efforts
reduced the scale's density. There is a
history of successful biological control against invaded pests by organisms
actively secured in areas other than the native home. Famous examples are the
European rabbit, Oryctolagus
cuniculus (L.), regulated by
a myxomytosis virus of South American origin and the black scale, Saisettia oleae (Bernard), of probable northern African origin (D.
P. Annecke, pers. commun.), regulated by Metaphycus
helvolus (Compere) from
extreme southern Africa. Also, the sugarcane borer, Diatraea saccharalis
(Fab.), of the Neotropics regulated by Apanteles
flavipes (Cameron) from
northern India; the coconut moth, Levuana
iridescens Bethune-Baker,
native to Fiji, regulated by a parasitoid, Bessa remota
(Aldrich) secured from Malaya; and Oxydia
trychiata (Guenée) in
Colombia regulated by Telenomus
alsophilae Viereck from
eastern North America (Bustillo & Drooz 1977, Drooz et al. 1977). Many
other examples exist (Pimentel 1963, Pimentel et al. 1984), which has led to
some speculation that the best
natural enemies for biological control might be those that have not
experienced close evolutionary contact with the target organism. The theory
considers that the host and its natural enemies coevolve to a balanced point
where the host may exist at a relatively higher density then where no co
evolutionary balance has had a chance to evolve. Although the examples of
drastic impact on a host by natural enemies without recent preevolutionary contact
are numerous and impressive, there are equally impressive and thorough
examples of host reduction in which natural enemies were obtained from the
native home where coevolution has occurred. The cottony-cushion scale and
Comstock mealybug, Pseudococcus
comstocki (Kuwana) (Ervin et
al. 1983) successes illustrate this clearly. Thus, we would not want to
deemphasize the native home as suggested by Hokkanen and Pimentel (1984). In
fact, in a recent analysis Hokkanen (1985) concluded that there are no
differences of biocontrol success according to the origin of the pest, and
that native hosts can be completely controlled by introduced natural enemies
exactly as exotic ones. However, as time goes on, one would expect examples
of the latter to drop proportionally because the former include accidental
invasions and random acquisitions, as well as planned biological control
attempts. There are
some antagonists to the approach endorsed by Hokkanen and Pimentel,
especially among the biological control of weeds researchers (Goeden &
Kok 1986, Schroeder & Goeden 1986). They argue that the record in the
case of weeds supports the native home approach most strongly. However, the
record does not seem wholly convincing evidence against Hokkanen's position,
because historically most biological weed control efforts emphasized the native home in the
search for natural enemies. Thus, statistically the argument is biased and
seems weak. Hazards & Awkward Situations
Encountered in the Field Researchers exploring for natural enemies in the field
frequently encounter situations that are hazardous or at least unpleasant. The account by Dr. Alfred Boyce of his discovery of the citrus
red scale parasitoid, Aphytis melinus in northern Pakistan clearly
illustrates this. Dr. Boyce had
obtained the original cultures from that region that was typically undergoing
intense political unrest. He was able
to bring out living cultures “in a hail of bullets” as he once described it
to Dr. Fred Legner. In his search for natural enemies of
common house and stable flies, Dr. Legner
often found himself in awkward and even dangerous circumstances,
stemming from suspicions by local inhabitants of techniques involved in the
retrieval of host material from animal waste habitats. Once in San Jose, Costa Rica while
processing samples of cattle manure for the presence off ly puparia
the director of the National Museum complained bitterly about such
“uncivilized” activity. In Central
Africa while searching for nests of the mountain gorilla, a prime breeding
site for Musca domestica, the Congo guides accompanying Legner and an
associate from Harvard University became suspicious about the collectors’
motives, probably suspecting witchcraft.
Legner’s Harvard companion who understood the local dialect alerted
the team to the unsettling remarks being made by the guides. This prompted an immediate cessation of
collection activity and a rapid return to base camp at Travelers’ Rest in
southwestern Uganda. That night
Congolese rebels stormed into the premises confiscating the area around the
bar; and after becoming quite drunk tore up the premises by breaking
furniture and urinating liberally around the place. While
exploring in and around the area of Queen Elizabeth National Park in
western Uganda, the accommodations at the park had been pilfered such that
there were no mattresses on the beds, and the water faucets had been removed
from outside taps. At night elephants
and hippos converged on the area to obtain water seeping from the broken
taps, which precluded one’s ability to visit adjoining bathroom
facilities. In the same
area of western Uganda while driving on a dirt road in tall grass country, a
mother elephant and her young crossed directly in front of the
automobile. After seeing the vehicle
the elephant returned to face the challenge!
A stand-off ensued whereby every time the car moved the elephant returned
with great agitation to ward off the intruder. Finally after part of an hour of this conflict, Legner and his
family who had accompanied him on this expedition, made a quick dash ahead on
the road to escape the irate elephant. One time the
automobile sustained a flat tire in a remote region of northern Uganda, where the dress code of
the inhabitants was sparse or nonexistent.
During the 20 minutes that it took to change the tire, people had
gathered in large numbers around the area to view the procedure. On completion and placing the damaged tire
into the trunk of the car, Dr. Legner was tapped on the shoulder by one of
the throng The man held up a
screwdriver that had mistakenly been left on the road bed, and in perfect
King’s English commented, “Sir, you left this on the ground!” In 1966 just
prior to the reign of Idi Amin, ominous drums were sounding in and around
Kampala, the research station that housed the researchers searching for
natural enemies of house flies and other pest insects. Local European residents talked frequently
of witch doctors predicting terrible events.
This made carrying on one’s collections difficult to say the least as
great efforts were made not to upset the inhabitants residing in remote
areas. A search in
Israel for fly breeding sites during late 1966 happened to take Dr. Legner
and colleague Dr. Dan Gerling to the border area between Israel and Jordan,
just north of Beersheba. A person was
noticed on the road ahead, and the custom was for motorists to offer rides to
pedestrians. However, on closer
examination it was noted that the person was wearing the uniform of a
Jordanian soldier, and no ride was offered.
That night they learned that the road they had been driving on earlier
in the day was booby trapped, resulting in the death of several Israeli patrol guards. This started an immediate build-up of
armed personnel in Beersheba and soon afterwards led to the 6-day war. Attempts to visit collection sites in the
Sinai and Eilat were then thwarted by Egyptian MIG jets that were strafing
cars on the highways. Judgments
of Natural Enemy Capability Ways in which
the capabilities of a biological control agent are judged, as well as the
environmental threats, vary for different groups of organisms. If we consider
in descending order of environmental risk, the terrestrial vertebrates first,
followed by zoopathogens, phytopathogens, phytophagous arthropods,
terrestrial scavengers (e.g., scarab beetles), aquatic vertebrates and
invertebrates, and finally parasitic and predatory arthropods, it would be
logical to screen the first group more thoroughly than the last. However,
judgments of potential effectiveness would probably require more effort for
the last group and least effort for the first, because of problems in
measuring dispersal and other behavioral traits, as will be explained. Terrestrial Vertebrates.--such as the mongoose, myna bird and giant toad are more
readily observed in their places of origin because of their size, so their attributes
may be more easily viewed. However, they are also capable of becoming
conspicuous additions to the general landscape, and without natural predators
of their own may have the capacity to soar in numbers in the areas of their
introduction. Thus, they may pose the threat of becoming pests themselves
because of their numerical abundance and often nonspecific feeding behavior,
as well as the side effects this can have on native and other desirable fauna
and flora.
Phytophagous Arthropods and Phytopathogens.--have traditionally evoked the most thorough of
preintroduction studies to safeguard desirable plant species in the areas of
introduction (Ehler & Andres 1983. Goeden 1983, Harris 1973, Huffaker
1957, Klingman & Coulson 1982). Host-plant specificity is strongly
emphasized (Harris & Zwolfer 1968, Zwolfer & Harris 1971). Past
screening has been so successful that among the numerous importations of
beneficial phytophagous arthropods and pathogens around the world (Julien
1982), there has never been any widespread occurrence of harmful behavior
shown by the organisms imported. Rare reports of beneficial phytophagous
arthropods feeding on desirable plant species after importation (Greathead
1973), usually involved temporary, geographically restricted alterations of
behavior during the establishment phase when the colony contracted in size
due to its inability to survive and reproduce on alternate host plants. Such
dire reports have been, nevertheless, magnified way out of proportion and
cited out of context as was done recently by Pimentel et al. (1984). Or an
obscure wild plant or recently cultivated variety or relative of a targeted
weed may come under attack by a phytophagous arthropod introduced earlier to
combat the weed, as presently has occurred in northern California with
imported natural enemies of Klamath weed, Hypericum
perforatum L., and a species
of Hypericum used for
roadside planting (Andres 1981). The benefits
derived from importation of phytophagous arthropods and phytopathogens are so
vast and levied against weeds that cannot be controlled effectively,
economically nor safely with any other strategy, that biological control will
continue to be a major effort. The risk involved has been and should continue
to be minimized by the extensive studies required prior to importation. Terrestrial Scavengers.--include scarab beetles that remove excess cattle dung
accumulated in grazing areas to improve pastures, and to control the
symbovine flies, Haematobia irritans (L.), Musca autumnalis deGeer, and Musca
vetustissima Walker,
primarily. Although dramatic successes have been achieved in the removal of
dung by the importation of several species of exotic scarabs in Australia,
Hawaii, California and Texas (Legner 1978a, Legner &
Warkentin 1983, Macqueen 1975,
M. M. Wallace, pers. commun., Waterhouse 1974), there apparently have been no
widespread concurrent practical reductions of fly densities (Legner 1984, M. M. Wallace,
pers. commun.). In some instances fly densities may have actually increased
in the presence of established dung-burying scarabs. Although laboratory and
field experiments predicted practical fly reductions by the dung scattering
activities of the scarabs, in pastures several forces interplay to thwart
experimentally based predictions. Elimination of predatory arthropods and
increase of available larval breeding habitat could be two of the principal
causative factors (Legner & Warkentin 1983, Legner 1984). Terrestrial
organisms that alter large habitats, such as scarab beetles, are especially
risky biological control candidates because their activity may overlap
portions of the niche of other species so that potential disruptive
side-effects among organisms in different guilds exist. The outcome for
future symbovine fly control may be undesirable in that some potentially
regulative natural enemies, such as certain predatory arthropods, may not be
difficult to establish in the disrupted habitat (Legner 1986a). In California
and Texas the predatory staphylinid genus Philonthus
is severely restrained from colonizing the drier dung habitat created by Onthophagus gazella F. (Coleoptera:
Scarabaeidae) activity (Legner 1986b, Legner &
Warkentin 1983, Roth et al.
1983). Furthermore, various nongraminaceous weed species often invade
California irrigated pastures that sustain large populations of exotic scarab
beetles, so that mechanical pasture renovation again is required (Legner
& Warkentin 1983 ).
Aquatic Vertebrates and Invertebrates.--include herbivorous fish for biological aquatic weed and
arthropod control and Turbellaria and Coelenterata for arthropod control. The
minnows Gambusia and Poecilia are used worldwide in
the biological control of mosquitoes (Legner & Sjogren 1984, Legner,
Sjogren & Hall 1974). However, the
threat to endemic fish has caused widespread concern so that alternatives in
the use of native fishes are under consideration (Legner, Medved &
Hauser 1975, Walters &
Legner 1980). Because fish
can be manipulated readily, the potential for resident species to increase
their effectiveness as natural enemies is greater than with terrestrial
organisms where widespread natural dispersion may have already covered most
possibilities. A group of
cichlid fish has been imported from Africa to the southwestern United States
for the biological control of mosquitoes, mosquito habitats and chironomid
midges. Although the degree of control achieved by the three species imported
varied in different parts of the targeted area, circumstances beyond the
control of researchers preempted a broader success. The fish species referred
to are Tilapia zillii (Gervais), Sarotherodon (Tilapia) mossambica (Peters), and Sarotherodon (Tilapia)
hornorum Trewazas, which were
imported to California for the biological control of emergent aquatic
vegetation that provides a habitat for such encephalitis vectors as the
mosquito Culex tarsalis Coquillet, and as
predators of mosquitoes and chironomid midges. Careful studies
under natural, but quarantine, areas in California showed that the different
fish species each possessed certain attributes for combating the respective
target pests (Legner & Medved 1973). Tilapia zillii was best able to perform both as a habitat reducer
and an insect predator. It also had a slightly greater tolerance to low water
temperatures, which guaranteed its survival through the winter months in
southern California, while t the same time it did not pose a threat to salmon
and other game fisheries in the colder waters of central California. It was
the superior game species and most desirable for eating. Nevertheless,
the agencies supporting the research (mosquito abatement and county
irrigation districts) acquired and distributed all three species
simultaneously throughout thousands of kilometers of irrigation system, storm
drainage channels and recreational lakes. The outcome was the permanent and
semipermanent establishment of the two less desirable species, S. mossambica and S.
hornorum over a broader
portion of the distribution range. This was achieved apparently by a
competitive superiority rendered by an ability to mouth-brood their fry,
while T. zillii did not have this
attribute strongly developed. It serves as an example of competitive
exclusion such as conjectured by Ehler & Hall (1982). In the clear waters
of some lakes in coastal and southwestern California, the intense predatory
behavior of S. mossambica males on the fry of T. zillii could be easily observed, even though adults of the
latter species gave a strong effort to fend off these attacks. This outcome
was not too serious for chironomid control because the Sarotherodon species were quite capable of permanently
suppressing chironomid densities to below annoyance levels (Legner, Medved
& Pelsue 1980). However, for
control of higher aquatic weeds, namely Potamogeton
pectinatus L., Myriophyllum spicatum var. exalbescens (Fernald) Jepson, Hydrilla verticillata Royle and Typha
species, they showed no capability whatsoever (Legner & Medved 1973). Thus,
competition excluded T. zillii from expressing its
maximum potential in the irrigation channels of the lower Sonoran Desert of
California and in recreational lakes of southwestern California. Furthermore,
as the Sarotherodon species
were of a more tropical nature, they died out annually in the colder waters
of the irrigation canals and recreational lakes. Although T. zillii populations could have been restocked, attention
was later focused on a potentially more environmentally dangerous species,
the white amur Ctenopharyngodon
idella (Valenciennes), and
other carps. The substitution of T.
zillii in storm drainage
channels of southwestern California is presently impossible because the Sarotherodon species are
permanently established over a broad geographic area.
Parasitic and Predaceous Arthropods.--(Insecta and Acarina) are in a distinct category which
usually defies accurate prejudgment of biological control potential.
Theoretical guidelines based on laboratory studies and mathematical models
are not always useful to judge performance in nature. The extremely small
size of parasitic and predaceous arthropods, their high dispersal capacity,
unique sex determination mechanisms, differential response to varying host
densities and climate, distribution patterns and size, unreliable sample
techniques, dependence on alternate hosts and possibilities of rapid genetic
change at the introduction site (Attique et al. 1980, Eikenbary & Rogers
1973, Legner 1986b, McMurtry et
al. 1978, Messenger 1971, Mohyuddin et al. 1981), make predictions of their
performance highly uncertain. Even sophisticated population models such as
those developed for the winter moth, Operophtera
brumata (L.), could not
predict the exceptional performance of the tachinid parasitoid Cyzenis albicans Fallen against this host in Canada (Embree 1971,
Hassell 1969a, 1969b, 1978, 1980; Waage & Hassell 1982). Because risks
to the environment posed by parasitic and predaceous arthropods are very low,
as previously considered, the inability to predict their impact has not been
a major obstacle to their deployment in successful biological control. At the
same time selection of these biological control agents for importation has
not been unsophisticated and lacking in scientific judgment as was suggested
by van Lenteren (1980). There are valid scientific criteria for deciding
probable good candidates, which are especially useful for elimination of
those with little likelihood for success or which possess certain undesirable
characteristics such as hyperparasitism (Luck et al. 1981). Coppel & Mertins (1977) proposed a list
of 10 desirable attributes of beneficial organisms to aid in assessment of
their capabilities prior to widespread dissemination, which are closely
interrelated and difficult to separate. Nevertheless, they categorize
organisms and weigh their potential according to a scientific plan. The list,
obviously developed from efforts on prior biological control projects,
considers ecological capability, temporal synchronization, density
responsiveness, reproductive potential, searching capacity, dispersal
capacity, host-specificity and compatibility, food requirements and habits,
hyperparasitism and propensity for culture. Additional categories of
importance to a broad understanding of the niche of a potential biological control
agent are systematic relationships and morphological attributes (e.g.,
fossorial structures, heavy sclerotization), and anatomical (e.g., type of
spermatheca), and physiological attributes (e.g., synovigenic or
proovigenic), cleptoparasitism, and genetic data (e.g., number of founders,
collection locality, strain characteristics). Consideration of these
attributes in a more holistic approach to natural enemy acquisition is
desirable because it incorporates interrelations among qualifications which
cannot be detected from an accumulation of single, even well quantified sets
of data. Biological
control researchers have traditionally acquired data under these various
categories whenever possible, which has greatly aided in the interpretations
of the dynamics in biological control successes. However, there has not been
a systematized plan for data acquisition and storage, which can be attributed
to funding primarily. The
collection of facts in the different groups is meant not only to form a data
base for more accurate predictions of success, but also to raise biological
control pursuits to a more intercommunicative realm. The absence of a data
collection system in the past has unquestionably resulted in the loss of
valuable information about biological control organisms. Critics of a more
systematized data acquisition scheme point to the weaknesses of data secured
in experimental fashion, but overlook the fact that even incomplete data can
elevate one's understanding to the "educated empiricism" of Coppel
and Mertins (1977). Although
there is ever more room for additional information about the natural enemies
of a given host to help the foreign explorer, it is becoming increasingly
possible to appraise performance in nature. This knowledge comes from a
combination of laboratory and field studies both in the places of origin of
the respective natural enemies and at their introduction sites. For example,
in the parasitoids attacking endophilous synanthropic flies, we find the
species naturally distributed within certain ecological zones. Cooler, more
humid environments harbor different species and strains than those found in
hot and drier areas. Marked seasonal abundance, host and microhabitat
preferences are demonstrated by the different species (Legner 1986b). Thus, it is
possible to estimate which species is best suited for biological control in a
given area based on knowledge of its ecological requirements. Additionally,
accumulated information on temporal synchrony with the target hosts allows
for a high degree of certainty in the prediction of which species is most
capable of exerting a regulative effect on single host species in a
multi-host habitat (Legner 1986b). Data on
density responsiveness, reproductive potential under different ambient
temperatures and RH, searching capacity in different environments and strata
of the host habitat, dispersal rates, host specificity and parasitoid food requirements,
systematics, synovigenic characteristics, mass production and genetic
variability provide a base from which to judge the likely performance of
particular species in different areas. It also provides a means to properly
sample for host destruction (Legner 1986b). In Australia
there is a widespread opinion that predatory and parasitic natural enemies of
symbovine flies have been duly tested in biological control based on previous
attempts at introduction. However, basic information now available on species
that were tried, such as Aphaereta
pallipes (Say), Aleochara tristis Gravenhorst and Heterotylenchus, suggest that these candidates were never
suited to their targeted hosts in the introduced environment. They were
incapable of exerting much pressure against their hosts because of different climatic preferences
(Legner 1986b). Research that
has been performed on the pink bollworm, Pectinophora
gossypiella (Saunders),
navel orangeworm, Amyelois transitella (Walker), and carob
moth, Ectomyelois ceratoniae (Zeller) has already
laid the foundation for accurate decisions on which species of natural enemies
have a potential for reducing their respective host densities (Legner 1986b, Naumann &
Sands 1984, Sands & Hill 1982). Clues were found to which regions of the
world might be searched for additional candidates. A common
criticism of a systematized approach that there is lack of adequate funding,
suggests that a certain degree of adequacy in financial support ought to be
secured before new projects are embraced. More biological control researchers
are faced with the necessity for holistic studies with the outcome that basic
data are obtained more frequently. In a recent example, the biological
control of chestnut gall wasp, Dryocosmos
kuriphilus Yasumatsu, in
Japan was guided largely by data acquired about the natural enemies in their
place of origin, China (Murakami & Ao 1980, Murakami et al. 1977).
Another example is the Comstock mealybug biological control success in
California (Meyerdirk & Newell 1979, Meyerdirk et al. 1981), which was
guided by basic research on natural enemies in Japan (Murakami 1966). Also,
the success of the North American egg parasitoid, Telenomus alsophilae,
against the South American geometrid, Oxydia
trichiata (Bustillo &
Drooz 1977) was based on results of studies on the parasitoid in North
America (Fedde et al. 1979). Considerations
on Geographical Origin of the Pest A well
conducted natural enemy introduction program requires an initial realistic
appraisal of the pest problem and the chances for success. Natural enemies
are sought in the native home of the pest and/or in an area which includes a
climate similar to that of destination. Species which restrict their attack
to the pest or a close relative of the pest are preferred, and those natural
enemies that possess the highest degree of preference for the pest are
usually chosen for final field release. During the search, natural enemies
are collected from all possible habitats to insure the inclusion of cryptic
forms and races. Special attention is given to the species of host plant on
which the pest is problematic. Biologies, host associations and species
attributes are ascertained automatically during the rearing and transfer
process. The Nearctic
insect fauna is large, including 30 orders, 500 families and about 12,000
genera and 150,000 species (Ross 1953). Knowledge about the origin and
dispersal patterns of insects is in reality very spotty; therefore, the
origin and evolution of the North American insect fauna is largely a subject
to speculate and for making assumptions. It is generally held that the
Palearctic was the center or origin for many of the ancestors of new North
American insects. Although much
can be inferred about the past history of species from studies of their
present range and recent changes in distribution, the fossil records provide
the most objective. Although the staphylinid beetle Oxytelus gibblus
is presently restricted to the western Caucasus Mountains, the fossil records
show that this species was extremely abundant in Britain during the last
glaciation. The western Caucasus, then, probably represents the last stand of the species rather
than its place of origin. The carabid
subgenus, Cryobius, has its
center of distribution in northeastern Siberia and northwestern America.
However, it was represented by a greater number of species in western Europe
during the Wisconsin glaciation, although none are found there at the present
time. The evolution
of the mosquito genus Culex
has been studied through observations of progressive changes in structures of
the male genitalia. The genus apparently spread through Africa where it gave
rise to a leading line, guardi.
Seven different lines were formed, all but one remaining confined to Africa.
The exception, Culex pipiens L., which apparently
spread to India, produced new lines. Some of these eventually reached North
America via South America giving rise among others to the species Culex tarsalis Coquillett. Were it not for the survival of connecting
links in Africa and Asia, we might think that this species group originated
in South America. Much of our
native insect fauna is so old that we have no basis for even discussing its
place of origin. For biological control it suffices that this is the native
home. If species invade other areas and become pests, natural enemies ought
to be sought here. The concentration of species in a particular area may, in
fact, reflect more their common environmental needs than the center of
dispersal after relatively recent speciation in that area. A large complex of
natural enemies has been stated to indicate the site of longest residence
(native home) of a species, and especially if one or more natural enemies are
host specific. For biological
control it may not be necessary to pin-point the place of evolution of a
species but rather its place of recent
origin in order to locate the best natural enemies. "Recent" is a
dubius term. It could be a few hundred or thousands of years. However, natural
enemy complexes efficient in regulation of a targeted pest may evolve quicker
than previously imagined, so that paleoentomology may not be too important in
biological control. The region of
origin of certain pest species is known without any doubt. Included are Icerya purchasi, Hypericum
(Klamath weed), Opuntia
cactus in India and Australia, rabbits in Australia, eucalyptus snout beetle,
Dutch elm disease, chestnut blight, olive scale, walnut aphid, navel
orangeworm (Amyelois transitella), grape leaf
skeletonizer (Harrisina brillans Barnes &
McDunnough), and many more. The origin of other pests is open to speculation,
with the following examples illustrating some of the difficulties involved. Saissetia oleae (Bernard), the black scale
on citrus, originally was believed to have originated in East Africa. Harold
Compere imported 30 species of parasitoids from East and South Africa during
1936-1937, with five becoming established in the United States. No
appreciable reduction in host density was caused by most species. However, Metaphycus helvolus (Compere) from Capetown, South Africa proved to
be very successful in permanently lower black scale densities. The host scale
is now believed to have originated in northern Africa and not within the
range of its most effective parasitoid. Circulifer tenellus (Baker), the beet
leafhopper, is a serious pest of sugar beets, tomato and other crops as it
vectors the curly top virus. This leafhopper possesses strong migratory
habits, moving hundreds of miles each spring and early summer to cultivated
crops. Systematics have played an important role in determining the origin of
this insect. It was originally thought to be from Australia under the genus
name Eutettix. Parasitoids
shipped from Australia failed to establish in North America. Oman (1948)
reclassified the leafhopper as Circulifer
and thought it originated in the arid portions of the Mediterranean and
Central Asian regions. A total of 36 shipments of parasitic material was
received during 1951-1954 (Huffaker 1954). Twelve parasitoid species were
involved, but the identity of all species is still not positive. Only two
released species were ever recovered as Lymaenon "A" and Polynema
"A." Further searches in Asia might be worthwhile.
Heliothis zea (Boddie) has been described as several species due to a great
variability in color and markings. It is widespread within 40o N. to 49o S.
Lat. Originally it was thought to have originated in the West Indies because
of the fact that it feeds there particularly on American plant genera, such
as tomato, corn, etc. It is of rare occurrence in Europe and this would
indicate to some that it originated there where a better natural enemy
complex might exist. Various parasitoids were imported against it with some
Braconidae and Tachinidae showing some success after their discovery in
India. Native predators are presently thought to be most useful against H. zea, but not usually to a satisfactory control level.
Further searches in Asia and possibly Africa might be worthwhile. Cydia (Carpocapsa) pomonella (L.), the codling
moth, damages pome fruit and some stone fruits. This species was known in
Europe in ancient times (around the 4th Century B.C.). The codling moth is
widely distributed throughout the world, but it is still apparently of little
consequence in parts of China and Japan. Its northern limit is determined by
minimum heat in summer. The presence of the moth in America, Australia, South
America, South Africa and eastern U.S.S.R. is thought to be of recent
occurrence. Its probable point of origin is in the central Palearctic where
wild apples, Malus silvestris, occur. A long list
of natural enemies is known, which undoubtedly reflects the large number of
researchers who have studied this pest. Some natural enemies were introduced
from Spain, eastern North America and the Middle East to various countries,
but only a few became established without showing any control (Clausen 1978).
Considering the great importance of this insect, it is surprising that a
greater effort has not been made to control it biologically. In Europe, for
example, several potentially very effective parasitoids are kept from
expressing themselves fully due to hyperparasitism. The primary parasitoids
might be imported to America where their capabilities could be fully
expressed. Also, an intensive search in northern China, southern Siberia and
Japan might turn up some effective natural enemies. Aonidiella aurantii (Maskell), the California
red scale, is found between 40o N. and 40o S. Lat., but it is at pest status
only in the subtropics. This scale has a wide host range but prefers citrus
and roses. Using the last of three leads for tracing the area of origin [ (1)
the area where the preferred host originated--citrus, (2) the area where the
pest is present but kept at low densities by natural enemies--the Far East,
and (3) the area where an abundance of related species exists--Neotropics], a
search was begun in South America in 1934-1936, where 17 species of Chrysomphalus were reported.
None of these were effective in biological control. The Far East was searched
subsequently, but it took from 1905 to 1941 to establish a parasitoid, Comperiella bifasciata Howard. Problems
that hindered finding the right species included the sago palm host plant
approach for locating parasitoids, which failed because it made host scales
immune to attack. There were also numerous host misidentifications. Aphytis lingnanensis Compere and A. melinus
DeBach were later successfully imported from the Far East. The resulting
success was attributed to the fact that parasitoids were sought from the
whole geographic range of the host in the East. Tetranychus spp. Tetranychid
mites are pests of worldwide distribution which are primarily found in the
subtropics. There are 200-250 species of economic importance, but there is
limited information on the native home of any species. Thee have been some
inferences made on the native homes of the European red mite, Panonychus ulmi (Koch) and citrus red mite (Panonychus citri
(McGregor), however (J. A. McMurtry, pers. commun.). Introductions
of predatory mites have been rather scanty, and should be continued before
any conclusions are drawn. Phytoseuilus
persimilus Athan-Henriot was
released in periodic mass liberations, and has been successfully used to
prevent phytophagous mite outbreaks in greenhouses and on strawberries. Two
phytoseiid species were established on citrus mite pests in Israel, and 12
phytoseiids were released on avocado and 9 against citrus red mite in
southern California. Results are too recent in the California introductions
to determine permanency. Insecticide resistant strains of Typhlodromus occidentalis Nesbitt have been
produced and released in various agroecosystems in California and Oregon
(Croft 1970, Hoy et al. 1982). Phthorimaea operculella (Zeller), the potato
tuberworm, is presently cosmopolitan. Some workers consider America to be the
native area based on the origin of its principal food plants, most of the
wild progenitors of which originated in South America. However, the first
successful introduction against this presumably American species was to
France of Bracon gelechiae Ashmead, where
infestations were reduced. This parasitoid was later successfully introduced
in Australia, Asia, Africa and other portions of North America. Other species
are currently being tried and it is probable that some new finds in South
America will provide even greater control. Nezara viridula (L.), the southern
green stinkbug, is probably of African origin. This pest is especially
important in tropical and subtropical areas of the world (Davis 1967). It
attacks 60 or more unrelated species of plants. The Egyptian parasitoid, Microphanurus basalis (Wollaston) was
successfully introduced into Australia, Fiji and Hawaii, where it was
effective only in the more tropical portions. Further attempts should be made
to secure parasitoids from the cooler portions of the pest's range in Africa. Pectinophora gossypiella (Saunders), the pink
bollworm is now cosmopolitan. However, recent advances in the taxonomy of the
genus have placed its origin in the vast Australasian region. Importations of
natural enemies from the northwestern part of Australia and southeastern
Indonesia and Malaysia have just begun. But this was preceded by 60 years of
work with parasitoids that were secured from Europe, Africa and India,
yielding poor results (Common 1958, Legner & Medved 1979, Wilson 1972).
Unfortunately, some ignore this breakthrough in locating the probable area of
origin and hold the previous 60 years of failure as proof for being unable to control pink bollworm biologically.
Diptera of
Medical and Veterinary Importance, include the genera Musca,
Fannia, Culex, Aedes,
and Anopheles, which are
becoming more difficult to control with insecticides and by cultural means. Emphasis
is now being placed on biological control. Unlike agricultural pests, medical
and veterinary pests are more difficult to evaluate because economic loss
data is not readily available. Therefore, although significant achievements
have been made with biological control of several species, the actual
population drops have not often been measured. [see Reviews] Future
Direction and Emphasis Hokkanen
(1985b) has recently addressed the question of "where do we go from
here" to maximize success in biological control. In a concluding
statement, Hokkanen (1985) considered the following: "Several
authors emphasize the importance of intellectual and material resources, as
well as a functioning institutional and administrative framework to any
successful biological control program. In a discussion about the factors
affecting establishment of exotic entomophages, van den Bosch proposed that
'a significant, perhaps even major, proportion of the introduced entomophages
never had a chance of establishment and never should have been introduced.'
He considered that biocontrol failures most often result from inadequate
information about the pest-natural enemy system, or from lack of persistence
in attempts, coupled with indifference in attitudes, as well as from
technical problems such as difficulties in propagating and liberating the
control agents." "Beirne
discussed these problems in greater detail, emphasizing inadequacies in
selection and shipment of parasitoids, inadequacies in colonization
procedures, and administrative constraints. The latter include financial
support, which tends to terminate when projects do not show signs of quick
success. Sailer added to this a discussion on the influence of economic
policy and national priorities in research, as well as organizational
structure of the importation agency and the increasing concern for
environmental protection." "DeBach
concluded that 'one thing is clear: highly trained professional entomologists
enthusiastically devoted to their science furnished the main ingredient for
success in case after case,' to which Sailer added 'almost without exceptions
these people had the opportunity to receive the kind of professional training
needed and the opportunity to be employed by organizations well suited, if
not designed, to facilitate their work.'" "These
authors clearly pointed out that much of the potential of classical
biological control remains to be realized, and will come about with adequate
infrastructure and resources. They also give examples of missed
opportunities, where the failure to appreciate the role and potential of
natural enemies in suppression of pests and the defects of program management
have prevented or greatly delayed the biological control of important
pests."
"Classical biological control introductions can be viewed as
grand scale experiments, which provide unique opportunities for developing
and testing ecological principles, as well as for gaining economic benefits.
Detailed studies on already existing results from past biocontrol attempts
should be carried out to improve our understanding of the factors that affect
both the economic and ecologic success in biocontrol. However, the chain of
events and choices most likely leading to biocontrol success in any project
will hardly ever be a dull routine that can be programmed into a computer: it
takes experienced, innovate scientists to identify the weak or missing links,
and to put the pieces of art together, scientifically." Past classical biological control
successes have relied heavily on the interaction with other international
organizations, especially the Commonwealth Institute of Biological Control
with headquarters in Curepe, Trinidad.
Various permanent and temporary laboratories of this organization
existed in all parts of the world.
Researchers there would host, assist and otherwise interact with those
of the United States Department of Agriculture and the University of
California to obtain beneficial species.
As independence from the British Commonwealth developed among the
different countries that maintained laboratories, local support for their
continuance diminished, and in many cases ceased entirely. This has resulted in a greater than 90%
decrease in classical biological control activity worldwide. Natural
enemies for use in biological control may be categorized into separate risk
groups. Parasitic and predaceous arthropods fit into the lowest risk
category, but are the most difficult to study and to assess for potential
success. The policy of certain
countries, e.g., Australia, of requiring intensive studies on native
organisms before allowing them to be exported is especially devastating to
the deployment of biological control.
A recent case of invading Australian wood borers that attack
eucalyptus in America has already caused the death of over half of the trees
in California, while the importation of effective natural enemies continues
to move at a crawl. Yet progress is
being made with increased attention to basic ecological and behavioral
research. The rate of biological control successes may drop initially as the
style of "educated empiricism" (Coppel & Mertins 1977) becomes
more widely adopted, as has apparently already begun (Hall & Ehler 1979,
Hall et al. 1980). Success rates could be expected to increase as the data
base enlarges and intercommunication possibilities expand. Certainly the
trend will ever more propel the activity of exotic natural enemy importation
into a solid scientific base. Exercise 25.1--List the steps you would take to initiate
a foreign exploration for natural enemies of an newly invaded pest insects in
California. Exercies 25.2--What if the pest were confined to northern
Minnesota or Maine? Exercise 25.3--How would you study potentially valuable
natural enemies before their introduction? Exercise 25.4--If following the introduction and
establishment of two natural enemy species the pest still remained at
densities above the economic threshold, what might be done to improve
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