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BIOLOGICAL CONTROL OF NOXIOUS PLANTS AND WEEDS
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Introduction In the United
States there are 500 major species of noxious plants (weeds) that cause an
estimated annual loss of around $8 billion (Chandler 1980, Goeden &
Andrés 1999). These plants infest cropland, rangeland, and recreational and
aquatic sites and vary in their economic importance and need for control.
Cultural and chemical controls for noxious plants are effective but temporary
at best, if not uneconomical. There is also a growing awareness that
herbicidal treatments might be harmful to the environment including humans.
Herbicide resistance spreads rapidly when very effective compounds are used.
Thus, control of noxious plants is no longer a matter of improved plant kill;
planning and ingenuity are required to minimize immediate losses without
inviting incursions by replacement weed species. Aldrich (1984) commented
that noxious plants (weeds) are part of dynamic ecosystems continually
evolving in response to natural and cultural control pressures. Biological
control is a proven method of controlling noxious plants, and there an a
large volume of literature devoted this approach. Quadrennial international
symposia and their proceedings continually update knowledge about this
discipline. Goeden & Andrés (1999) suggest that biological control
continues to offer promise and expanded application in reducing losses due to
noxious plants. Biological
noxious plant control involves the study of relationships among such plants,
their associated organisms and the environment, followed by the manipulation
of selected species (natural enemies) to the detriment of the target plant
species. Attention focuses on those plant/natural enemy relations that have
coevolved to the degree that natural enemies cannot exist or would have
little environmental impact in the absence of their host. Goeden & Andrés
suggest that coevolved natural enemies that have developed a high degree of
host specificity have proven the safest to use, are least likely to damage
nontarget plant species and are most suitable for regulating the plant's
abundance. Biological control researchers go to considerable effort to match
natural enemies to their host plants in problem environments, seeking
combinations and devising manipulations most detrimental to the target
plants. Presently all biological control activity involving plants in
Australia is regulated by the Biological Control Act of 1984, which details
how plants targeted for biological control are to be open to public review
before the release of biological control agents (Cullen & Delfosse 1984,
Turner 1985). The natural
enemies used in biological control are self-perpetuating only in the presence
of their plant hosts and then only within the limits set by the environment.
According to definitions for biological control (Smith 1919, 1948, DeBach
1964), the ability of natural enemies to regulate noxious plant or arthropod
populations in a self-sustaining, density-dependent manner sets biological control
apart from other methods of control. The
methodology used in biological plant control consists of six parts: (1)
assuring proper identification of the target plant, (2) charting its
geographic range, (3) characterizing the habitats it infests, (4)
ascertaining the losses caused by the plant, (5) determining the degree of
control required, and (6) compiling a list of natural enemies already present
or reported elsewhere. Both
advantages and disadvantages are, however, associated with biological control
of noxious plants. Advantages include (1) the introduced agents perpetuate
and distribute themselves throughout the plant's range, (2) the impact of
host-specific agents is focused on a single plant species without harm to
other plants, (3) the cost of developing biological control is relatively
inexpensive compared to much higher costs for other approaches (Harris 1979,
Andrés 1977), (4) the agents are non polluting, energy efficient and
biodegradable, (5) the knowledge generated during pre release and evaluation
studies contributes to a broader understanding of plant ecosystems and
environmental factors regulating natural communities. Disadvantages are (1)
once established in an area an introduced agent cannot be extirpated from the
environment, (2) a host specific agent will control only one species in a
noxious plant species complex, (3) impact of the agent is usually slow,
requiring 3-4 years before control is achieved, (4) an agent may expand its
host range to include closely related nontarget plants and (5) the
establishment, spread and impact of a biological control agent is determined
by the quality of the environment and the host, and cannot be predicted. Introducing
New Natural Enemies For Biological Control Naturalized noxious plants often have few
host specific natural enemies capable of effectively regulating their
abundance. Additional species of natural enemies may be sought in the plant's
native range and introduced to the problem areas. This approach is common
worldwide, and has led to the introduction of numerous plant-feeding insects
and mites, and recently, plant pathogens and nematodes (Julien 1982, 1987).
Finding and introducing phytophagous organisms requires thorough
preintroduction studies to assure that control can be achieved and that
economically and ecologically important plants will not be adversely
affected. Several authors have listed the guidelines for introducing such
natural enemies (Zwölfer & Harris 1971, Frick 1974, Andrés et al. 1976,
Klingman & Coulson 1983, Schroeder & Goeden 1986). Goeden &
Andrés list the following steps in this approach: (1) Project Selection. Once released, introduced natural enemies
cannot be restricted to parts of the plant's geographic range. Before
undertaking studies that may lead to natural enemy introduction there must be
assurance that the plant has few, if any, redeeming virtues and that there is
little or no public opposition to the project (Turner 1985). (2) Search For Natural Enemies. A list
of organisms recorded from the target plant is compiled from literature and
museum records, which is followed by field surveys and studies of associated
organisms in selected parts of the plant's native range. Such organisms are
collected, identified and checked in the literature and museum records.
Candidate species are selected for further study. (3) Host Range & Biological Studies. Biological studies
involving various aspects of behavior such as feeding and oviposition, are
conducted in the laboratory in efforts to determine host plant range. Tested
are cultivated and ecologically important plant species, with special
attention on close taxonomic relatives (Wapshere 1974a). (4) Summary of Host Range Studies. A
summary of the natural enemy's taxonomy, behavior, biology and host plant
relationships is prepared in a special report. These reviews are prepared in
the United States by the U. S. Department of Agriculture, Animal & Plant
Health Inspection Service, Technical Advisory Group (USDA, APHIS-TAG),
Hyattsville, Maryland, as well as relevant state Departments of Agriculture
and universities. A recommendation is made on whether or not to import the
candidate organism. (5) Importation
and Release. After approval, natural enemies are collected from the
same field populations that constituted the test material and transferred to
a domestic quarantine. Univoltine or difficult to culture species are
identified and examined to assure that they are free from parasites and
entomogenous pathogens. Species that are amenable to culture are reared for
one generation before being liberated. Quarantine processing is labor
intensive and often restricts the number of biological control agents that
can be examined. Release sites are selected on the basis of climate, habitat,
freedom from disturbance and other factors in order to enhance chances for
establishment. (6) Evaluation.
Evaluations are given on success of establishment, field reproduction and
damage inflicted against the target plant. Noxious plants may have dozens of associated
natural enemies, with some species attacking only the flowers and fruit,
while others attack the leaves, stems, branches, crowns or roots. Zwölfer
(1988) reported that the guild of agents attacking the flower heads and
achenes of an asteraceous thistle may include monophagous to oligophagous
species that vary in their impact on the host plant. Pre introduction studies
help determine which species are sufficiently host specific for biological
control purposes and suggest the best sequence for importation. An agent's
host finding capability and competitiveness with other flower head infesting
species can be very important (Zwölfer & Harris 1971, Harris 1973, Goeden
1983). Conservation
of Biological Control Organisms Sometimes indigenous or exotic natural
enemies feed and reproduce on a noxious plant host yet do not provide
effective biological control. However abundance of an associated agent might
be effectively enhanced to provide localized reduction of a plant. For
example, DDT has been used selectively to eliminate Exochomus sp., a coccinellid beetle predator that
restricted the potential of an introduced cochineal insect, Dactylopius opuntiae (Cockerell), to
control the prickly pear cacti, Opuntia
ficus-indica (L.) Miller and O.
tardispina Griffiths in
South Africa (Annecke et al. 1969, Moran & Zimmerman 1984). Reducing the
coccinellid predators allowed the cochineal insects to increase in number and
the cacti were effective controlled. On the other hand many indigenous
species have complements of natural enemies which themselves are controlled
by parasitoids, predators and environmental factors. A thorough understanding
of plant/natural enemy/environment relationships is required in order to
manipulate aspects of the relationship to favor the agent's impact. In
general this is not been feasible because of the high cost involved. Augmentation of
Natural Enemies Supplemental releases of a natural enemy may
increase its abundance and time its impact against a noxious plant. This
approach was not widely practiced until the discovery that an endemic fungus,
Colletotrichum gloeosporioides (Penz.) Sacc.
(Melanconiales), could be cultured and applied to northern jointvetch, Aeschynomene virginica (L.) B.S.P. (Leguminosae),
a noxious plant of rice in the southeastern United States (Daniel et al.
1973). This work stimulated the search for indigenous pathogens associated
with other noxious plants (Charrudattan & Walker 1982, Templeton et al.
1978). The impact of the moth, Bactra verutana
Zeller could be enhanced by supplementing existing field populations with
large numbers of this insect against purple nutsedge, Cyperus rotundus
L. in cotton (Frick & Chandler 1978). Although experimental control was
demonstrated, the method proved too expensive to be practical. History
of Plant Control with Natural Enemies Following is a summary of historical events
in the biological control of noxious plants derived from Goeden & Andrés
(1999): For most of its history, the biological
control of noxious plants was the domain of a rather small, dedicated group
of broadly versed entomologists (DeBach 1964), which began rather by
accident. The earliest record of the biological control of a noxious plant
involved the intentional introduction of the cochineal insect, Dactylopius ceylonicus (Green) to northern
India from Brazil in 1795 in the false belief that it was D. coccus Costa, a species cultured commercially as a source
of carmine dye. Instead of reproducing well on the cultivated, spineless
prickly pear cactus, Opuntia
ficus-indica (L.) Miller, D.
ceylonicus readily
transferred to its natural host plant, O.
vulgaris Miller, that had
become widespread in India when it escaped cultivation in the absence of its
South American natural enemies. Once the value of D. ceylonicus
as a biological control agent was recognized, it was introduced in 1836-1838
to southern India, where it brought about the first successful, intentional
use of an insect to control a noxious plant. Shortly before 1865, D. ceylonicus also was transferred from India to Sri Lanka
which resulted in the successful control of O. vulgaris
throughout the island (Goeden 1978, Moran & Zimmerman 1984). Attention next shifts to Hawaii where Albert
Koebele was hired as foreign explorer after helping to achieve the
spectacularly successful biological control of the cottony-cushion scale on
citrus in California during the late 1890's (Doutt 1958, 1964). Koebele
explored the jungles of southern Mexico during 1902 for insects feeding on
lantana (Lantana camara L., Verbenaceae).
Lantana was an ornamental plant of Central and South American origins that
had escaped from cultivation to become a serious pest in Hawaii. Koebele
shipped 23 species of insects to Hawaii (Goeden 1978). Koebele suffered
problems of extreme temperatures, unscheduled shipping delays, pathogens and
other contaminants (Perkins & Swezey 1924). Upon their arrival by ship in
1903, the insects Koebele collected were liberated directly on lantana plants
in the field without host specificity tests. Eight species, including some of
the most effective natural enemies of lantana, were reported established on
this plant throughout the islands by 1905 (Andrés & Goeden 1971, Goeden
1978, Julien 1982). Procedures for exploration of natural
enemies of an alien plant in its country or countries of origin were
pioneered in the lantana project. The lantana seed fly, Ophiomyia lantanae
(Froggatt) (Diptera: Agromyzidae), was transferred from Hawaii to New
Caledonia in 1908-1909 and to Fiji in 1911 (Rao et al. 1971). These shipments
marked the beginnings of a tradition of transfer projects (DeBach 1964),
involving biological control agents of proven worth to other countries with
the same noxious plants. Three more species of lantana insects were then
transferred from Hawaii to Fiji during 1922-1928 (Rao et al. 1971). An attempt was made to introduce D. ceylonicus to Australia from Ceylon and India in 1903,
without success (Goeden 1978, Moran & Zimmerman 1984). Then an intensive
Australian effort on the biological control of prickly pear cacti (Opuntia spp.) began in
1913-1914, when the two membered Prickly-Pear Travelling Commission surveyed
the insects and pathogens associated with these plants in Java, Sri Lanka,
India, East Africa, South Africa, the Canary Islands, littoral Mediterranean
countries, the United States, Mexico and parts of Central America, the West
Indies, South America and Hawaii (Johnston & Tryon 1914). This effort of
worldwide exploration for natural enemies of a group of noxious plants
remains unequalled in scope of geographic coverage. Biological control of the prickly pear
cacti, Opuntia inermis deCandolle and O. stricta Haworth in Australia ranks as one of the most
successful projects in biological control of noxious plants. The project
followed the initial efforts of the Prickly Pear Travelling Commission, which
first recognized the potential value of what was later to become the
principal natural enemy, the moth, Cactoblastis
cactorum (Berg)
(Lepidoptera: Pyralidae). The principal entomological effort in this
biological control project occurred during the 1920's when North and South
America, particularly the southern United States, Mexico and Argentina, were
thoroughly explored for potentially useful, cactus-feeding insects. More than
150 species of cactus insects eventually were collected and studied, many of
which were new to science. From 1921 to 1925, 48 species were imported into
Australia, of which 19 were liberated and 11 became established. A single consignment of C. cactorum
was imported from Argentina in 1925. Large scale mass culture and host plant
specificity tests with useful and weedy plant species were undertaken for the
first time in a biological control project. Cactoblastis cactorum
became widely established following the distribution of more than 2.7 billion
mass reared and field collected eggs between 1925 and 1933. Almost 90% of the
original stands of O. inermis and O. stricta were destroyed by 1934 through larval feeding by
this moth, supplemented by airborne, soft-rot bacteria for which the borers
provided entrance wounds into infested plants. Virtually complete control of
the cacti was achieved in Queensland and northern New South Wales involving
24 million ha of formerly infested land that was restored to agricultural use
(Dodd 1940, Goeden 1978, Moran & Zimmerman 1984). The spectacular success of Cactoblastis cactorum tended to eclipse the
benefits derived from other cactus insects used in biological control,
notably several species of cochineal insects (Moran & Zimmerman 1984).
For example, Dactylopius ceylonicus was successfully
reintroduced to Australia during 1913-1915 and virtually eliminated O. vulgaris as a rangeland weed in Queensland. Both C. cactorum and Dactylopius
spp. were transferred during the mid 1920's and 1930's to countries where
prickly pear cacti also were introduced pests: Indonesia, Mauritius, New
Caledonia, Reunion and South Africa (Rao et al. 1971, Greathead 1971, Goeden
1978). In South Africa and Mauritius these early successful transfer projects
led to the independent development of other successful research projects in
the biological control of noxious plants (Greathead et al. 1971, Goeden 1978,
Julien 1982, 1987). Another noteworthy success following the
prickly pear cactus effort was the biological control of Koster's curse, Clidemia hirta (L.) D.Don (Melastomataceae). This shrub, native to
the West Indies and tropical America, became a serious problem on Fiji.
Following a preliminary survey of insects attacking C. hirta
and allied Melastomataceae, Liothrips
urichi Karny was selected as
a promising biological control agent and its life history and host plant
relationships intensively studied in Trinidad during 1927-28. Potted C. hirta infested with the thrips were shipped in cold
storage to Fiji in 1930. Upon arrival the thrips were transferred directly to
plants in the field. Field releases continued throughout 1930. By 1932-33,
several hundred hectares of thrip stunted C.
hirta had been overgrown by
plant competitors of greater forage value. Shaded and greatly weakened by
thrips attack these weeds were soon defoliated and killed. Regrowth was
readily located and attacked by L.
urichi. By 1937 the
competitive ability of the C.
hirta was permanently
impaired by continued thrips attack except in a few shaded and wet areas
(Simmonds 1937, Rao et al. 1971, Goeden 1978, Julien 1982). New Zealand joined the list of Commonwealth
countries sponsoring original research on biological control of noxious
plants in the 1920's. Studies were initiated of insects attacking gorse, Ulex europaeus L. (Leguminosae) in England in 1926 and the
introduction and successful colonization of the seed weevil, Apion ulicis Forster was accomplished during 1929-31. Surveys of
insects attacking blackberries (Rubus
spp., Rosaceae) in Europe and North America were conducted during the mid
1920's, but no species were thought safe enough for introduction and the
project was abandoned. Beginning in 1927 and continuing into the 1930's,
diapausing pupae of the cinnabar moth, Tyria
jacobaeae (L.), a defoliator
of tansy ragwort, Senecio jacobaea L. (Asteraceae), were
introduced into New Zealand from England, but establishment was not attained.
Australia also received T. jacobaeae from New Zealand
during 1929-32, beginning a series of colonizations that continued into the
1950's and 1960's. Efforts to establish this moth were precluded by the predatory
activity of native insects, mainly scorpion flies (Goeden 1978, Julien 1982). In Australia the success of the prickly pear
biological control stimulated an attack on other widespread introduced
rangeland noxious plants, including cocklebur, Xanthium strumarium
L. (Asteraceae). Explorations for natural enemies began in the United States
during 1929. St. Johnswort, Hypericum
perforatum L. (Hypericaceae)
was targeted in 1926 with explorations in England. The cocklebur project
yielded only partially successful biological control while varying results
with different introduced insects were obtained with St. Johnswort (N. Clark
1953, L. R. Clark 1953). Basic studies performed on the St. Johnsworth
project set a pattern for contemporary Australian projects in biological
control. The natural enemies and technology transferred from this project
contributed to the development of biological control of noxious plants in
North America. Prior to World War II, a framework was developed for the rapid
expansion of biological control efforts that were conducted after 1950. The
first biological control effort against plants in the continental United
States was developed by H. S. Smith, which involved the introduction of
insects to control native prickly pear cacti, Opuntia littoralis
(Engelmann) Cockerell, and O.
oricola Philbrick, and
various hybrids, on rangeland of Santa Cruz Island in southern California
(DeBach 1964). Phytophagous insects originating from the California mainland
and in Texas were introduced to Santa Cruz island beginning in 1940, but
successful biological control was attained only after Dactylopius opuntiae,
native to Mexico and the southern California coast, was introduced in 1951
from Hawaii ex Australia ex Mexico (Goeden et al. 1967, Goeden 1978, Goeden
& Ricker 1981). Biological control efforts were reduced
during World War II to a few transfer projects. For example, the leaf beetle,
Chrysolina hyperici (Förster) was
transferred from Australia to New Zealand in 1943 for the biological control
of St. Johnswort. In 1944 the introduction of several insect species from
Australia was made to California for specificity testing and release during
1945-46 for the biological control of St. Johnswort, which became known as
Klamath weed. J. K. Holloway of the USDA and H. S. Smith of UC, Riverside
organized the work. Successful biological control of Klamath weed, primarily
caused by the defoliating leaf beetle, Chrysolina
quadrigemina (Suffrian),
rivaled the Australian success with prickly pear cacti. This success
primarily was responsible for fostering the establishment and expansion of
biological noxious plant control in North America (Huffaker 1957). The first
intentional introduction of an insect for plant control was made in Canada in
1950 with the importation of Chrysolina
quadrigemina and C. hyperici from California to control St. Johnswort (Smith
1951). Within a decade after the liberation of C. hyperici
and C. quadrigemina, the Klamath weed had been reduced in status
from an extremely important rangeland scourge to that of an occasional
roadside plant, and now occurs at less than 1% of its former density and has
been removed from the list of noxious plants in California (Holloway &
Huffaker 1949, 1951). Its present
occurrence primarily along roadsides is linked to disturbance there of the
phytophagous biological control agents (E. F. Legner, unpub. data). Efforts were resumed in Hawaii in 1945 to
control Eupatorium adenophorum Sprengel
(Asteraceae) with the introduction of the stem gall forming fly, Procecidochares utilis Stone, which presumably
was recommended for introduction to Hawaii by A. Koebele 20 years earlier.
This successful introduction was followed by a series of projects undertaken
by the Entomology Division of the Hawaii Department of Agriculture, making
Hawaii a center of activity in the biological control of noxious plants
during the 1950's and 1960's. Plants that were targeted for biological
control in the 1950's were the Christmas berry, Schinus terebinthifolius
Raddi (Anacardiaceae); elephant's foot, Elephantopus
mollis Humboldt, Bonplaud
& Kuth (Anacardiaceae); sourbush, Pluchea
odorata (L.) Cassini
(Asteraceae); melastoma, Melastoma
malabathricum L.
(Melastomataceae); firebush, Myrica
faya Aiton (Myricaceae); and
emex, Emex australis Steinheil and E. spinosa Campdera (Polygonaceae). Substantial to complete
biological control of emex was achieved at 600-1,200 m elevations with the
weevil, Apion antiquum (Gyllenhal) introduced
from South Africa in 1957 (Davis 1966). The rest of these six projects and
several contemporary transfer projects on biological control in Hawaii are
unfortunately poorly documented (Goeden 1978, Julien 1982, 1987). A successful project was begun near the end
of World War II on black sage, Cordia
macrostachya (Jacquin)
Roemer & Schultes (Boraginaceae), an introduced plant pest in sugarcane
fields on the island of Mauritius. Preliminary surveys of the insect fauna of
black sage and related plant species were conducted in the West Indies during
1944-46. Following detailed life history studies and host specificity tests,
the leaf beetle, Metrogaleruca
obscura DeGeer was
introduced to Mauritius from Trinidad in 1947 (Simmonds 1950). The beetle
populations multiplied rapidly and by 1950 had spread over the entire island,
causing heavy defoliation which killed or weakened plants so that they were
replaced by competing plant species. A seed feeding wasp Eurytoma attiva
Burks also was introduced in 1959-50 (Williams 1960). Defoliation and seed
destruction by these introduced insects have continued to prevent the
regeneration of black sage (Simmonds 1967, Goeden 1978, Julien 1982). Until this time most plants targeted for
biological control were introduced and perennial in relatively undisturbed
rangeland. However, starting in Australia, Canada and the United States in
the late 1950's, projects were initiated on aquatic and semiaquatic weeds,
annuals and biennials, cropland and ruderal weeds. For example, the first
aquatic plant targeted for biological control with insects was alligatorweed,
Alternanthera phylloxeroides (Martius)
Grisebach (Amaranthaceae). The first annual plant targeted for biological
control with insects in North America was puncturevine, Tribulus terrestris
L. (Zygophyllaceae). A classic Commonwealth of Biological Control study by
Zwölfer (1965) of the insect fauna of Canada thistle, Cirsium arvense
(L.) Scopoli and other wild Cynareae (Asteraceae) in Europe which was
sponsored by the Canada Department of Agriculture during the early 1960's,
formed the foundation for a number of biological control projects targeted on
introduced thistles in Canada and the United States (Schröder 1980). These
included annual, biennial and perennial species of Carduus, Centaurea,
Cirsium and Silybum (Goeden et al. 1974a).
These projects have continued with mixed results. Especially interesting is
the successful biological control of musk thistle, Carduus nutans
L., attained with the introduced seedhead weevil, Rhinocyllus conicus
L. in Canada (Harris 1984a) and Virginia (Kok & Surles 1975). Introduced spurges, Euphorbia spp. (Euphorbiaceae), of Eurasian origins, were
targeted for biological control in Canada in the early 1960's with the first
introduction of the hawk moth, Hyles
euphorbiae L. from Germany
in 1965 (Harris 1984b). The biological control of rush skeletonweed, Chondrilla juncea L. (Asteraceae) in Australia is another important
advancement. This project was the first to involve the intentional
international transfer of a phytopathogen for the biological control of a
plant, i.e., the rust fungus Puccinia
chondrillina Bubak &
Sydenham (Uredinales) between Italy and Australia in 1971 for the successful
biological control of a noxious plant. This project also was one of the first
to target a plant pest of cropland (dryland wheat). It established procedures
for testing phytopathogens for host specificity under quarantine conditions
and involved the first intentional importation in 1971 of a phytophagous
mite, Eriophyes chondrillae for biological
control (Cullen 1974, 1978). Scope
of Biological Noxious Plant Control Target Plants.--101 species of plants have been targeted for biological
control (Julien 1982, Julien et al. 1984). Thirty-three plant families are represented
among the 101 species, 25 of which belong to the Asteraceae and 19 of which
are Cactaceae. The other 31 families are represented by five or fewer species
(Julien et al. 1984). Only about 22% of what Holm et al. (1977)
consider to be 18 of the world's worst plant pests have been targeted for
biological control, i.e. Convolvulus
arvenis L. (Convolvulaceae),
Cyperus esculentus L. and C.
rotundus (Cyperaceae), and Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae)
(Pemberton 1980). Success was achieved only against the water hyacinth, E. crassipes, while the other three projects have been
unsuccessful (Julien 1982). It was also noted that biological control of
plant pests has not yet had a single project against a noxious grass
(Pemberton) despite grasses (Graminaceae) comprising 10 of the world's 18
worst plants according to Holms et al. (1977). Two grasses,
Digitaria sanguinalis (L.) Scopoli and Panicum
dichotomiflorum Michaux,
were included among the some 80 species or species groups of plants listed by
Goeden et al. (1974a) as having been targeted, without success, for
biological control in the United States and Canada. Noxious grasses traditionally have not been
considered suitable for biological control because many are close relatives
of important cultivars. The chances of finding arthropod natural enemies able
to discriminate among such closely related, potential host plants are
considered remote. However, phytopathogens offer promise for control of
noxious graminaceous plants, as some are very host specific (e.g., rust
fungi). Noxious plants that are least amenable to biological control include
those in highly disturbed habitats, submersed aquatic plants (with apparently
few host specific natural enemies), highly toxic plants for which tolerable
densities are too low to be obtained by natural enemies, minor plants of
limited distribution that do not threaten to invade other areas, and plants
whose eradication is sought (Harris 1971, Frick 1974, Goeden 1977). Most noxious plants successfully controlled
with introduced natural enemies were introduced plant species (Julien 1982,
Julien et al. 1984). Only four species of native plant pests have been
successfully controlled with intentionally introduced organisms: Opuntia dillenii (Ker-Gawler) Haworth (Cactaceae) on the island of
Nevis in the West Indies (Simmonds & Bennett 1966); O. littoralis
and O. oricola on Santa Cruz Island off southern California
(Goeden et al. 1967, Goeden & Ricker 1981); and O. triacantha
(Willdenow) Sweet on the islands of Antigua, Monserrat and Nevis in the West
Indies (Simmonds & Bennett 1966, Bennett 1971). All four of these native
plants are prickly-pear cacti (subgenus Platyopuntia)
which along with other Cactaceae as Moran & Zimmerman (1984) observed,
are unusual among terrestrial plants as regards their insect relations. Natural Enemies Employed.--Julien (1982) listed 174 biological control projects
directed against the 101 noxious plant species previously noted. Of these,
151 (87%) used exotic organisms introduced against 82 plant species, and 23
(13%) used native organisms against 26 plant species. There were 171 species
of insects in seven orders and 38 families comprising 98% of all releases of
natural enemies and 96% of all species of natural enemies released for
biological control of these 101 plants. Most released species were in the
Coleoptera, Lepidoptera, Diptera and Hemiptera (Homoptera & Heteroptera),
in decreasing order. There were very few species of Orthroptera, Thysanoptera
and Hymenoptera utilized. Of the 69 colonized species of Coleoptera,
60 species of Lepidoptera, 20 species Diptera and 16 species of Hemiptera,
65%, 55%, 70% and 66% became established and 29%, 20%, 19% and 44% were
effective as biological control agents, respectively. The 10 families of
insects that contained the most species released for biological control of
plants in decreasing order were Chrysomelidae, Curculionidae, Pyralidae,
Dactylopiidae, Tingidae, Tephritidae, Cerambycidae, Noctuidae, Apionidae,
Agromyzidae, Gelechiidae and Tortricidae. Moran & Zimmerman (1984)
reported that 63 species of cactophagous insects were introduced worldwide
for biological control of 22 species of Cactaceae, with 19 (30%) being
successfully established. In Australia there were 54 species and South Africa
24 species of insects introduced for cactus control. Phytopathogens.--The decade of the 1970's saw increased efforts to use
phytopathogens, especially fungi, for biological control of aquatic and
terrestrial noxious plants (Charudattan 1978, Freeman & Charudattan 1981,
Charudattan & Walker 1982). Julien (1982) listed four phytopathogens that
were imported for biological control of noxious plants worldwide. Two of
these pathogens were accidental introductions, while the other two, both rust
fungi, provide examples of successful control of alien terrestrial plants
with intentionally introduced natural enemies. The introduction of Puccinia chondrillina into Australia in 1971 for the biological
control of rush skeletonweed has already been mentioned. The high degree of
host specificity exhibited by P.
chondrillina prevented the
direct transfer of the Australian material to control the two forms of rush
skeletonweed found in the western United States. Surveys in Europe uncovered
a strain of P. chondrillina that attacked the
predominant form of the plant in the United States (Emge et al. 1981). Another example is the successful biological
control of weedy blackberries, Rubus
constrictus Lefevre &
Mueller & R. ulmifolius Schott (Rosaceae),
with Phragmidium violaceum (Schultz) Winter
introduced from Germany to Chile in 1973 (Oehrens 1977). Biological control
of Hamakua pamakani, Aegeratina riparia
(Regel) King & Robinson (Asteraceae) was obtained with the pathogen Cercosporella sp. (Uredinales)
imported from Mexico to Hawaii in 1975 (Trujillo 1985). Charudattan (1984) discussed plant pathogens
that could be used as microbial herbicides. This strategy employs alien or
native pathogens that are mass cultured and applied as inundative inocula on
target plant pests. Successful examples include Collectotrichum gloeosporioides
f.sp. aeschynomene,
registered and sold as CollegoTM for control of northern
jointvetch, Aeschynomene virginica on rice and soybeans
in Arkansas (Templeton et al. 1978); Phytophthora
citrophthora (R.E. &
E.H. Smith) Leonian (Peronosporales), registered and sold as DevineTM
for control of milkweed vine, Morrenia
odorate (Hook. & Arn.)
Lindle in Florida citrus (Ridings et al. 1978). Misc. Natural Enemies.--There have been two species of mites (Acarina) used
successfully in the biological control of noxious plants. Tetranychus opuntiae Banks was an
accidental introduction on prickly pear cacti in Australia. Eriophyes chondrillae was the first mite species intentionally
transferred between continents for biological control of plants (Cullen 1974,
1978). An eriophyid mite, E.
boycei Keifer was exported
from California to the Soviet Union in 1971 and 1972 for the biological
control of ragweeds, Ambrosia
spp. (Asteraceae), but it was not released (Goeden et al. 1974b). Comroy
(1982) gave additional examples of native and introduced mites attacking
weeds in an attempt to demonstrate their efficacy as biological control
agents. Although nematodes are well known as plant
pest few species are used as biological control agents. Only Paranguina picridis Kirjanova & Ivanova has been used (Julien
1982, Julien et al. 1984). This species, obtained from the Soviet Union, was
released in restricted field trials in Quebec, Saskatchewan, Canada in 1976
to control Russian knapweed, Centaurea
repens L. (Asteraceae). The
nematode was successfully transferred from central Asia to the Crimea and
reportedly yielded good control of Russian knapweed (Kovalev 1973).
Experimental use was made of Nothanguina
phyllobia Thorne by
augmenting its naturally occurring populations with large number of
infectious larvae to control silverleaf nightshade, Solanum elaeagnifolium
Cavanilles (Solanaceae) in Texas (Orr 1980). The introduction of N. phyllobia into Australia and South Africa is under
consideration. Other invertebrate natural enemies have
limited use as nonselective grazers in biological control of aquatic plants,
including crayfish, snails and tadpole shrimp (Andrés & Bennett 1975,
Takahashi 1977). A vertebrate herbivore, the grass carp, Ctenopharyngodon idella
(Curvier & Valenciennes) (Pisces: Cyprinidae), has yielded mixed results
in different countries when introduced against mixes of aquatic plants
(Julien 1982). Seven other fish
species and the manatee complete the list of vertebrates that have limited
use in aquatic plant control of which the cichlid genera Tilapia and Sarotherodon
have shown the greatest control potential (Pictures) (Andrés &
Bennett 1975, Julien 1982, Legner 1983, 1986; Garcia & Legner 1999, Misc.
Research ). One report
discusses how fish converted noxious pests to edible protein (Legner 1980) Geese, sheep and goats have long been used as managed
grazers of terrestrial plants (King 1966). Biological
Control Organizations in Noxious Plant Control Goeden & Andrés (1999) detail the
various world organizations devoted to biological control of noxious plants.
They disclose more than 70 countries, which were involved in 499 releases of
introduced natural enemies for biological control. References pertaining to
organizations are Julien (1982) Julien et al. (1984), Coulson (1985), Schroeder &
Goeden (1986) The biological control of noxious plants has
deployed phytophagous insects from seven insect orders. Of these the least
successful have been Lepidoptera, which because they are so large, are
thought to be more susceptible to generalist predators (Bernays & Moran
1996). Although of initial importance for accelerating control of target
plants, the intrinsic rate of increase is not thought to be very important
for establishment. However, the level of specialization on the host plant may
be important. This degree of specialization may also be due to relatively
greater use of the plant for protection by highly adapted, specialized
species of phytophagous insects. Protection from abiotic events and predators
may involve internal feeding, galling, host-specific crypsis and many
specialized uses of particular features of the host plant. Monophagy (host
specificity) has to precede complex adaptive specializations, and specificity
is very important for restriction of the biological agent to the target
plant. Extreme specialization is not as common as monophagy among insects and
is not easy to measure. Characteristics of plants, which make them vulnerable
to biological control, are not easy to identify, especially as biological
noxious plant control has been dominated by relatively few target plants (= Opuntia spp., Lantana spp. and Compositae).
Finding patterns of biological attributes of insect herbivores that were used
in biological control and among their host plants has not been possible. This
is thought to be in part because of the problem of trying to generalize among
taxa and life forms with unequal representation, which. Bernays & Moran
(1996) state, "We conclude that biological control of weeds remains an
art dependent for success on the judgment of the gifted naturalist with
relevant experience." Bernays & Moran (1996) reviewed the
biological attributes of phytophagous insects, deriving their information
primarily from Julien (1982, 1987), Julien et al. (1984), and Moran (1986).
The attributes of plants that contribute to their status as weeds have been
discussed by Groves & Burdon (1986), McDonald et al. (1986), Mooney &
Drake (1986), Kornberg & Williamson (1987) and Joenje et al. (1987).
Crawley (1983) listed the main groups of vertebrate and invertebrate
herbivorous fauna and recorded the frequency of herbivory in these groups.
Also, the mode of feeding and the parts of the plant eaten were
characterized. Phytophagous species predominate among insects. Strong et al.
(1984) estimated that ca. 25% of all known living species of animals and
plants are plant-feeding insects. The biological control of noxious plants
has involved insects 96% of the time. Establishing
Phytophagous Biological Control Agents There is no agreement about the proportions
of phytophagous species represented in each of the seven orders of insects
that include phytophagous species and which have been used in biological
control. Chapman's (1982) estimate for the phytophagous insect fauna of Great
Britain is considered the most reliable based on authoritative records of the
feeding habits of each species, and includes almost the entire British insect
fauna. Bernays & Moran (1996) show various tables which detail these
data. In making such lists, it is difficult to define phytophagy precisely,
especially for species that are mainly wood and seed feeding. There are
differences in the proportions of phytophages in different taxa in different
geographical regions, and limited knowledge of the fauna and their feeding
habits. The data suggest that high proportions of releases of Lepidoptera and
Coleoptera have failed to establish, and there have been an exceptionally
large number of introductions of phytophagous Coleoptera species worldwide.
Diptera have a good record of successful establishment. Julien et al. (1984) provided an estimate of effectiveness of each of the
releases of biological control agents from the success ratings of Moran &
Zimmermann (1985). Apparently the Dactylopiidae (Homoptera) used widely as
biological control agents against cacti, have the best record of insect
biological control agents. Also indicated is that large percentage of
releases of Pyralidae (Lepidoptera) are effective, but the data are biased by
the outstanding and repeated success of the cactophagous moth, Cactoblastis cactorum against Opuntia spp. in many parts of
the world. Other pyralids have not been very successful. There have been 111 species of noxious
targeted for biological control using phytophagous insects, including 25
species against which native phytophagous organisms have been used (Julien
1987). These plants are in 33 families with the majority of target species in
the Compositae and Cactaceae (25 & 22 species, respectively), and with
five or fewer target plants in each of the other families (Julien 1984). No
success was achieved against target weed species in 11 plant families. Almost
2/3rds of all successful biological control attempts using insect herbivores
have been targeted at Lantana
camara (Verbenaceae), cacti
and Asteraceae Compositae. About 25% of all the releases that resulted in
establishment of phytophagous insects for biological plant control have been
on Lantana camara, ca. 23% on 22 species
of Opuntia (Cactaceae), 10%
on 12 species of thistles and knapweeds (Asteraceae) and about 6% on St.
John's wort, Hypericum perforatum. There is a
correlation between the proportion of phytophagous insects species introduced
for biological control of noxious plants and the proportion that successfully
established: the greater the number of species introduced the greater the
chances of establishment. The overall probability of successful establishment
of a phytophagous insect for biological control of noxious plants has been
ca. 0.71. This estimate was also given by Julien et al. (984) for all
invertebrate organisms and fungi used in noxious plant suppression. The overall
establishment rate has been greater in the case of composite plants other
than thistles and knapweeds (0.88), and in biological control of aquatic
plants (0.90). Phytophagous
Insect Feeding Habits Southwood (1978) and Strong et al. (1984)
point out that there are special evolutionary problems facing herbivorous
insects. The most general of these problems is the low nutrient content of
plant tissues, especially protein (McNeill & Southwood 1978, Lawton &
McNeill 1979, Lee et al. 1983). Phytophagous insects feed on a variety of
plant diets, from seed and fruit tissue, which is relatively rich in
nutrients, to mature leaves and wood, which have low levels of all available
nutrients. Many phytophagous insects are so well adapted to a generally low
protein diet that high levels may even be harmful (Broadway & Duffey
1986). Phloem feeding insects encounter different problems as there are
extreme variations in nutrients and secondary compounds within a plant and
within a leaf. Other problems include genetic variation, age, climate, soil
and time of day. Diet variability may be the greatest
difficulty confronting plant feeding insects. Somatic mutations were
considered by Denno & McClure (1983) as an evolutionary advantage for
plants in their ability to change under selection pressure by insect
herbivores. Mobile insects may have an advantage over sessile species in
making selections, and there are many examples of adaptability of herbivores
(Jermy 1987) and few examples of learning ability in relation to nutrient needs
(Waldbauer et al. 1984, Lee & Bernays 1988). Herbivorous insects that have been used in
the biological control of noxious plants include sap-suckers, miners,
chewers, borers and gall-formers that damage leaves, stems or cladodes,
reproductive parts or roots. Of all successful establishments, species where
the immature stages and the adults damage the host have been involved in 60%
of the cases; for 37% the immature stages alone are involved, and species in
which adults along cause damage accounted for only 3% of the cases. The
distribution of the main feeding motes for the immature stages of insect
herbivores used in the biological control of weeds is given in table form by
Bernays & Moran (1996). Apparently feeding habit does not greatly affect
the chances of establishment of the herbivorous insects used in biological
control. The differences in the proportions for immature insects that feed by
sucking simply reflects the large number of cases where released sucking
insects have established against Lantana
and against the Opuntia spp.
For all weeds combined, stem-boring species established more often than they
failed and species which feed on the leaves or cladodes of their host plants
failed more often than they established. These patterns do not include Lantana nor Opuntia, however. There has been a significantly higher rate
of establishment for phytophages whose immature stages feed on seeds or
fruits and a highly significant increase in the number of releases that
failed when herbivorous insects were used whose immature stages feed on the
roots of their hosts, and relatively small number of releases have involved
species whose immature stages feed on stems. These statistics all exclude Opuntia spp. and Lantana spp. as previously. Specificity
to Host Plant Host plant specificity is stressed in
biological control of noxious plants. Most insect herbivores are more or less
host specific, i.e., they feed on plant species within a family or subfamily
or lower taxon (Chapman 1982). This may be influence partly by ecological
factors rather than an inability to feed and develop on certain plants. Some
insects have microhabitat limitations that greatly restrict the avai8lable
foods, and some herbivorous species are restricted in their use of plants by
the action of predators (Smiley & Wisdom 1985). Such ecological
restriction is shown by the fact that host plant range is usually greater
under laboratory conditions than in the field even with some seemingly
monophagous species. In the laboratory 53% of insect biological control
agents were shown to increase the number of species of host plants on which
they could complete their development; the remaining 47% were assumed to be
restricted entirely by plant characteristics. Plant physical appearance can be limiting,
but for the most part, host plant acceptance and rejection is determined by
chemical factors, especially plant secondary compounds (Dethier 1982). The
most specific feeders may respond positively to chemicals typical of their
host plants, but they are also inhibited from feeding by features of nonhosts
(Bernays & Chapman 1987a). Sensitivity to deterrents increases markedly
with specificity and it is likely, although unproven, that dependence on key
host compounds also increases with specificity. The absolute dependence on
particular compounds by monophagous species is considered rare, because of
the ability of most specific insects to eat nutrient mixtures without host
specific chemicals and to oviposit on neutral substrates. Feeding or oviposition inhibition may be due
to repellents acting from a distance, but more usually it is by deterrents in
the surface waxes of the plant, or within the living tissues. In some host
specific insects, volatile compounds, wax components or internal constituents
are specific attractants, phagostimulants or oviposition stimulants (Juniper
& Southwood 1986). The neural basis of host plant choice has been
reviewed by Dethier (1982), Hanson (1983) and Schoonhoven (1987). Such
mechanisms are considered variable, although specificity appears dominated by
deterrent inputs from nonhost chemicals. Information transmitted by certain
deterrent cells, which are sensitive to a wide range of plant secondary
compounds, can lead to rejection behavior or, in some cases, the compounds
may cause a decline in the input from cells signalling favorability (Dethier
1982, Mitchell 1987). During the course of evolution, loss of
sensitivity to certain deterrents may be associated with a change in host us
to plants containing these compounds. A taxon that has been well studied this
way is a species group in the lepidopteran genus Yponomeuta (van Drongelen 1979). In this group, the
ancestral host plants and more recent host associations are well understood.
Loss of sensitivity to particular secondary compounds in recently adopted
plant host species could accompany the behavioral switch to these hosts.
Therefore, it seems that the derived insect species have been permitted to
use the new plant species, and in these insects there do not seem to be
essential compounds in the chosen hosts. Schoonhoven (1982) found that specific
signalling compounds that occur in the hosts of some insect herbivores evoke
responses from highly sensitive receptors. There are not enough examples to
generalize, but the clearest cases of this phenomenon occur in the most
recent specialized insect herbivores, such as Chrysolina spp. (Rees 1969). The tarsi have receptors
particularly sensitive to the host chemical, hypericin, which occurs on the
leaf surfaces of Hypericum
spp. Genetic or experimentally based variation in
sensitivity to deterrents may yield some information on the importance of the
rejection response; for instance, population variation or cases of
habituation, imply a limited importance of deterrents, and probably a relatively
low degree of obligate specialization on the host plants. Unvarying
deterrence, on the other hand, would indicate a greater specialization. Up to
now variation has been greater in herbivorous insects with a wider host range
(Jermy 1987). But, adequate genetic variance was found in the specific
bruchid beetle Callosobruchus
maculatus for artificial
selection of larvae onto a new host in 16 generations (Wasserman &
Futuyma 1981). The diversity of compounds involved in behavioral
responses of herbivorous insects may be viewed as convenient cues that enable
the insects to reject the nonhost with minimum delay and improve host finding
(Van Emden 1978). In more cases they are seen as plant defenses, which
insects may overcome, and if they do they may turn the defense into a useful
positive cue. On evolution and poly/monophagy, Huffaker et
al. (1971) stated, "Evolution has served both to promote monophagy and
to promote and maintain polyphagy in relation to the organisms and their
environment. The value of a broad diet is obvious; if one food (prey) is
scarce another can be substituted. By their very nature, specialists are
better adapted to utilize a specific prey at low prey densities in
maintaining their own populations. They are more closely synchronized in
their habits, haunts and seasonal life phases, and are normally better
attuned in nutritional needs, reproductive potential and searching behavior
to effectively utilize their prey at a minimal prey density. Thus, they are
more effective and reliable biological control agents..." The earliest
insect herbivores are believed to have been polyphagous, having arisen from a
variety of ancestors feeding on mixtures of pollen, fungi and decaying plant
and animal matter (Strong et al. 1984). Now the vast majority of phytophagous
insects have relatively restricted host ranges; probably over 90% feed on
only one or two plant families, while over 70% feed on only one or two genera
(Chapman 1982), and a significant proportion feed on only one species with
the accompanying risk of resource limitation. There are thought to be
important advantages for insects in having a limited host range, and it is
usually assumed that specialization for dealing with plant chemistry is the principal
one (Berenbaum 1986). Becoming adapted to detoxify the defenses of one group
of plants and being nutritionally specialized for dealing with them, is
expected to involve a loss of capability to deal with other groups of plants
(Levins & MacArthur 1969, Scriber 1983). The theories of chemical
coevolution of plants and their insect herbivores have been the subject of
many reviews (Feeny 1975, Rhoades 1979, 1983). May factors may drive evolution of host
plant specialization. The adaptive link between deterrence of herbivorous
insects by plant secondary compounds and their detrimental effects on insects
is rather weak (Mitter & Futuyma 1983, Bernays & Chapman 1987b).
There are many biologically active compounds that deter feeding by insects,
but which appear to have no detrimental effects when they are ingested.
Because deterrents have a major role in the behavior of host plant selection,
the implication is that rejection is triggered something other than avoiding
toxins. Supporting this reasoning are, e.g., force-feeding on nonhosts often
allows adequate growth and development (Waldbauer et al. 1984); there have
been numerous host switches by insect herbivores to unrelated plants (Strong
et al. 1984); artificial selection for host changes has been shown (Futuyma
& Gould 1979, Wasserman & Futuyma 1981); insects have versatile and
effective means of dealing with plant secondary compounds 9Bernays 1981,
1982); and habituation to deterrents has been shown in a number of insect
herbivores (Jermy et al. 1982). From the nutritional standpoint, monophagous
species of insects do not appear to have any advantage in terms of growth
rates, and may even be at a relative disadvantage (Futuyma & Philippi
1987). If the present patterns of restricted host
range do not always result from the need to specialize because of plant
chemistry, other pressures and reasons must exist that give the specialist
insect herbivore an edge. They believe that these reasons should be sought in
the major causes of mortality in herbivorous insects. Differential mortality
on different hosts due to variation in protection from natural enemies is
considered a possibility and has been shown in some cases (Smiley &
Wisdom 1985, Price et al. 1986, Lawton 1986). Host
Plant Ranges Weed control specialists formulated a
hypothesis for selection pressure which influences a restricted host range.
In it generalist natural enemies are thought to drive the process forward,
and it was believed possible to demonstrate that specialized prey specific
parasitoids are relatively more important. Evidence stems from work by
Bernays & Graham (1988), and Bernays (1988). Another option for
herbivores under pressure from prey specific parasitoids is to switch host
plants. Maintenance of sufficient flexibility may allow switching to occur
repeatedly and if tracking by the parasitoid is effective, polyphagy may
result. Thus in some cases a return to polyphagy would be driven by
specialist parasitoids. The overall proportions of specialist and generalist
herbivores might be a reflection of the relative mortality from specialist
parasitoids and generalist predators. Switching to other hosts is a problem in the
introduction of herbivorous insects used in biological control. However,
switching may be unlikely, or at any rate would be a lengthy evolutionary
process. This is because the herbivore, having been introduced without its
specialist parasitoids, may lack the normal pressures to cause a switch.
Species that have developed extreme dependence on one plant species have
shown only limited adaptability for switching, however. Analysis
of Biological Control Projects It is estimated the proportion of
introductions that resulted in successful establishment of insect biological
control agents for each insect family. It is regular for the number of such
establishments to be about double the number of failures. A detailed analysis
from Julien (1982) was made of more than 500 case of definite establishment
or definite failure to establish. Insect families represented by less than
three introductions were omitted from the analysis, and the remainder were
divided into those that are mainly or wholly monophagous/oligophagous, such
as Chrysomelidae, Tephritidae and Pyralidae, and those with polyphagous
species such as Noctuidae, Arctiidae, Gelechiidae and Agromyzidae. The ratios
of successful establishments to failures were 2.6 "0.1 for the first
group and 0.7 " 0.1 for the second, although all species analyzed were
specialists because as biological control agents they were originally
accepted for their host specificity. It is possible that species from
families in which polyphagy is common are less specialized and less adapted
to making use of host plants for protection from an array of mortality
factors. Differences in these ratios are not thought
to be due to nutritional factors, since all the developmental characteristics
and the fecundity of insect herbivore species analyzed from a data base
gathered at Silwood, England showed no significant differences between insect
species that successfully established and those that failed to establish.
Relative to biological control and to factors in the new environment of the
introduced herbivore, generalist predators and parasitoids and abiotic
factors will be of primary importance since the specialist natural enemies
have been carefully excluded. In the majority of cases for biological control
agents, the impact of natural enemies is unknown. Julien (1982), however,
noted the importance of predation in many of the cases where the agent was
from one of the more polyphagous families and Goeden & Louda (1976)
provided a summary of what was known 15 years ago. Crawley (1986) listed
generalist predators along with abiotic factors as the major causes of
establishment failure, and several analyses indicated that climate affected
species that failed more than species that established. If degree of specialization is important in
the establishment of a biological control agent, then it might be expected
that monophagous species would be more successful than oligophagous species.
The Silwood, England data showed that of 540 introductions for biological
control of plants, 36% involved agents that fed in their native habitats on
only one plant species or subspecies; 52% were restricted to a single genus
or species group and the remainder fed on more than one genus. But there are
no significant differences in these patterns when the cases of establishment
are compared with those that failed. Reasons could relate to the additional
complication that in a number of instances the agent was deliberately
introduced onto a plant species other than the original host plant (Goeden
& Kok 1986, Moran et al. 1986). The numerous insect species used in
controlling Lantana camara appear to be monophagous
but this plant is probably a species complex (Stirton 1977, Spies &
Stirton 1982), and the degree of specificity of its herbivores may have to be
categorized differently. Also an insect that feeds on one species of host
plant rather than on five is not necessarily more extreme in its level of
specialization. Important plant features for a highly specialized insect may
be shared by related plant species in one genus. For example, many species of
Hypericum contain hypericin,
which is used by species of Chrysolina
as a recognition factor, and the data indicate that this is a relatively
extreme case of specialization. On the other hand, Lantana camara
varieties differ in their complexes of volatile compounds as determined by
small to humans, and such differences may explain why many insect herbivores
are so selective among these varieties. It is possible, however, that the
level or degree of specialization of the phytophagous insect species is
important in the eventual process of establishment of a biological control
agent, although measuring this parameter is impossible. It may only be
possible to detect and test this by such means as chemoreceptor screening, or
by tests for predator avoidance. Insects feeding on fruits and seeds or that
form galls are more specific than leaf feeders (Janzen 1978, 1981). This may
be associated with such factors as small size, which favors success
biological plant control agents or level of specialization required to deal
with phenological and chemical factors that may provide extreme constraints
(Janzen 1969, Huffaker et al. 1976). Herbivore
Ecological Attributes It was concluded by Crawley (1986) from an
analysis of Silwood, England data that "the most pronounced patterns to
emerge from an analysis of wed control agents relate to the insects' intrinsic
rate of increase. Species with higher values of Ri are more likely to depress
weed abundance to low levels." It was also maintained that the
likelihood of establishment is most influenced by Ri. A reexamination of the
same data was made from the Silwood Project to investigate quantitatively the
components of Ri and to detect differences between introduced insect
herbivore species that established and those that failed to establish. No
significant differences were found in fecundity, generation time, voltinism
or longevity. On the other hand, size was a significant factor in the
successful establishment or failure to establish: small insects were much
more likely to establish than large insects. Crawley (1986) made the point
that intrinsic rate of increase showed a close negative correlation with body
size, but the importance of size relates to something other than the
intrinsic rate of increase since the generation time is not different in the
two groups and yet it is this component of Ri that should be most influenced
by size. Alternative reasons for the importance of
size in establishment of introduced herbivores may relate to causes of
mortality. Climatic factors should affect large insects less (Scriber &
Feeny 1979), and diseases seem unimportant in general. Since specialist
natural enemies have been excluded, the answer may be related to the
influence of generalist predators and parasitoids. Of all the failures of
insect herbivores to establish among the biological control projects that
were examined from an analysis of data from Silwood, England, only one was
shown to be mainly limited by a parasitoid, so that predation may be the
important factor. This makes intuitive sense when considering that the
attractiveness of large prey to a variety of predators such as birds and
lizards. Only speculation is possible on the identity of the main predators
of relatively large insect herbivores, most of which are Lepidoptera. The
importance of predators among species of this order compared with those of other
groups may be grater because the large exposed eggs are particularly
vulnerable to ant predation (Hoffmann 1981, Robertson 1985), the surface
feeders, which predominate among the Lepidoptera, are vulnerable to
generalist predators and the larger late instars may be particularly
vulnerable to vertebrate predation (Dempster 1984). Considering established biological control
agents, a higher proportion tend to be distributed over a wider geographic
range in their native home than is the case for those that did not establish.
They also tend to be found at many sites on a local level and to be very
abundant in general in their country of origin. Biological attributes
underlying these differences are unknown, but it is thought that wider
distribution involves at least a generally greater tolerance of climatic
extremes. The role of natural enemies in regulating the herbivores in their
native homes was compared for species that became successfully established as
biological control agents and for those that failed. Of 453 cases analyzed,
there were no significant patterns or trends of any kind. Plant
Attributes Bernays (1985) from a comparison of the
number of species in different plant families and the number of major weed
species in the same families, suggested that most important weed species are
in the more recently evolved plant families. For example, the Polygonaceae
and Cyperaceae have more weedy species than expected by chance and the
Poaceae contains a relatively large number of noxious plant species. There are
probably many reasons for this including physiological aggressiveness and
resistance to grazing damage. There are also good biological reasons for not
attempting to control graminoid plants with introduced arthropods. They are
not rich in numbers of insect species and tend to have a smaller proportion
of specialist herbivores. Only one attempt has been made to control a noxious
grass and it failed (Julien et al. 1984). Other than grasses, the greatest number of
noxious plants are in the Asteraceae but not in much greater proportion than
is expected by chance (Bernays 1985). But many noxious plant species occur in
the Cactaceae, and most of these are in the genus Opuntia (Julien et al. 1984, Moran & Zimmermann 1984).
Several factors may have contributed to the aggressiveness of cacti and may
have contributed to their status as pests: (1) they are successful
competitors, especially in dry areas, poor soils or in mismanaged or
botanically disturbed areas; (2) they are succulents and have morphological
and physiological adaptations including a waxy cuticle, shallow roots, and
CAM photosynthetic mechanisms to resist drought; (3) they reproduce and
distribute readily by seed and/or vegetative propagules; and (4) the thorns
provide a very successful protection from grazing animals. In addition, as
with other alien pestiferous species, it is perhaps of major importance that
they were released into their respective areas of introduction without the
associated natural insect fauna (Moran & Zimmermann 1984). No phytophagous
insects outside the new world (the native home of all cacti) have adapted to
Cactaceae as permanent hosts (Moran 1980) Qualities of noxious plants that make them
likely candidates for successful control by herbivores are variable. Moran et
al. (1986) summarized some of the attributes of such plants that may be
important in the context of biological control with herbivorous insects. They
commented that native plants have less chance of being controlled than
aliens, although there are some exceptions such as the control of the native Leptospermum scoparium in New Zealand
(Julien 1982), and that perennials provide a stable permanent habitat for
herbivorous insects and appear to have been more susceptible to biological
control than annuals. However, there are also exceptions such as Tribulus terrestris in the southwestern United States. Also of importance in biological control is
the taxonomic isolation of the plant. Moran et al. (1986) discussed the
practical value of this attribute in relation to screening insect herbivores
for biological control, but there may also be biologically relevant traits of
taxonomically isolated species. There may be a greater number of specialist
herbivores on taxonomically isolated plants because of a longer
coevolutionary history of insects with them (Strong et al. 1984). But this
may only be important if the taxonomic isolation is accompanied by
structural, chemical or phenological differentiation relevant to the insect
herbivores. A monospecific genus in the Poaceae may be more similar to its
other family members than a monospecific genus in the Asteraceae or
Verbenaceae. This may be one of the reasons why grass feeders tend to feed on
many more genera than do phytophages that feed on dicotyledonous plants
(Bernays & Barbehenn 1987). There is some controversy about whether
greater specialization of a phytophagous insect to a particular host
increases or diminishes its chances of success as a biological control agent
against an alien plant. It is considered that a greater degree of
specialization may provide better protection from a wide range of mortality
factors, and this may enhance the phytophages' chances of survival. Hokkanen
& Pimentel (1984) argued that insect biological control agents that
utilize different but related species to the target weed may be more
successful biological control agents of plants than insect herbivore species
introduced onto conspecific host plants. Goeden & Kok (1986) and Moran et
al. (1986) have contested these ideas, as has been discussed in a previous
section. Dennill & Moran (1988) provided evidence from insect-plant
associations in agriculture that highly specialized insects (those with an
old evolutionary association with their host plants, as was the terminology
used by Hokkanen & Pimentel 1984) and less specialized herbivores (those
with the potential of forming new associations with target plants that are
not among their original hosts) can be equally damaging to their hosts and
thus have utility as biological control agents. Dennill & Moran (1988)
also contended that because many of the insect herbivores involved in recent
associations have a restricted host range, there need not be any additional
risk associated with their introduction. PHYTOTOXINS FROM
PLANT PATHOGENS Genetically modified bacteria have been
inoculated into American elm trees to protect the trees from Dutch elm
disease, but the same technology could be applied to hastening the
destruction of target noxious plants (see Strogel 1991). The innate weed-killing
powers of living microorganisms have intrigued researchers in the quest for
environmentally compatible alternatives to chemical herbicides. Strobel
(1992) discussed the biological approach that would bypass the need to
release whole organisms and would reduce risks that pathogens might later
adapt to nontarget host plants. In place of organisms, the approach deploys
substances produced by microbes, namely weed-damaging compounds called phytotoxins. After the phytotoxins
are extracted from pathogens, the toxins can be studied individually for
their modes of attack. Once the chemical structure of these compounds are
known, they might be synthesized, thus escaping the need to collect or
maintain colonies of pathogens to produce weed-killing materials (Strobel
1991). It is that possible that many derivatives also might be synthesized to
improve the effectiveness of the original toxins. For further details on this
innovative approach to biological weed control, please also refer to
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