BIOLOGICAL CONTROL OF NOXIOUS PLANTS AND WEEDS
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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.
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).
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.
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.
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).
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).
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.
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.
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 Charudattan (1990), Charudattan & Walker (1982), Hoagland (1990), Kenfield et al. (1988), Stierle et al. (1988).
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