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Classical biological control, i.e. the introduction and release of exotic insects, mites, or pathogens to give permanent control, is the predominant method in weed biocontrol. Inundative releases of predators and integrated pest management are less widely used. The United States, Australia, South Africa, Canada, and New Zealand use biocontrol the most. Weeds in natural ecosystems are increasingly becoming targets for biocontrol. Discussion continues on agent selection, but host-specificity testing is well developed and reliable. Post-release evaluation of impact is increasing, both on the target weed and on non-target plants. Control of aquatic weeds has been a notable success. Alien plant problems are increasing worldwide, and biocontrol offers the only safe, economic, and environmentally sustainable solution.

Biocontrol involves harnessing disease-suppressive microorganisms to improve plant health. Disease suppression by biocontrol agents is the sustained manifestation of interactions among the plant, the pathogen, the biocontrol agent, the microbial community on and around the plant, and the physical environment. Even in model laboratory systems, the study of biocontrol involves interactions among a minimum of three organisms. Therefore, despite its potential in agricultural applications, biocontrol is one of the most poorly understood areas of plant-microbe interactions. The complexity of these systems has influenced the acceptance of biocontrol as a means of controlling plant diseases in two ways. First, practical results with biocontrol have been variable. Thus, despite some stunning successes with biocontrol agents in agriculture, there remains a general skepticism born of past failures (Cook and Baker, 1983; Weller, 1988). Second, progress in understanding an entire system has been slow. Recently, however, substantial progress has been made in a number of biocontrol systems through the application of genetic and mathematical approaches that accommodate the complexity. Biocontrol of soilborne diseases is particularly complex because these diseases occur in the dynamic environment at the interface of root and soil known as the rhizosphere, which is defined as the region surrounding a root that is affected by it. The rhizosphere is typified by rapid change, intense microbial activity, and high populations of bacteria compared with non-rhizosphere soil. Plants release metabolically active cells from their roots and deposit as much as 20% of the carbon allocated to roots in the rhizosphere, suggesting a highly evolved relationship between the plant and rhizosphere microorganisms. The rhizosphere is subject to dramatic changes on a short temporal scale-rain events and daytime drought can result in fluctuations in salt concentration, pH, osmotic potential, water potential, and soil particle structure. Over longer temporal scales, the rhizosphere can change due to root growth, interactions with other soil biota, and weathering processes. It is the dynamic nature of the rhizosphere that makes it an interesting setting for the interactions that lead to disease and biocontrol of disease (Rovira, 1965, 1969, 1991; Hawes, 1991; Waisel et al., 1991). The complexity of the root-soil interface must be accommodated in the study of biocontrol, which must involve whole organisms and ultimately entire communities, if we are to understand the essential interactions in soil in the field. The challenge in elucidating mechanisms of biocontrol is in reducing the complexity to address tractable scientific questions. One of the most effective approaches toward the identification of critical variables in a complex system has been genetics. The study of mutants can be conducted in simplified laboratory systems or in the field, thus making accessible the examination of particular genetic changes and the associated biochemical characteristics in the real world. This review presents recent advances in our understanding of the biocontrol of root diseases. We emphasize research aimed at enhancing our understanding of the biology of the interactions that result in disease suppression. It is this understanding that will make possible the practical use of microorganisms in the management of plant disease in agroecosystems. Numerous recent reviews present comprehensively the variety of microbial biocontrol agents (Chet, 1987; Weller, 1988; Whipps and Lumsden, 1991; O'Sullivan and O'Gara, 1992; Cook, 1993; Goldman et al., 1994; Cook et al., 1995; Lumsden et al., 1995). In this discussion of current and future directions in biocontrol, our goal is to present key themes in the discipline, drawing on the bacteria Pseudomonas and Bacillus and the fungi Trichoderma and Gliocladium as examples representing a range of life strategies and mechanisms of disease suppression. We address the principles of interactions of the biocontrol agent with the pathogen, the host plant, and the microbial community, illustrating each principle with some well-studied examples of successful biocontrol agents (DBO).

Coccinellids have been widely used in biological control for over a century, and the methods for using these predators have remained virtually unchanged. The causes for the relatively low rates of establishment of coccinellids in importation biological control have not been examined for most species. Augmentative releases of several coccinellid species are well documented and effective; however, ineffective species continue to be used because of ease of collection. For most agricultural systems, conservation techniques for coccinellids are lacking, even though they are abundant in these habitats. Evaluation techniques are available, but quantitative assessments of the efficacy of coccinellids have not been done for most species in most agricultural crops. Greater emphasis is needed on evaluation, predator specificity, understanding colonization of new environments, and assessment of community-level interactions to maximize the use of coccinellids in biological control.

Empirical research has not supported the prediction that populations of terrestrial herbivorous arthropods are regulated solely by their natural enemies. Instead, both natural enemies (top-down effects) and resources (bottom-up effects) may play important regulatory roles. This review evaluates the hypothesis that higher-order predators may constrain the top-down control of herbivore populations. Natural enemies of herbivorous arthropods generally are not top predators within terrestrial food webs. Insect pathogens and entomopathogenic nematodes inhabiting the soil may be attacked by diverse micro- and mesofauna. Predatory and parasitic insects are attacked by their own suite of predators, parasitoids, and pathogens. The view of natural enemy ecology that has emerged from laboratory studies, where natural enemies are often isolated from all elements of the biotic community except for their hosts or prey, may be an unreliable guide to field dynamics. Experimental work suggests that interactions of biological control agents with their own natural enemies can disrupt the effective control of herbivore populations. Disruption has been observed experimentally in interactions of bacteria with bacteriophages, nematodes with nematophagous fungi, parasitoids with predators, parasitoids with hyperparasitoids, and predators with other predators. Higher-order predators have been little studied; manipulative field experiments will be especially valuable in furthering our understanding of their roles in arthropod communities.

Transgenic plants expressing Bacillus thuringiensis (Bt) toxins are currently being deployed for insect control. In response to concerns about Bt resistance, we investigated a toxin secreted by a different bacterium Photorhabdus luminescens, which lives in the gut of entomophagous nematodes. In insects infected by the nematode, the bacteria are released into the insect hemocoel; the insect dies and the nematodes and bacteria replicate in the cadaver. The toxin consists of a series of four native complexes encoded by toxin complex loci tca, tcb, tcc, and tcd. Both tca and tcd encode complexes with high oral toxicity to Manduca sexta and therefore they represent potential alternatives to Bt for transgenic deployment

The potential harmful effects of non-indigenous species introduced for biological control remain an important unanswered question, which we addressed by undertaking a literature review. There are few documented instances of damage to non-target organisms or the environment from non-indigenous species released for biological pest control, relative to the number of such releases. However, this fact is not evidence that biological control is safe, because monitoring of non-target species is minimal, particularly in sites and habitats far from the point of release. In fact, the discovery of such impacts usually rests on a remarkable concatenation of events. In addition to trophic and competitive interactions between an individual introduced species and a native one, many effects of introduced species on ecosystems are possible, as are numerous types of indirect interactions. Predicting such impacts is no mean feat, and the difficulty is exacerbated by the fact that introduced species can disperse and evolve. Current regulation of introduced biological-control agents, particularly of entomophages, is insufficient. At the very least, strong consideration should be given to the likely impact of both the pest and its natural enemy on natural ecosystems and their species, and not only on potential costs to agriculture, silvi-culture, and species of immediate commercial value.

Few data exist on the environmental risks of biological control. The weevil Rhinocyllus conicus Froeh., introduced to control exotic thistles, has exhibited an increase in host range as well as continuing geographic expansion. Between 1992 and 1996, the frequency of weevil damage to native thistles consistently increased, reaching 16 to 77 percent of flowerheads per plant. Weevils significantly reduced the seed production of native thistle flowerheads. The density of native tephritid flies was significantly lower at high weevil density. Such ecological effects need to be better addressed in future evaluation and regulation of potential biological control agents

▪ Abstract  Suppression of the blossom-blight phase of fire blight is a key point in the management of this destructive and increasingly important disease of apple and pear. For blossom infection to occur, the causal bacterium, Erwinia amylovora, needs to increase its population size through an epiphytic phase that occurs on stigmatic surfaces. Knowledge of the ecology of the pathogen on stigmas has been key to the development of predictive models for infection and optimal timing of antibiotic sprays. Other bacterial epiphytes also colonize stigmas where they can interact with and suppress epiphytic growth of the pathogen. A commercially available bacterial antagonist of E. amylovora (BlightBan, Pseudomonas fluorescens A506) can be included in antibiotic spray programs. Integration of bacterial antagonists with chemical methods suppresses populations of the pathogen and concomitantly, fills the ecological niche provided by the stigma with a nonpathogenic, competing microorganism. Further integration of biologically based methods with conventional management of blossom blight may be achievable by increasing the diversity of applied antagonists, by refining predictive models to incorporate antagonist use, and by gaining an improved understanding of the interactions that occur among indigenous and applied bacterial epiphytes, antibiotics, and the physical environment.

Fragmentation of habitats in the agricultural landscape is a major threat to biological diversity which is greatly determined by insects. Isolation of habitat fragments resulted in decreased numbers of species as well as reduced effects of natural enemies. Manually established islands of red clover were colonized by most available herbivore species but few parasitoid species. Thus, herbivores were greatly released from parasitism, experiencing only 19 to 60 percent of the parasitism of nonisolated populations. Species failing to successfully colonize isolated islands were characterized by small and highly variable populations. Accordingly, lack of habitat connectivity released insects from predator control

The evolution of resistance by insect and weed pests to chemical pesticides is a problem of increasing importance in applied ecology. It is striking that the evolution of resistance by target pest species in biological control is much less frequently reported, particularly in control involving parasitoids and predators, rather than pathogens. Although it is conceivable that this reflects biases in reporting or frequency of application, we suggest that there is a puzzle here worthy of scrutiny, and we outline several potential underlying causes. In order of discussion (not necessarily of importance), these are: (1) lack of genetic variation; (2) genetic constraints on selection; (3) weak selection; (4) temporally varying selection; and (5) coevolutionary dynamics. We, in particular, focus on the potential for weak selection on the host for increased resistance, despite effective control. The very spatial mechanisms (e.g., refuges, metapopulation dynamics) believed to facilitate the persistence of many natural enemy-victim systems with strong biological control may also incidentally provide an environment where selection is weak on target pests to evolve improved resistance to control agents, thereby biasing coevolution toward the enemy. The basic insight is that in a spatially heterogeneous environment, a strong limiting factor on a population can be a weak selective factor. The hypotheses presented here provide ingredients needed to predict which biological control systems might be evolutionarily stable, and which not. Our aim in this thought piece is to stimulate more attention to the evolutionary dimension of biological control systems.