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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.

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.

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).

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.

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.

▪ 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.

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.

The current laboratory study was designed to evaluate the effect of abiotic and biotic factors such as temperature, light intensity, relative humidity and host age on biological and ecological characteristics of Aphelinus asychis parasitizing Aphis gossypii. The traits studied were developmental duration, mortality, sex ratio, longevity, fecundity and host feeding. A. asychis can completely develop and reproduce at temperatures 18 deg C and 30 deg C, light intensities of 1,000 and 7,000 lux and relative humidities of 30% and 60%. Temperature had a significant effect on the developmental duration as well as on the percentage and longevity of females, while mortality from mummification to emergence, fecundity and host feeding were only slightly affected. Relative humidity affected the developmental duration. High light intensity resulted in a shorter developmental duration, higher incidence of females and longer life span of the females. A high tolerance to climatic variations and life cycle well adapted to this aphid host are properties that make it likely that A. asychis could be used for the biological control of the cotton aphid in greenhouses.

Trichoderma harzianum T39 and T. longibrachiatum strain 6 were applied on grapevine to determine their effectiveness against Phaeomoniella chlamydospora on vine cuttings and pruning wounds. Cuttings were dipped in a Trichoderma suspension either before or after callusing. Pre-callusing dips were carried out for 3 years and yielded contradictory results. By contrast, post-callusing Trichoderma dips led to significant growth of hairy roots and a reduction in the longitudinal discolorations caused by P. chlamydospora inoculated into the rootstock after dipping. Trichoderma spp. were also applied to pruning wounds of grafted potted vines, which were then inoculated by placing drops of a conidial suspension of P. chlamydospora on the wound surface. Trichoderma application prevented black goo and necrosis in the wood below the wound. In the vineyard, T. harzianum T39 was sprayed after pruning for two consecutive years. The biocontrol agent was re-isolated from the wood close to the sprayed pruning wounds for up to 2 months after spraying. Although further investigations are necessary, our findings suggest that Trichoderma could be one of the steps in the control of esca [Sono stati effettuati trattamenti alla vite con Trichoderma harzianum T39 e T. longibrachiatum ceppo 6 per determinarne l´efficacia nei confronti di Phaeomoniella chlamydospora su talee e ferite da potatura. Le talee sono state immerse in sospensioni di Trichoderma, sia prima, sia dopo la formazione del callo. I trattamenti prima della formazione del callo sono stati effettuati per 3 anni e hanno fornito risultati contrastanti. Al contrario, quelli successivi alla formazione del callo hanno determinato una crescita significativa delle radici pilifere e una riduzione delle necrosi longitudinali causate da P. chlamydospora inoculata nel portinnesto dopo l´immersione nella sospensione. Le specie di Trichoderma sono state anche applicate a ferite da potatura di viti innestate coltivate in vaso, che sono state successivamente sottoposte a inoculazione ponendo gocce di una sospensione di conidi di P. chlamydospora sulla superficie della ferita. Il trattamento con Trichoderma ha prevenuto lo sviluppo di gommosi nera e di necrosi nel legno al di sotto della ferita. Nel vigneto, T. harzianum T39 è stato irrorato dopo la potatura per due anni consecutivi. L´agente di controllo biologico è stato reisolato dal legno contiguo alle ferite da potatura irrorate fino a due mesi dopo il trattamento. Sebbene siano necessarie ulteriori ricerche, i nostri risultati fanno ritenere che Trichoderma può essere uno dei passaggi nel controllo del mal dell´esca.]

At five sites in Hungary and Italy, traps baited with phenylacetaldehyde (PA) caught significantly higher numbers (10 to 100 times more) of green lacewings than unbaited traps, which demonstrates that this compound is an attractant. Traps with three bait dispensers usually caught more than those with one dispenser, but the difference was significant only at two out of five test sites. There was no difference in the numbers caught by sticky delta and funnel traps baited with PA. However, funnel traps could be adapted to catch living green lacewings. The vast majority of the specimens belonged to the Chrysoperla carnea species complex. Ch. carnea sensu lato dominated the catches at all sites. At some sites 3-11% of the insects caught were Ch. lucasina. PA-baited traps were attractive to both sexes, but generally more females were caught than males. Funnel traps baited with 3 dispensers of PA caught green lacewing adults throughout the season in Hungary.