[Biological control, biological equilibrium and the needs of agriculture].
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Agriculture↗
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Results for “biological control”
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A review of helisoma duryi in biological control.
Biological control of schistosomiasis by means of introduction of the north American planorbid snail, Helisoma duryi, as a competitor of the intermediate host snails has been proposed. The systematics of the genus Helisoma and the geographic distribution of the different species is described. Papers dealing with laboratory experiments or field observations on the competition between H. duryi and different intermediate host snails have been reviewed. The status of H. duryi as intermediate host of trematodes has been evaluated by searching the literature for all the trematode species that are recorded from the genus Helisoma. The list does not include trematodes of medical or veterinary importance and despite many attempts it has not been possible to infect H. duryi with Schistosoma mansoni and S. haematobium. Finally this paper makes a few comments on the experiments that should be performed in the laboratory, under semifield conditions and and field conditions before H. duryi should be actively dispersed in Africa. The aspects to be considered include the nature of the competitive interactions, the relation between H. duryi and different medical and veterinary important trematodes and the effect of H. duryi on the biotope.
Animals↗
Biological control. Aspects of biological control--with special reference to arthropods, protozoans and helminths of domesticated animals.
Biological control describes situations in which a living antagonist (a predator, parasite, parasitoid or a pathogen) is distributed by man to lower pest (parasite) populations to acceptable sub-clinical densities or to keep the population at a non-harmful level. Ideally, biological control has no negative effects on the environment, whereas chemical control is not always so harmless. Laboratory and field observations have revealed many organisms, such as viruses, bacteria, fungi, protozoans, turbellarians, nematodes, earthworms, tardigrades, insects, copepods and mites as antagonists to parasitic arthropods, protozoans and helminths of domesticated animals. However, only very few of these antagonists have shown promising qualities as biological control agents within veterinary science. The lack of success should be linked to the lack of knowledge about complex natural biological systems and the antagonists that may be found there. This situation has restricted the interest of industry in developing biological products. In the future, however, industry may become more interested in biological control considering the increasing problems with parasite resistance to drugs in combination with the increasing cost of developing new chemical products, and because of increasing public concern about chemical residues in animal products and in the environment.
Animals↗
Assessment of the importance of similarity in carbon source utilization profiles between the biological control agent and the pathogen in biological control of bacterial speck of tomato.
Bacterial speck of tomato, caused by Pseudomonas syringae pv. tomato, was used to determine whether similarity in carbon source utilization between a preemptive biological control agent and the pathogen was significant in determining the ability of the bacterium to suppress disease. Similarity in carbon source utilization was quantified as the ratio of the number of tomato carbon sources utilized in vitro by the biological control agent to the number of tomato carbon sources utilized in vitro by the target pathogen (the niche overlap index [NOI]). Suppression of the disease was quantified as the percent reduction in disease severity compared to the pathogen-only control when nonpathogenic bacteria were applied to foliage 48 h prior to the pathogen. In the collection of 36 nonpathogenic bacterial strains, there was a significant (P < 0.01), but weak (r(2) = 0.25), correlation between reduction in disease severity and similarity in carbon source utilization, suggesting that similarity in carbon source use was significant in determining ability to suppress disease. The relationship was investigated further using catabolic mutants of P. syringae strain TLP2, an effective biological control agent of speck. Catabolic mutants exhibited lower levels of similarity (NOI = 0.07 to 0.90) than did wild-type TLP2 (NOI = 0.93). With these catabolic mutants there was a significant (P < 0.01), and stronger (r(2) = 0.42), correlation between reduction in disease severity and similarity in carbon source utilization. This suggests that similarity in carbon source utilization was a more important component of biological control ability for the catabolic mutants than for the nonpathogenic bacteria. Together, these studies indicate that suppression of bacterial speck of tomato was correlated with nutritional similarity between the pathogenic and nonpathogenic bacteria and suggest that preemptive utilization of carbon sources was probably involved in the biological control of the disease by both the naturally occurring nonpathogenic bacteria and the catabolic mutants.
Carbohydrate Metabolism↗
Replacement control and biological control.
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Animals↗
Management of ectoparasites with biological control organisms.
Biological control is not a new concept, but for many reasons it is gaining interest for control of livestock ectoparasites. These reasons will be discussed, both from a political view and from environmental and economic views. The US government has vowed to reduce pesticide use by the year 2000, but other forces may drive this change even faster. Pesticide costs are high, and efficacy against some pests is questionable. Also, many producers are concerned about the environment, and are anxious to do their part to reduce chemical pollution. Specialised training is required to reduce on-the-farm difficulties involved with the use of biological-control organisms. Otherwise, how do producers or veterinarians purchase and use biocontrol organisms, and how do they critique what has been purchased? Included is a short summary of the three most common ectoparasites of livestock, and the type of biological-control strategies being developed to combat them. Much of the classical work has been done on filth fly control, most likely because of the nuisance status of flies, and because of the availability of candidate beneficial organisms, particularly parasitic wasps. And finally, two fly-control success stories will be briefly described. Tremendous strides have been made in house-fly and stable-fly control with parasitic wasps on feedlots, but more work is needed to better understand the habits of immature fly populations. A predaceous fly is being tested for pest fly control in dairies. Larvae of this fly can kill 15-20 house-fly larvae daily, and the adults do not become pestiferous on farms or around homes. Biological control will be an important part of livestock pest control in the future, but its implementation will require a corps of educated producers who are confident that biological control can work for them.
Animals↗
A ratio-dependent food chain model and its applications to biological control.
While biological controls have been successfully and frequently implemented by nature and human, plausible mathematical models are yet to be found to explain the often observed deterministic extinctions of both pest and control agent in such processes. In this paper we study a three trophic level food chain model with ratio-dependent Michaelis-Menten type functional responses. We shall show that this model is rich in boundary dynamics and is capable of generating such extinction dynamics. Two trophic level Michaelis-Menten type ratio-dependent predator-prey system was globally and systematically analyzed in details recently. A distinct and realistic feature of ratio-dependence is its capability of producing the extinction of prey species, and hence the collapse of the system. Another distinctive feature of this model is that its dynamical outcomes may depend on initial populations levels. Theses features, if preserved in a three trophic food chain model, make it appealing for modelling certain biological control processes (where prey is a plant species, middle predator as a pest, and top predator as a biological control agent) where the simultaneous extinctions of pest and control agent is the hallmark of their successes and are usually dependent on the amount of control agent. Our results indicate that this extinction dynamics and sensitivity to initial population levels are not only preserved, but also enriched in the three trophic level food chain model. Specifically, we provide partial answers to questions such as: under what scenarios a potential biological control may be successful, and when it may fail. We also study the questions such as what conditions ensure the coexistence of all the three species in the forms of a stable steady state and limit cycle, respectively. A multiple attractor scenario is found.
Animals↗
Construction of a range of derivatives of the biological control strain agrobacterium rhizogenes K84: a study of factors involved in biological control of crown gall disease
The biological control strain Agrobacterium rhizogenes K84 is an effective agent in the control of Agrobacterium pathogens, the causative agents of crown gall disease. A number of factors are thought to play a role in the control process, including production of the specific agrocins 84 and 434, which differ in the spectra of pathogenic strains that they inhibit in vitro. A range of derivatives of strain K84 has been developed with every combination of the three resident plasmids, pAgK84, pAgK434, and pAtK84b, including a plasmid-free strain. These derivatives produced either both, one, or neither of the characterized agrocins 84 and 434 and were isolated by plasmid curing, conjugation, and Tn5 transposon mutagenesis. The ability of the derivative strains to inhibit gall formation on almond roots was compared to that of the wild-type K84 parent. Treatment with the plasmid-free derivative did not result in a significant level of control of an A. rhizogenes pathogen based on numbers or dry weight of galls formed on injured almond roots. The presence of plasmid pAgK84, pAgK434, or pAtK84b significantly enhanced the biological control efficacy of K84 derivatives, and the highest level of control was observed with strains harboring two or more plasmids. The results observed with strains deficient in agrocin 434 production suggest that this product may play an important role in the biological control of A. rhizogenes pathogens. The involvement of plasmid pAgK84b in biological control has not previously been reported. This study supports the conclusion that multiple factors are involved in the success of strain K84 as a biological control agent.
Journal Article↗
Impulsive control strategies in biological control of pesticide.
By presenting and analyzing the pest-predator model under insecticides used impulsively, two impulsive strategies in biological control are put forward. The first strategy: the pulse period is fixed, but the proportional constant E(1) changes, which represents the fraction of pests killed by applying insecticide. For this scheme, two thresholds, E(1)(**) and E(1)(*) for E(1) are obtained. If E(1)>or=E(1)(*), both the pest and predator (natural enemies) populations go to extinction. If E(1)(**)<E(1)<E(1)(*), the pest population converges to the semi-trivial periodic solution while the predator population tends to zero. If E(1) is less than E(1)(**) but even if close to E(1)(**), there exists a unique positive periodic solution via bifurcation, which implies both the pest and the predator populations oscillate with a positive amplitude. In this case, pest population is killed to the maximum extent while the natural enemies are preserved to avoid extinction. The second strategy: the proportional constant E(1) is fixed (E(1)<E(1)(*) firstly), but the pulse period changes. For this scheme, one threshold tau(0) for the pulse period tau is obtained. We can reach the same target as above by controlling the period impulsive effect tau<tau(0), even if close to tau(0). Our theoretical results are confirmed by numerical simulations.
Animals↗
Population regulation of a classical biological control agent: larval density dependence in Neochetina eichhorniae (Coleoptera: Curculionidae), a biological control agent of water hyacinth Eichhornia crassipes.
The release of classical biological control agents has reduced the economic, environmental and social problems caused by water hyacinth, Eichhornia crassipes; however, additional control measures are needed in some locations. Water hyacinth plants were treated with different densities of eggs of the weevil Neochetina eichhorniae Warner, one of the main control agents, under different nutrient regimes in a controlled experiment. Plants were destructively sampled and the development of N. eichhorniae was assessed. The survival of first and second instars declined as larval density increased. Plant nutrient status did not directly affect the mortality rate of larvae, but at higher nutrient concentrations larvae developed faster and were larger at a given developmental stage. It is argued that the density dependence operating in N. eichhorniae occurs through an interaction between young larvae and leaf longevity. Consequently, events which disrupt water hyacinth leaf dynamics, e.g. frost or foliar herbicides, will have a disproportionately large effect on the control agents and may reduce the level of control of the host.
Animal Nutritional Physiological Phenomena↗
Biological control of helminths.
As a potential component in future integrated parasite-control strategies, biological control by means of predacious fungi seems to be moving from a promising possibility toward becoming a reality, and the netforming nematode-destroying fungus Duddingtonia flagrans appears to be the candidate of choice. Not only has this fungus been found in, and isolated from, fresh sheep, cattle and horse faeces, but it also appears to be the only fungus that is able to consistently and significantly reduce the number of infective trichostrongyle larvae in faeces from animals fed fungal spores. Results from the last few years have shown that D. flagrans is able to trap and destroy free-living stages of the most important and common trichostrongylid larvae with very similar external life-cycles, as well as larvae of parasites with a slightly different transmission biology (Nematodirus spp., Dictyocaulus viviparus). The introduction of microfungi for biological control could be as part of a feed supplement or incorporated in feed-blocks presented to animals which are raised under relatively intensive conditions and constant surveillance. Apart from the special niche for organic farmers, such a product would be suited for horses, small ruminants (as either milking herds or housed daily for other reasons), cattle in special situations and free-roaming pigs. The most important constraint, still, for a major breakthrough in biological control in the latter is the lack of good antagonists against the long-lived and rather resistant infective stages of parasites, being transmitted as larvae inside the egg. Since the first Conference on Novel Approaches to the Control of Helminth Parasites of Livestock in Armidale, Australia, 1995, there has been a steady evolution within the area of biological control of parasitic nematodes. Today this principle is being exploited and tested out in almost all parts of the world, under various climatic conditions and production systems. Where, in the past, a large part of the work focused on cattle and to a lesser degree horse and sheep parasites, the focus of the research in many of the newly involved countries is on small ruminants, because of their importance to primarily small-scale farmers in local communities. Today research and trials are either on-going or being planned in many developing countries, as well as in countries in transition. The involvement of multinational agencies in addition to national and industrial interests is very welcome and should increase the chances and keep up the momentum for development and implementation of biological control in future animal production around the world.
Animals↗
[Alternative methods of vector control: genetic and biological control].
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Animals↗
Biological control of Sclerotinia sclerotiorum attacking soybean plants. Degradation of the cell walls of this pathogen by Trichoderma harzianum (BAFC 742). Biological control of Sclerotinia sclerotiorum by Trichoderma harzianum.
Two experiments of biological control of Sclerotinia sclerotiorum, one in the greenhouse and the other in the field, were carried out with soybean and Trichoderma harzianum as host and antagonist, respectively. Significant control of disease was achieved in both experiments, but there were no significant differences in plant growths. In the greenhouse, the application of T. harzianum as alginate capsules, increased the survival of soybean plants more than 100% with respect to the disease treatment. In the field, T. harzianum treated plants survived 40% more than those from the disease treatment, showing a similar survival level to control plants. Besides, a significant reduction (62.5%) in the number of germinated sclerotia was observed in the Trichoderma treated plot. Chitinase and 1,3-beta- glucanase activities were detected when T. harzianum was grown in a medium containing Sclerotinia sclerotiorum cell walls as sole carbon source. In addition, electrophoretic profiles of proteins induced in T. harzianum showed quantitative differences between major bands obtained in the media induced by S. sclerotiorum cell walls and that containing glucose as a sole carbon source.
Journal Article↗
The role of helminths in the biological control of mammals.
Biological control of invertebrates has been successful while that of vertebrates has been, with the exception of myxomatosis in rabbits, unsuccessful; reasons for this are discussed. Demographic studies of small mammals suggest that population regulation occurs by several different mechanisms, more than one of which may be acting at the same time. Coevolution is an important phenomenon in host-parasite associations, nevertheless parasites may limit host population abundance. The basis of the regulatory effect on the host population is that parasite-induced host mortality or reduction in fecundity is density-dependent. Increasing evidence of the density-dependent effects of helminths on host survival and reproduction is forthcoming from laboratory studies but has not been confirmed in the field. The theory that a helminth parasite may regulate mammal population abundance has been verified recently in the laboratory. A multidisciplinary research programme aimed at understanding the mechanisms responsible for formation of house mouse (Mus domesticus) plagues and seeking strategies to reduce mouse numbers is discussed. One aspect of the work involves investigation of the potential of the nematode, Capillaria hepatica, as a biological agent in the control of wild mice in the cereal-growing regions of Australia. Biological control of mammals is viewed within the context of integrated pest management. A helminth species which reduces host survival or fecundity at an increasing rate as host abundance increases has a role in host population regulation. There is potential to capitalize on that role and apply the helminth as a biological agent in the control of mammals which have attained pest status.
Animals↗
[Biological control agents against mosquitoes].
The control programs of mosquitoes are physical; chemical, mechanic control, control by sterilization, genetic, public health rules and biological control. Biological control is the most efficient and convenient one because of resistant organisms against insecticides, economic problems and degeneration of the eco-system. This control employs protozoal, fungal, bacterial pathogens, mosquito predator fish, nematoda parasites as bioagents. Safety tests, sensitivity to environmental factors, effectiveness against non-target organisms, production, stability and field trials are all giving promising results.
Culicidae↗
Biological control of fungal pathogens.
Biological control of soil-borne plant pathogens is a potential alternative to the use of chemical pesticides, which have already been proved to be harmful to the environment. Several strains of the fungus Trichoderma have been isolated and found to be effective biocontrol agents of various soil-borne plant pathogenic fungi under greenhouse and field conditions. Different application approaches have been used including integration of Trichoderma with reduced doses of chemical agents. Biochemical and molecular biology studies carried out to explore the mechanisms involved in biological control revealed that Trichoderma is a rather specific mycoparasite. Lectins were found to be involved in the recognition between Trichoderma and its host fungi, whereas chitinase is involved in the degradation of the host cell wall. Genetic engineering techniques were employed in order to increase the effectiveness, stability, and biocontrol capacity of Trichoderma spp. as well as other biocontrol agents, such as Pseudomonass spp. and Rhizobium.
Chitinases↗
Biological control of nematode parasites in sheep.
In a world in which sheep producers are facing increasing problems due to the rapid spread of anthelmintic resistance, the battle against gastrointestinal parasitic nematodes is a difficult one. One of the potential new tools for integrated control strategies is biological control by means of the nematode-destroying microfungus Duddingtonia flagrans. This fungus forms sticky traps that catch developing larval stages of parasitic nematodes in the fecal environment. When resting spores (chlamydospores) of this fungus are fed daily to grazing animals for a period of time, the pasture infectivity and thus, the worm burden of grazing animals are lowered, especially in young lambs. Research has been conducted throughout the world covering many different climates and management systems. An Australian parasite model showed that if the fungus performs efficiently (> or =90% reduction in worm burden) for 2 or 3 mo, it should contribute significantly to a reduction in the number of dead lambs otherwise occurring when managed only by anthelmintic treatment and grazing management. Feeding or field trials have clearly demonstrated that dosing with a few hundred thousand spores per kilogram of live BW not only reduced the number of infective larvae but also increased the BW of the lambs compared with controls not given fungus. Initial Australian work with feeding spores by means of a block formulation or a slow-release device has shown some promise, but further work is needed to fully develop these delivery systems. In tropical Malaysia, small paddock trials and field studies resulted in significant improvements, in terms of lower worm burdens and increased live BW, when feeding half a million spores daily to grazing lambs. Additional benefits have been observed when the fungus is employed in combination with a fast rotational grazing system. Research has also demonstrated that spores can be delivered in slightly moist feed block material, but only if such blocks are consumed rapidly, because of their very short shelf life. In the northern, temperate Danish climate it has been demonstrated that daily feeding of half a million spores per kilogram of live BW can lead to significant production benefits, with increased live BW gain in fungus-exposed animals. Biological control of parasitic nematodes in sheep seems to hold promise for the future, but to be able to assist producers, the optimal delivery system needs to be refined and further developed. In addition, more work will be needed to define the best use of this technology in different geographic regions.
Animals↗
Applications of biological control in resistant host-pathogen systems.
Insect pest species can have devastating effects on crops. Control of these insect pests is usually achieved by using chemical insecticides. However, there has been much cause for concern with their overuse. Consequently, research has been carried out into alternative forms of control, in particular biological control methods. Recent laboratory studies have indicated that these natural forms of control can induce resistant strains of insect pest. In this paper we present a discrete-time host-pathogen model to describe the interaction between a host (insect species) that can develop a resistant strain and a pathogen (biological control) that can be externally applied to the system. For this model we use a single-state variable for the host population. We show that the proportion of resistance in the population impacts on the viability of the host population. Moreover, when the host population does persist, we explore the interaction between host susceptibility and host population levels. The different scenarios which arise are explained ecologically in terms of trade-offs in intrinsic growth rates, disease susceptibility and intraspecific host competition for the resistant subclass.
Animals↗