Plant disease can be defined as anything that prevents a healthy plant from performing to its maximum potential. This broadly defines biotic and abiotic plant diseases. Plant pathology is the scientific study of diseases found in plants that are caused by pathogens and some other environmental conditions. Organisms that can cause infectious disease include bacteria, fungi, oomycetes, viruses, virus-like organisms, protozoa, phytoplasmas, nematodes, and parasitic plants. Plant diseases need to be controlled in order to maintain the quality and abundance of the food, feed, and fibre that are produced by growers all around the world.
Different approaches may be used to prevent, or mitigate, or control plant diseases. Beyond good agronomic and horticultural practices, the growers often rely majorly on chemical pesticides and fertilizers. Such types of inputs to agriculture have contributed more significantly to the spectacular improvements over the past 100 years in crop productivity and quality. However, the environmental pollution that is caused by the excessive use and misuse of certain agrochemicals, has led to considerable changes in the attitude of the people towards the usage of pesticides in agriculture.
Additionally in natural ecosystems, the spread of plant diseases may prevent the successful application of chemicals, because of the scale to which such applications might have to be applied. Consequently, some of the pest management researchers have focused mainly on their efforts in order to develop alternative inputs to the synthetic chemicals for controlling pests and diseases. The biological control of plant diseases is the alternative way to control pests. A variety of biological controls are easily available for use, but further development and effective adoption of these techniques will require a greater understanding of the complex interactions among people, plants, and the environment.
Entomopathogenic nematodes (EPN) are a group of nematodes also called threadworms, these can cause death to insects. The term entomopathogenic has a Greek origin, where the ‘entomon’ means insect, and pathogenic means causing disease. These animals are present in between microbial pathogens, parasitoids, and predators. These are habitually grouped with pathogens because of their symbiotic relationship with bacteria. Although many other parasitic threadworms can cause diseases in living organisms, the entomopathogenic nematodes are specific in infecting only insects.
Entomopathogenic nematodes (EPNs) live inside the infected insect host parasitically, and thus they are termed endoparasitic. They can infect many other different types of insects that are living in the soil like flies, butterflies, the larval forms of moths, and beetles. EPNs have been found all over the world and in a range of ecologically diverse habitats. They are highly diverse, complex, and specialized. The most commonly studied EPN’s are those that can be used in the biocontrol of harmful insects, which are the members of Steinernematidae and Heterorhabditidae. They are the only insect-parasitic nematodes that possess an optimal balance of biological control attributes.
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Biological Pest Control Types
There are three basic biological management strategies:
Importation: Importation, also called classical biological control, involves the introduction of natural enemies of pests to a new locale where they are not capable of occurring naturally. Some of the early instances were often found unofficial and not based on research, and some introduced species became serious pests themselves. To be most effective at controlling a pest, a biological control agent requires a colonizing ability that allows it to keep pace with changes to the habitat in space and time. Control is the greatest if the agent has the temporal persistence to the cause. So that it can maintain its population even in the absence of the target species.
Augmentation: Augmentation involves the release of natural enemies in a supplemental form that occurs in a particular area, it involves boosting the naturally occurring populations. In an inoculative release, control agents are released in small numbers at intervals to allow them to reproduce. To set up longer-term control and by keeping the pest down to a lower level. In inundative release, a large number of enemies are released to rapidly reduce a damaging pest population. Augmentation can be effective, but it is not guaranteed as it depends on the precise details about the interactions present between each pest and control agent.
Conservation: The conservation of natural enemies that are existing in an environment is the third method of biological pest control. Natural enemies that are already adapted to the habitat and the target pest, and the conservation of these enemies can be simple and cost-effective when the nectar-producing crop plants are grown in the borders of rice fields. These crop plants can provide nectar in order to support the predators and parasitoids of planthopper pests. These have been demonstrated to be more effective that the farmers sprayed about 70% fewer insecticides and enjoyed yields that are boosted by 5%.
Biological Weed Control Agents
The biological control of insect pests to maintain pest populations below damaging levels by the use of living organisms. Natural enemies of arthropods fall into three major categories such as predators, parasitoids, and pathogens.
Predators: Predators are mainly consuming prey in a large number directly during their whole lifetime, these are free-living species. Given that major crop pests are insects, where many of them are considered as predators that are used in biological control are insectivorous species. Lady beetles, particularly their larvae which are active between the month of May and July in the regions of the northern hemisphere and also consume mites, scale insects, and small caterpillars.
Parasitoids: Parasitoids can lay their eggs on or inside the body of an insect host, which can further be used as food for the developing larvae. Whereas the insect host is killed ultimately. Most of the insect parasitoids are flies or wasps, and many of them have a very narrow host range. The most important groups are the ichneumonid wasps, which use caterpillars as their main hosts. The braconid wasps can attack caterpillars and a wide range of other insects that include aphids. The chalcid wasps can parasitize eggs and larvae of many of the insect species. Parasitoids are most effective at reducing pest populations while their host organisms have limited refuges to hide from them.
Pathogens: Pathogenic microorganisms include a wide range of fungi, bacteria, and viruses. These microorganisms can kill or debilitate their host body and are relatively host-specific. Various microbial insect diseases can occur naturally, but may also be used as biological pesticides.
Biological Control of Mosquitoes
Biocontrol is the use of the natural enemies of pests like mosquitoes in order to manage the population of pests. There are several types of biocontrol, this includes the direct introduction of parasites, predators, and pathogens to target mosquitoes. Effective biocontrol agents include certain kinds of predatory fish that feed on mosquito larvae such as mosquitofish also known as Gambusia affinis and some cyprinids or carps and minnows and killifish. The direct introduction of tilapia and mosquitofish into ecosystems around the world have been found to have disastrous consequences. However, utilizing a controlled system through aquaponics provides control of mosquitoes without adverse effects on the ecosystem.
Other predators include dragonflies, which can consume mosquito larvae in the breeding waters. Whereas adult dragonflies can eat adult mosquitoes and some species of lizard and gecko. Biocontrol agents that have had lesser degrees of success include the predator mosquito Toxorhynchites and predator crustaceans such as Mesocyclops copepods, nematodes, and fungi. Predators such as bats, birds, lizards, and frogs have been used, but their effectiveness is only anecdotal. Invertebrate pathologists study the diseases that can affect mosquitoes in the hope that some of them can be utilized for mosquito management. Microbial pathogens of mosquitoes include bacteria, viruses, fungi, nematodes, protozoa, and microsporidia.
Dead spores of the soil bacterium Bacillus thuringiensis, especially Bt israelensis (BTI) can interfere with the larval digestive systems. It can be dispersed by hand or dropped by helicopter in large areas. After the larvae turn into pupae BTI loses effectiveness because they stop eating. Two species of fungi can kill adult mosquitoes, they are Metarhizium anisopliae and Beauveria bassiana.
Integrated pest management (IPM) is the use of the most environmentally appropriate method to control pest populations. Typical mosquito-control programs by using IPM involve conducting surveys, in order to determine the species composition that is affecting, relative abundance, and distribution of adult and larval mosquitoes that occur seasonally. After all these analyses the control strategy can be defined.
Biological control agents are non-polluting ones and thus these are environmentally safe and acceptable. Usually, they are the species that are specific to targeted pests and weeds. Biological control discourages the use of chemicals that are unsuitable to the environment and ecologically by establishing natural balance. As both biological control agents and the pests are in the complex race of evolutionary dynamism the problems of increased resistance in the pest will not arise. Because of the chemical resistance developed by the CPB (Colorado potato beetle), its control has been achieved by the use of bugs and beetles.