Agriculture has been facing the destructive activities of numerous pests like fungi, weeds and insects from time immemorial, leading to radical decrease in yields. Pests are continuously being introduced to new areas either naturally or accidentally. Global trade native pest species are being introduced to new areas. Controlling these species is a tough challenge worldwide. Agriculture and forests are an important resource to sustain global economical, environmental and social system. For this the global focus is to secure high and quality yields and to produce environmentally compatible crops. Chemical means of plant protection is leading in integrated pest management and diseases of plants, but pesticides cause toxicity to humans and warm-blooded animals. Despite many years of effective control by conventional agrochemical insecticides, so many factors are threatened by effectiveness and continues use of these agents. Which include the development of insecticide resistance and use cancellation or de-registration of some insecticides due to human health and environmental concerns. Therefore, an eco-friendly and safe alternative is the need of the hour. One method to improve pest control strategies to generate higher quality and greater quantity of agricultural products. Therefore, there is a need to develop biopesticides which are effective, biodegradable and do not leave any harmful effect on environment.
The First documentation of insect diseases is usually attributed to the descriptions of honeybee maladies recorded by Aristotle between 330 and 323. However, those observations of diseased silkworms were recorded in China as far back as 2700. The majority of early descriptive insect pathology concentrated on these two domesticated insects the honeybee (Apis mellifera) and the silkworm (Bombyx mori). The husbandry of these species dates back to the advent of written language and probably beyond. Aristotle also recorded these diseases of other invertebrate life forms, includes ants, oysters, scallops, and lobsters. He may be considered to be the first invertebrate pathologist. However, research and development of pathogens to control weeds (Te Beest et al. 1992) and plant diseases (Sivan and Chet 1992) have increased dramatically in the past decade; there are currently over 20 pathogens commercially available for weed and plant disease control (Copping 1998).
Microbial insecticides are composed of microscopic living organisms like viruses, bacteria, fungi, protozoa, nematodes or the toxins produced by these organisms. A microbial toxin can be defined as a biological toxin material derived from a microorganism, like bacterium or fungus. Pathogenic effect of those microorganisms on the target pests is so specific. The effect by microbial entomopathogens occurs with the invasion through the integument or gut of the insect, followed by multiplication of the pathogen resulting in the death of the host, e.g., insects. Studies have demonstrated that the pathogens produce insecticidal toxin important in pathogenesis. Most of the toxins produced by microbial pathogens which have been identified are peptides, but they vary greatly in terms of structure, toxicity and specificity
Advantages of microbial insecticides
The organisms used in microbial insecticides are essentially nontoxic and nonpathogenic to wildlife, humans, and other organisms not closely related to the target pest. The safety offered by microbial insecticides is their greatest strength. The toxic action of microbial insecticides is often specific to a single group or species of insects and this specificity means that most microbial insecticides do not directly affect beneficial insects in treated areas. Most microbial insecticides can be used in conjunction with synthetic chemical insecticides because the microbial product is not deactivated or damaged by residues of conventional insecticides. Their residues present have no harm to humans or other animals; microbial insecticides can be applied even when a crop is almost ready for harvest. The pathogenic microorganisms can become established in a pest population or its habitat and provide control during subsequent pest generations or seasons. They also enhance the root and plant growth by way of encouraging the beneficial soil micro flora.
Disadvantages of microbial insecticides
A single microbial insecticide is toxic to only a specific species or group of insects, each application may control only a portion of the pests present in a field and garden. Other types of pests are present in the treated area; they will survive and may continue to cause damage. Conventional insecticides are subject to similar limitations because they too are not equally effective against all pests it is because of selectivity indeed and this negative aspect is often more noticeable for predators, chemicals and microbial. Heat desiccation (drying out), or exposure to ultraviolet radiation reduces the effectiveness of several types of microbial insecticides. Proper timing and application procedures are especially important for some products. Special formulation and storage procedures are necessary for some microbial pesticides. Several microbial insecticides are pest-specific; the potential market for these products may be limited.
Bacterial biopesticides are the most common and cheaper form of microbial pesticides. As an insecticide they are generally specific to individual species of moths and butterflies, beetles, flies and mosquitoes. They are effective when they come into contact with the target pest. Bacteria in biological pesticides survive longer in the open. Bacterial pathogens used for insect control are spore-forming, rod-shaped bacteria in the genus Bacillus. They occur commonly in soils, and most insecticidal strains are isolated from soil samples. Bacilli are present in an extremely large area of environments ranging from sea water to soil, and are even found in extreme environments like hot springs. This bacterium could be one of the major sources of potential microbial biopesticides because it retains several valuable traits. Bacterial insecticides must be eaten by target insects to be effective; they are not contact poisons. Insecticidal products composed of a single Bacillus species may be active against an entire order of insects, or they may be effective against only one or a few species. For example, products containing Bacillus thuringiensis kill the caterpillar stage of a wide array of butterflies and moths. The microbial insecticides most widely used in the United States from 1960s are preparations of the bacterium Bacillus thuringiensis (abbreviated as Bt). Bt products are produced commercially in large industrial fermentation tanks. As the bacteria live and multiply in the right conditions, each cell produces (internally) a spore and a crystalline protein toxin called an endotoxin. Most commercial Bt products contain the protein toxin and spores, but some are cultured in a manner that yields only the toxin component. Bacillus popilliae and Bacillus lentimorbus, unlike Bt, do cycle in the environment if a substantial grub population is present at the time of application. When grubs killed by these bacteria break apart, a new batch of spores is released into the soil. These spores can survive (waiting to infect another grub) beneath undisturbed sod for a period of 15 to 20 years. Apparently lawn applications of milky spore disease bacteria might not have to be repeated each year. Bacillus popilliae var. popilliae and Bacillus lentimorbus offer limited usefulness in most mid western states because the predominant lawn grubs in this region are annual white grubs, which are larvae of beetles called chafers (genus Cyclocephala). These larvae are not susceptible (or are only slightly susceptible) to milky disease caused by Bacillus popilliae var. popilliae.
There are more than 1600 different viruses which infect 1100 species of insects and mites. A group of viruses called baculovirus, to which about 100 insect species are susceptible. Baculoviruses are rod shaped particles which contain DNA. Most viruses are enclosed in a protein coat to make up a virus inclusion body. In alkaline condition of insect's midgut dissolves the protein covering and the viral particles are released from the inclusion body. These particles fuse with the midgut epithelial cells, multiply rapidly and eventually kill the host. Viral pesticides are more expensive than chemical agents, many baculoviruses are host specific. Therefore they cannot be used to control several different pests. The action of baculoviruses on insect larvae is too slow to satisfy farmers. These viral preparations are not stable under the ultraviolet rays of the sun. Efforts are being made to encapsulate baculoviruses with UV protectants to ensure a longer field-life. The larvae of many insect species are vulnerable to devastating epidemics of viral diseases. The viruses that cause these outbreaks are very specific, usually acting against only a single insect genus or even a single species. Most of the viruses that have been studied for use as potential insecticides are nuclear polyhedrosis viruses (NPVs), in which numerous virus particles are "pack-aged" together in a crystalline envelope within insect cell nuclei, or granulosis viruses (GVs), in which one or two virus particles are surrounded by a granular or capsule like protein crystal found in the host cell nucleus. These groups of viruses infect caterpillars and the larval stages of sawflies. The well-known success of employing baculovirus as a biopesticides is the case of Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) used to control the velvet been caterpillar in soybean. In the early eighties this program was performed in Brazil. Since then, over 2,000,000 ha of soybean have been treated with the virus annually. The virus is obtained by in vivo production mainly by infection of larvae in soybean farms. The demand for virus production has increased tremendously for protection of four million hectares of soybean annually. Large scale in vivo production of baculoviruses encounters many difficulties the high demand for AgMNPV require studies dealing with inexpensive in vitro production of the virus. The use of AgMNPV brought about many economical, ecological and social benefits.
Protozoan pathogens naturally infect a wide range of insect hosts. One important and common consequence of protozoan infection is a reduction in the number of offspring produced by infected insects. Although protozoan pathogens play a significant role in the natural limitation of insect populations, few appear to be suited for development as insecticides. Entomogenous protozoa are an extremely diverse group with relationships ranging from commensally to pathogenic. They are generally slow acting and debilitating rather than quick and acute. Although they are undoubtedly important in natural biological regulation of insect populations, they do not possess the attributes necessary for a successful microbial insecticide. They can be extremely effective at reducing the fitness and fecundity of insects reared in culture so that they may have a very real negative effect on microbial insecticides produced in vivo. Most protozoan infections cause sluggishness, irregular or slowed growth, resulting in reduced feeding, vigor, fecundity, and longevity. Microsporidian infections in insects are thought to be common and responsible for naturally occurring low to moderate insect mortality. But these are indeed slow acting organisms, taking days or weeks to make harm their host. Frequently they reduce host reproduction or feeding rather than killing the pest outright. Microsporidia often infect a wide range of insects. Some microsporidia are being investigated as microbial insecticides, and at least one is available commercially, but the technology is new and work is needed to perfect the use of these organisms.
Nematodes are non-segmented, elongated roundworms that are colorless, without appendages, and usually microscopic. There are non-beneficial and beneficial nematodes. Non-beneficial nematodes are also called "plant parasitic nematodes" and cause damage to crops and other types of plants. Beneficial nematodes attack soil borne insect pests, yet are not harmful to humans, animals, plants, or earthworms, and can therefore be used as biological control organisms. Beneficial nematodes that cause disease within an insect are referred to as "entomopathogenic" and have the ability to kill insects. The keys to success with entomopathogenic nematodes are understanding their life cycles and functions; matching the correct nematode species with the pest species, applying them during appropriate environmental conditions, and applying them only with compatible pesticides. Entomopathogenic nematodes are living organisms; they require careful handling to survive shipment and storage as well as appropriate environmental conditions to survive in the soil after application.
Fungi, like viruses, often act as important natural control agents that limit insect populations. Most of the species that cause insect diseases spread by means of asexual spores called conidia. Although conidia of different fungi vary greatly in ability to survive adverse environmental conditions, desiccation and ultraviolet radiation are important causes of mortality in many species. Where viable conidia reach a susceptible host, free water or very high humidity is usually required for germination. Fungal pathogens differ in the range of life stages and species they are able to infect. Many important fungal pathogens attack eggs, immature, and adults of a variety of insect species. Others are more specific to immature stages or to a narrow range of insect species. Several factors have limited the development of fungal insecticides in the United States. Although fungal pathogens (at least some species) can be produced on artificial media, large-scale production of most pathogens has not yet been accomplished. Precise production and storage conditions must be established and maintained to ensure that infective spores are produced and stored without loss of viability before they are applied. Once applied, pathogenic fungi often are effective only if environmental conditions are favorable; high humidity or rainfall usually is important. Where fungal pathogens are incorporated into soil to control belowground pests, the adverse effects of ultraviolet radiation and desiccation are minimized, but other microorganisms that act as competitors or antagonists often alter pathogen effectiveness. Verticillium lecanii a fungus sold under the trade name Vertelec has been used in greenhouses in Great Britain to control aphids and whiteflies. Lagenidium giganteum a aquatic fungus is highly infectious to larvae of several mosquito genera. It cycles effectively in the aquatic environment (spores produced in infected larvae persist and insect larvae of subsequent generations), even when mosquito density is low. Its effectiveness is limited by high temperatures.
The demand for bio-pesticides is rising steadily in all parts of the world. When used in Integrated Pest Management systems, biopesticides efficacy can be equal to or better than conventional products, especially for crops like fruits, vegetables, nuts and flowers. By combining performance and safety, biopesticides perform efficaciously while providing the flexibility of minimum application restrictions, superior residue and resistance management potential, and human and environmental safety benefits. Hopefully, more rational approach will be gradually adopted towards biopesticides in the near future and short-term profits from chemical pesticides will not determine the fate of biopesticides
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