Everthing you need to learn about controlling insect pests using Biocontrol agents (BCAs).
Biological control is defined as the reduction in attack of a crop species by a pathogen achieves using another living organism, or organisms. Biological control covers the use of any organism to control a pathogen. This definition includes host plant resistance as a natural, and highly effective, form of biological control.
This concept itself includes both direct and indirect effects, due either to introduced antagonists, or manipulation of existing populations to reduce disease. Agronomic practices such as soil amendment with organic matter to boost microbial populations are therefore covered by this definition.
Introduction is not theoretically limited to insects and vertebrate predators; the use of the myxomatosis virus to control rabbits in Australia suggests that microbes may be able to play similar roles.
However, introductions have two fundamental limitations:
1. They have an extra dimension of unpredictability because an organisms, hitherto unknown, is being brought to a new area which it might find futile in unanticipated ways and/or may not acclimatized in changed conditions;
2. There is no clear way to make a profit from one-of introductions.
Substantial contributions can also come from fundamental biotechnological research.
Increases in potency by modification of the plasmid complement of the bacterium, which controls the synthesis of the protein both reduce the cost of production and provide the starting place for new, more easily used formulations.
Changes in the spectrum of activity of the toxic protein by genetic engineering can open wholly new markets.
Production of saprogenic strains may reduce production costs by avoiding the waste of metabolic energy on spore production and may make the product more acceptable in certain countries.
Types of Biological Control:
There are two main types of biological control, inundation and augmentation, both of which may be practiced with microbes or other organisms. They are essentially self-descriptive.
1. Inundation:
Involves the use of very large numbers of organisms over a short time to suppress or destroy a population.
2. Augmentation:
It is being the supplement of an existing natural population, which has the power to control a pest or disease.
Following are the approaches used for the biological control of plant pathogen:
Following are the species of viruses, fungi, bacteria, and protozoa that are considered the most likely for development into microbial biopesticides.
Both of these methods are employed in the control of insects by microbes. Many of the microbes used to suppress insect populations for the duration of crop would devastate the insects any way, unfortunately a little latter than is desirable for crop protection.
The numbers of propagules are, therefore, augmented at an early stage in the season and suppression of the pests begins and climaxes earlier, resulting in protected crop. The microbial insect control has aimed to bring forward the time when the pathogen controls the population rather than to introduce a new factor into the pest control system.
The fact that cultural practices such as crop rotation can reduce disease indicates that pathogen populations may be regulated by natural means, rather than by human intervention. During dispersal, survival and the early stages of infection on the surface of the host, pathogens are exposed to other organisms, which may affect their growth or viability. The starting point for biological control is to identify these natural constraints on pathogens and exploit them wherever possible to limit disease outbreaks.
The classical biological control is based on potential natural control agents and needs good understanding of the population biology and ecology of both the target pest or pathogen, and the biocontrol agent. A number of microbial products have been released onto the market for the control of insects and weeds.
Microbial pesticides are based on the toxin-producing bacterium, Bacillus thuringiensis, and insect-pathogenic fungi and viruses. All the products for weed control are based on plant-pathogenic fungi and are therefore described as mycoherbicides.
To date there are far fewer examples of biological agents developed for the control of plant pathogens. There have been numerous demonstrations of promising control in model systems, but scaling this up into crops in the field has often given disappointing results. Why is this? The following discussion will focus on the search for effective biocontrol agents (BCAs) for plant pathogens, and consider both the problems and prospects for commercialization of such agents.
Microbial Antagonism:
Soil-borne plant pathogens interact with other microorganisms found in the soil. The living organisms present in soils are capable of reducing the viability or infectivity of plant pathogens. These pathogens have a large influence on the incidence and severity of disease in the fields.
It has further indirect evidence for such microbial antagonism comes from surveys of disease in continuous monoculture, been known for many years that certain soil can reduce disease caused by vascular wilt fungi, such as Fusarium, and that the suppressive properties of such soils can be removed by sterilization. It is also possible to convert a conductive soil into a suppressive one by mixing in a proportion of the suppressive type.
The simplest explanation for this phenomenon is that as the pathogen population increases, there is a corresponding increase in the amount or activity of antagonistic microorganisms. The exact mechanism of such antagonism is still not clear.
One possible reason is that certain types of bacteria found in the root zone, or rhizosphere for instance fluorescent Pseudomonas, can interfere with infection of roots by fungi, and over time these types may come to predominate. Alternatively there may be increased microbial competition for the nutrient substrates on which the pathogen survives between crops, or even an increase in the level of predation or parasitism of the pathogen by other living agents, including viruses.
Disease suppression may well be due to a combination of several of these processes. This kind of natural antagonism may be exploited to control disease, either by managing the crop to maximize the impact of resident antagonists, or by isolating and identifying the most effective agents for use in specific biocontrol programmes.
Selection of Biocontrol Agents for Plant Disease Control:
There is little difference, in principle, between screening chemicals and testing biological agents for disease control. Both selection processes require an appropriate assay system to detect activity against the target pathogen, or pathogens.
Screening for biocontrol agents should, therefore, focus on specific natural sources chosen to optimize the changes of isolating strains with the correct biological properties. Random screening of various microorganisms for suppressive effects on plant disease has often been used to select the most promising strains for further development. The success of screening should be aimed on rational approach based on ecology.
For effective results, a biocontrol agent (BCA) should able to colonize a particular habitat, or to occupy a specific niche, in sufficient numbers to interfere with the growth or survival of the target pathogen. Rather than introducing a randomly selected microbial antagonist, it would be better to introduce one known to be adapted to the habitat concerned. Hence the best place to look for potential BCAs is in the specific environment in which they are to be used.
For example, if the target is a pathogen, which infects plant roots, the logical place to look for an antagonist is in the rhizosphere. Any microorganism isolated from this habitat is likely to have biological and physiological attributes enabling successful multiplication in the root zone, and is therefore, rhizosphere competent. Similar concepts apply to leaf surfaces or other substrates such as straw where a particular stage in the pathogen life cycle might be disrupted.
Production and Formulation of Biocontrol Agents:
BCAs are originated from living organism and usually needs to be metabolically active and effective. The commercialization of a BCA is the major difficult steps including its production, packing and delivery in sufficient quantities to the agent in a viable and stable form.
The other problems are as follows:
1. Variation is a normal feature of microorganisms, but each batch of a BCA needs to have similar activity. This can be a particular problem in scaling-up production, as microbial populations can change their properties during growth in fermenters. The BCA then has to be harvested and distributed in a formulation, which ensure viability.
2. BCA need to have a reasonable shelf life so that it can be stored for a period without significant loss of activity.
3. It needs to be applied to the crop, or into soil, in a way, which ensures that the antagonist grows and persists in the environment for sufficient time to exert control.
4. The above-cited problems have been solved to certain extent using several approaches including freeze-drying microbial cultures, or mixing cells with inert carriers such as clay or talc. Alternatively, biomass may be immobilized or encapsulated in an alginate polymer. A food source such as wheat bran can be added, which not only acts as a carrier, but also releases nutrients promoting growth of the microorganisms once applied.
There are a variety of application methods, for instance as liquid sprays or drenches, seed- dressings, and pelleted formulations, which slowly release the BCA into the environment. As with chemicals the choice depends upon the target pathogen, and the mode of action of the BCA.
Mode of Action of Bio-Control Agents:
The action of BCAs involves protection of an infection court by prior treatment with a microbial antagonist. For effective suppression of the pathogen establishment of a metabolically active threshold population of the BCA at the infection site is required. BCAs act against pathogens in soil or on plant debris and affect the survival of resting structures or propagules, rather than preventing infection. The exact mechanism of antagonism is still to be established. Following are the possible modes of actions.
Suppression by BCAs may be due to occupation of a particular niche by the BCA. It leads to physical exclusion of the pathogen, or to competition for essential nutrients. At one time it was believed that control of Heterobasidion by Peniophora was due to this type of effect. Now it is suspected that there is more direct interactions between the hyphe of the two fungi called hyphal interference.
Another possible mode of action is production of a toxic or inhibitory compound by the BCA. For example in case of agrocin.
Antibiotics have also been shown to be important in the biological control of root-infecting fungi including take-all. Strains of P. fluoresceins active against G. graminis produce an antibiotic known as phenazine; mutants which have lost the ability to produce this antibiotic give poor control.
This correlation has been confirmed by molecular genetic analysis in which inactivation of antibiotic biosynthesis genes by transposons in the bacterium abolished biocontrol activity, while introduction of a functional gene restores it. Such experiments suggest that more than 50% of the control effect is due to production of this compound. Different antibiotics have been implicated in the control of Pythium and other soil-borne fungal pathogens.
Siderophores:
Siderophores are low molecular weight compounds. They have a high affinity for iron and aid transport into cells. These chemicals are efficient scavengers of iron and may thus mop up all of the available supply in the immediate environment. Many pathogens require iron as an essential mineral nutrient for growth, and in some cases iron is required for virulence.
Hence, production of a sideropore by a BCA may reduce the growth of a pathogen, of its ability to attack the host. Fluorescent pseudomonads produce several siderophores, such as the pigmented compound pyoverdin, and the most convincing evidence that these contribute to disease control again comes from studies on non- producing mutants, which are less effective than wild-type strains.
Chemicals Diffusion:
Fungal pathogens form sclerotia as fruiting bodies. It is difficult to control as these propagules because they persist for long periods in soil. Several fungi, which invade sclerotia and act as parasites have now been identified. Sporidesmium sclerotivorum is an obiligate parasite of sclerotia of several important pathogens, including species of Sclerotinia, Sclerotium and Botrytis cinerea.
Multicellular conidia of the parasite are stimulated to germinate by chemicals diffusing from nearby sclerotia, and germ tubes infect the sclerotia, causing eventual lysis. Spores of this parasite added in sufficient quantities to soil have been shown to give good control of diseases such as lettuce drop caused by Sclerotinia minor. However, it is difficult to produce such BCA on a large scale in pure culture. Another sclerotial parasite, Coniothyrium minitans, has also demonstrated biocontrol potential, but only when added to soil at high inoculum rates.
Mycoparasites:
Mycoparasites are group of fungi, which infect other fungi. Mycoparasites have diverse host-parasite relationships, ranging from necrotrophy to biotrophy, and may infect different stages in fungal life cycles. Mycoparasites may be close relatives of plant-pathogenic fungi, for example, species of Pythium (P. nunn and P. oligandrum).
These fungi coil around and lyse hyphae of the damping-off fungus P. ultimum. The diversity of interactions occurring in natural environments means that it is usually possible to find an organism parasitic on the target pathogen or pest. Hence fungi attacking other fungi, nematodes and insects are all being studied as potential BCAs.
Hypovirulence:
A unique form of natural biocontrol has been observed in the pandemic of the highly destructive chestnut blight disease, caused by Cryphonectria parasitica. This fungus infects via wounds causing aggressive lesions, or cankers, which girdle the stem, leading to the death of shoots above the infection site. Significantly, strains of the fungus isolated from such cankers were found to be less virulent than the original pathogen (hypovirulent).
If these hypovirulent isolates were co-inoculated with highly virulent strains, the resulting cankers also healed. If the virulent strains could be converted to the hypovirulent phenotype by hyphen contact and fusion in culture, disease can be controlled. The possible reason may be that some transmissible factor moves from the hypovirulent strain into the more aggressive one.
The agents responsible were shown to be cytoplasmic and identified as double-standard (ds) RNA molecules. Several different-sized dsRNAs have been isolated from hypovirulent strains of the fungus, and the larger of these have sequence homology with certain plant virus genomes. It seems likely that hypovirulence is therefore a type of virus infection. The natural transfer of hypovirulence between strains of C. parasitica suggests that the disease might be managed simply by introducing hypovirulent isolates into the pathogen population.
Induction of Host Resistance:
BCAs can act by suppression of disease through the induction of host defence mechanisms. It has been known for many years that inoculation of plants with avirulent strains of pathogens, or agents causing necrosis, can trigger both local and systemic plant resistance. More recently, it has been shown that rhizobacteria applied to seeds or roots can induce a systemic resistance response expressed against pathogens infecting aerial tissues.
For example, treatment with Pseudomonas spp. increases resistance to the vascular wilt fungus Fusarium oxysporum f. sp. dianthi in carnation, leaf pathogens (Colletotrichum orbiculare and Pseudomonas syringae in cucumber, and bacterial blight (Pseudomonas syringae pv. Phaseolicola) in bean. This resistance appears to be associated with the induction of pathogenesis-related (PR) proteins as seen in a typical systemic acquired resistance (SAR) response. Presumably the colonizing BCA produces signal molecules, which activates the SAR pathway. However, the specific mechanism of induction is still not clear.
Constraints for Production of Biocontrol Agents:
Control of plant diseases using BCAs has been a disappointment. Few commercial products have reached the market place and much of the early promise has not been fulfilled in practical terms. Is this a fair assessment?
Following are the important constrains BCAs:
1. The inadequate availability of knowledge and understanding of microbial ecology and the factors leading to sustained performance of a BCA in a natural environment are yet not well established. Secondly, the expectation that natural control agents will substitute for chemicals in terms of instant results is unreasonable. There needs to be a more subtle approach in which these agents are seen as part of an integrated strategy for disease management.
2. The inconsistent performance of most BCAs in the field is the main obstacle for their widespread acceptance. Though BCAs are genuine alternative to chemicals but efforts are to be made to convince growers for their adaptability. Another constrain about the nature of BCA is to find out possible natural antagonists of pathogens, which work well under certain conditions. The challenge therefore is to improve the performance of these agents over a range of conditions, either by devising more effective methods of formulation and delivery, or by improving the properties of the BCA itself.
3. If BCAs are produced commercially then the microorganisms are subjected to repeat round of mutation and selection to obtain higher yielding or better-adapted strains. The critical difference between an industrial process and biological disease control is that a BCA has to function in a complex and variable environment, which includes interactions with other organisms.
In most cases only some of the factors are involved in effective antagonism of the pathogen. The genetic control of these different traits is likely to be complex, so that manipulation by mutation or, more precisely, recombinant DNA, may be difficult. Nevertheless, there is some hope that genetic engineering might provide improved strains giving more consistent control performance.
4. A further obstacle to the utility of genetically engineered strains of BCAs is public acceptability. The issue of transgenic organisms is controversial, and their use is regulated by strict legislation. While transgenic crops are now permitted, subject to controls, in several countries, there is greater concern about engineered microorganisms, and this may limit their application in the field.
Need for Genetic Improvement of Fungi:
Although there is extensive knowledge and expertise in the genetics of fungi important to existing industries like pharmaceuticals and brewing, relatively little is known about why one strain of an entomopathogen generates a successful epizootic with one another, equally pathogenic strain does not. We believe that the future development of successful fungal products for insect control rests as much with the geneticists, biochemists, physiologists and ecologists as it does with the fermentation and formulation specialists.
Biological Control to Control Other Pests:
Slugs:
Trapping, Natural Enemies and Biological Control:
Traps containing either milk or beer have been used for decades to catch slugs. These traps are labour-intensive and costly. Controlled refuges may be used, as an alternative or in addition. These are simply pieces of wood, tile or carpet placed on the soil among crops; they act in two ways.
They offer within-plot refuges for predatory beetles, which help to control slugs and many other pests. They also attract slugs, which use them as daytime refuges and sites for egg laying. Regular weekly inspection, and collection of any slugs or their eggs found, offers a quick and effective means of control.
Marsh flies (Tentanocera elata and Euthycera cribrata, Schiomizidae) are parasites of slugs and snails, and are attracted by the wildlife garden, with its ponds and flowers. These flies can lay up to 600 eggs in a season, while each fly larva may kill from nine to twenty-five slugs.
The newly hatched larva waits for a slug to pass by, then grabs hold of it with special mouth hooks, and burrows straight inside. At this stage it is a parasite, consuming the living slug tissue. Eventually the slug will die, and the larva then leads more of a predatory existence, waiting to ambush passing slugs, or following them along their slime trails.
Carabid beetles are effective biological control agents. Many carabids eat slugs, however, certain species, such as Pterostichus niger and Abax parallelepipedus, have been shown to be particularly important slug predators. Microorganisms, like nematode worms, a microsporidium, and certain strains of Bacillus thuringiensis (BT) all show promise, and may one day make organic slug control much simpler.
Birds, like thrushes, blackbirds, robins, starlings, crows, jackdaws, gulls and even owls, feed on slugs. Encourage these in the garden with shrubberies to nest in and provide winterberries, plus nest- boxes where appropriate. If you have space and opportunity, you can use chickens or ducks as biological control agents.
Target Pest and Its Biological Control:
The targeted pests are the members of Phylum Arthropod: the animal with a hard, outer skeleton and a jointed body and limbs. Arthropods make up a phylum of invertebrates that includes insects, such as ants, beetles, and butterflies; crustaceans, such as lobsters, shrimps, and crabs; and arachnids, including scorpions, spiders, and ticks. In terms of sheer numbers and the variety of niches they fill, arthropods are the most successful animals on Earth.
More than one million arthropod species have been identified- more than 20 times the number of known fish, amphibian, reptile, bird, and mammal species combined.
Insects are major target pests, with over 750,000 species grouped in 26 orders, insects form the largest class of animals living on land and in air.
Insects can be grouped into four groups:
1. Wingless forms
2. Having wings but cannot be folded
3. Wings can be fold over backs when not in use.
4. Wings can be folded of complete metamorphosis is exhibited.
Grasshoppers, aphid are examples of wings foldable. Butterfly, beetle exhibit complete metamorphosis (type of development). The important pests are- Grasshoppers, Pieris rapae (Artogeia rapae), Lymantria dispar, Spodoptera exigua, Tortricidae, Bemisia argentifolii, Bemisia tabaci, Teleogryllus commodus, Simuliidae, Helicoverpa spp., Heliothis spp.
2. Lepidoptera:
Lepidopteran families include: Alucitidae, Arctiidae, Batrachedridae, Coleophoridae, Cossidae, Drepanidae, Gelechiidae, Geometridae, Glyphipterigidae, Gracillariidae, Hesperiidae, Lasiocampidae, Limacodidae, Lycaenidae, Lymantriidae, Lyonetidae, Noctuidae, Notodontidae, Nymphalidae, Papilionidae, Phyllocnistidae, Pieridae, Psychidae, Pterophoridae, Pyralidae, Saturniidae, Sesiidae, Sphingidae, Tineidae, Tortricidae, Yponomeutidae and Zygaenidae. A wide variety of lepidopterous pests can be controlled by the bacterium Bacillus thuringiensis var kurstaki.
3. Other Important Target Pests:
Jassids (Amrasca bigutulla), Aphid (Lipaphis erysimi), Stem borer (Chillo partellus), Termite (Odontotermes obesus/Microtermes obesi) and Parasitoides and predators.
Principles of Biological Control and Botanical Pesticide, Biotic Agents and Microbial Pesticides:
Biological control is an important way to manage the plant diseases. The biocontrol agents do not destroy the crop. They destroy the insect pests called crop defendants or friendly insect beneficial.
Conservation of these naturally occurring farmer’s friends in the field or multiplying them in the laboratory and releasing in the fields is called Biological Control.
Biological control is likely to be inexpensive as compared with the production and application of chemicals which act as protectants and therapeutic agents. Furthermore, hyper parasites have a capacity for genetic variation and so are likely to produce new strains which are able to attack newly evolved resistant/virulent strains of the host pathogen, if and when such strains arise due to selection pressure in the population. The hyper parasites, in a sense, are there as living agents which are able to adopt themselves to changes in plants.
Meaning of Biological Control:
Although still grossly underused, biological control is gaining world recognition as a primary and often essential component of successful integrated pest management. Classical biological control involves deliberate introduction and establishment of natural enemies in areas where they did not previously occur. The approach is used largely against pests of foreign origin.
Biocontrol is a term which can include almost anything that is of biological origin and can mean using insects to control weeds or using microorganisms to control insects! Biological control has commonly had both pure and applied definitions and connotations.
Way for Biocontrol:
i. Collection and use of biotic agents, viz. parasitoids and predators.
ii. Isolation and non-production of antagonizing microbial organisms containing bacteria, fungi, viruses, nematodes, protozoa etc. which are known as microbial pesticide.
iii. Using plant based materials e.g. neem which are known as botanical pesticides.
Thus, microbial pesticides or botanical pesticides or biotic agents either alone or in combination are known as biopesticides.
Naturally Occurring Biological Agents:
The microorganisms used in biocontrol of plant diseases are termed as Bioagents or Antagonists. Exploitation of biocontrol agents and their artificial application and marketing for the large scale use of the growers, places them under the category of newly coined term biopesticides.
The two types of biopesticides are biochemical and microbial. Biochemical pesticides may have a similar structure to, and function like, naturally occurring chemicals, and have nontoxic modes of action. Insect pheromones, for example, are naturally occurring chemicals that insects use to locate mates. Microbial insecticides are another kind of biopesticide. They come from naturally-occurring or genetically altered bacteria, fungi, algae, viruses or protozoans.
They suppress pests by:
i. Producing a toxin specific to the pest;
ii. Causing a disease;
iii. Preventing establishment of other microorganisms through competition; or
iv. Other modes of action.
Ecology and Biological Control:
Biological agents include a wide variety of life form, including vertebrates, invertebrates, fungi and bacteria. Luck (1984) reviewed the use of three general types of mathematical model, those that treat continually growing predator-prey populations (overlapping generations), those that treat predator-prey populations with non-overlapping generations, and those that view the predation process as one of resource exploitation (i.e., foraging theory)”. Biological pest control strives to reestablish this balance in one of following three ways.
Importation:
Foreign exploration is conducted to identify and collect natural enemies in the county from which an exotic pest has been introduced. Following the discovery of a potential bio-control agent, it undergoes extensive evaluation to insure that its ecology and host range are compatible with the community to which it will be introduced and that it will not become a pest once it is released. Suitable candidates are reared and released in the new habitat in hopes that they will become established and suppress the pest population.
Conservation:
A variety of management activities can be used to optimize the survival and/or effectiveness of natural enemies. Conservation activities might include reducing or eliminating insecticide applications to avoid killing natural enemies, staggering harvest dates in adjacent fields or rows to insure a constant supply of host, or providing shelter, over-wintering sites, or alternative food sources to improve survival of beneficial species.
Augmentation:
Natural enemies that are unable to survive and/or persist in a new environment can sometimes be reared in large number and periodically released to suppress a pest population. In some cases, small numbers of a beneficial species are released in several critical locations to suppress local pest outbreaks (an inoculative release). In other cases, larger numbers are released in a single location to flood the pest population with natural enemies, an inundative release.
Herbal preparation in controlling pest would be more reliable than that of chemicals, consequently, the percentage of harmful effects on the crop environment would also be minimum. In the recent years, there have been concerted international efforts at developing plant derived pest-management control agents. These agents are relatively cheap, safe, biodegradable and eco-friendly.
The other ecological methods are:
1. Artificial Structures:
Suitable nesting boxes are kept for predatory birds and wasps (Polistes spp.) and burlap traps are placed around citrus trees for attracting coccinellid predatory (Cryptolaemus montrouzieri) of mealybug.
2. Food and Shelter:
Pollen-and nectar-bearing flowering plants are planted on the bunds to provide supplementary food to predatory like chrysopids, coccinellids, predatory mites, syrphid flies and parasitoids.
3. Controlling Ants:
Ants interfere with the activities of predators and parasitoids. Control and physical exclusion of ants by putting a band of some botanical around the trunk in citrus orchards increases the efficiency of coccinellid predatory (Cryptolaemus montrouzieri) and several parasitoids.
4. Cultural Practices:
Avoiding of trash burning in sugarcane protects the parasitoid Tetrastichus pyrillae of sugarcane pyrilla. Strip- cutting in alfalfa helps in shifting of the predators to nearby cotton crop. Regular removal of fallen fruits of apple and their destruction reduces the overall population of codling moth.
The egg parasitoids released in such orchards are much more effective. There are a number of examples wherein cultural lowering of the pest population gives more opportunity for the natural enemies to be effective. Thus, if the water shoots in citrus are removed, the populations of leaf-miner, scale-insects, citrus psylla and mealy-bug are reduced.
5. Trap Crops:
This is a strategy by which biological control could be made more effective and economic. The target host is attracted to the border rows of a trap crop which is much more preferred than the main crop. The attracted pest on trap crop is more amenable to be economically tackled through biological control in a restricted area.
Planting of castor around tobacco nurseries and marigold around main crop for attracting Spodoptera and Helicoverpa (Heliothis) respectively, and releasing egg parasitoids or spraying nuclear polyhedrosis virus could give effective suppression.
The principles of soil solarization involve trapping of solar heat energy through polythene covering, to raise the soil temperature to the levels where it becomes lethal to temperature sensitive or mesophilic soil microorganism. Polythene covering of soil induces greenhouse effect and raises soil temperature.
Details about Microbes in Biological Control:
I. Infectious Pathogenic Viruses:
1. Inclusion Viruses:
They are submicroscopic, obligate, intracellular, pathogenic organisms.
2. Nuclear Polyhedrosis Viruses (NPV):
Virus particles are rod- shaped and are encased in an outer envelope which may enclose one or several virus rods (dependent on the particular NPV). The viruses enclosed in the envelope are occluded (encased) in protein crystals called polyhedra. NPV is normally transmitted by oral ingestion of polyhedra. Ingested polyhedra dissolve, releasing virus rods into the lumen of the insect host’s midgut.
The symptoms of NPV infected host include:
1. Larval skin darkens and may have yellow patches or appear oily;
2. Skin becomes fragile;
3. Hemolymph becomes turbid;
4. Prior to death, infected larva usually climbs to highest point available and dies; and
5. After death, integument frequently ruptures, releasing millions of polyhedra.
3. Granulosis Viruses (GV):
GV particles are surrounded by an envelope similar to NPVs envelope. Particles surrounded by these membranes are occluded in a proteinaceous capsule similar to the polyhedral protein that occludes NPV’s. GV usually contains only one GV particle rather than many virus particles contained in NPVs and Cytoplasmic Polyhedral Viruses (CPVs). The fat body of lepidopterous larvae is the primary site of infection. GV’s are transmitted orally and via the egg.
Symptoms of a GV infected host include:
(1) Larvae frequently develop lighter color;
(2) Blood of infected larvae is usually turbid and
(3) Contains large numbers of capsules; and
(4) c. GV infections involving the epidermis cause liquefaction of larvae similar to NPV infections (but only when epidermis is infected).
Cytoplasmic Polyhedrosis Viruses (CPV):
Particles are not enclosed in membranes as are NPVs, but are occluded in protein crystals similar to those of NPVs. CPV infect the cytoplasm of the midgut epithelium of lepidopterous larvae.
Symptoms of CPV infected hosts include:
(1) Developmental times of host larvae are longer than normal;
(2) Infected larvae appear to have small bodies and large heads; and
(3) Body color may change.
II. Infectious Pathogenic Bacteria:
1. Non-Spore-Forming Bacteria:
(1) Potential pathogens,
(2) Lives in digestive tracts of most insects,
(3) Gain entrance to hemocoel due to stress factors (they can tolerate temperature extremes, other pathogens, parasites).
2. Spore-Forming Bacteria:
These are most important bacterial pathogens.
3. Bacillus Popilliae:
It causes milky disease in white grubs of scarab beetles such as the Japanese beetle. These are transmitted orally by ingestion of spore. After ingestion, spores germinate and penetrate the alimentary canal (probably through the Malpighian tubules). They are found in blood ca. 30 hrs after initial invasion (30°C), 7 to 10 days later, ca. 2-5 billion spores/larva.
Larva’s blood appears milky due to spores. Larva dies shortly thereafter. Commercial production of spores must be produced in living hosts -no artificial media available. They are very pathogenic to many lepidopterous larvae and immatures in 4 other orders. They are transmitted orally when the bacteria sporulates, it forms a toxic crystal (parasporal body). Different lepidoptera exhibits various responses to the various combinations of crystal and B. thuringiensis spores.
Some are susceptible to action of either the crystal or the spore alone (some to both):
i. Type I- develop a general paralysis and die 1 to 7 hrs after ingestion;
ii. Type II- do not develop a general paralysis and die 2 to 4 days after ingestion;
iii. Type III- susceptible to a combination of crystals and spores; and
iv. Type IV- some lepidoptera not susceptible.
After ingestion of spores, first symptom in both Type I and II is cessation of feeding. Activity of crystal is dependent on the pH of the larval foregut and midgut (pH 9-10.5) and the action of proteolytic enzymes within the gut. The crystal is a protoxin activated by enzymic hydrolysis.
4. Bacillus Thuringiensis:
They can be grown on artificial media. Hundreds of tons are manufactured in the U.S. The variety in the commercial preparation Dipel is B. thuringiensis var. kurstaki.
III. Infectious Pathogenic Fungi:
More than 36 different genera of fungi contain species which cause insect disease. Identification of fungus species is difficult. Most fungi are transmitted from one host to another by a spore, usually a conidium. Conidia germinate and form a special structure which penetrates the insect cuticle. Fungus then grows in insect’s body until the insect is filled with mycelia (insect is usually dead at this point).
Under favorable conditions, fungus continues to grow and produces structures which protrude through the cuticle and forms spores or conidia. Development of fungus infections is dependent on environmental conditions such as high humidity and temperature and high population densities. The important examples of fungi are: Metarrhizium anisopliae, Beauveria bassiana, Entomophthora spp. etc.
IV. Infectious Pathogenic Protozoa:
Flagellates, ciliates, amoebas, coccidians, and haplosporidians have pathogenic relationships with insects, but are considered the least important groups.
Neogregarines and microsporidians are the most important entomopathogenic protozoa.
1. They are transmitted orally from one insect to another (have a resistant spore).
2. They can be transmitted transversally from infected females to her progeny.
3. They produce diseases in insects which range from very pathogenic to chronic debilitating infections.
4. They can be important naturally occurring mortality factors.
5. They are obligate parasites which cannot complete their life cycles in artificial media.
V. Infectious Pathogenic Nematodes:
Several entomopathogenic nematode families (Mermithidae, Steinernematidae, Heterorhabditidae) contain species that are parasites of insects during at least part of the nematodes development. Normally, they have 4 molts between the egg and adult stages and between stages are referred to as juveniles. Most nematodes infect insect hosts as infective stage juveniles.
They may enter through the host cuticle or through the midgut, after entrance into the hemocoel, juvenile undergoes a period of rapid growth, then leaves the host, enters the soil and molts to form the adult nematode. Mating and oviposition occurs external to the host in the Mermithidae. Some species kill their hosts upon leaving.
Some species transport bacteria when they enter the cavity of the host and the insect dies from bacterial septicemia and the nematode feeds on the bacteria in the dead host tissue. Most nematodes are difficult to culture on artificial media. Only obligate endoparasitic nematodes are found in the genus Steinernema (= Neoaplectana).
Pre-Requisities of Microbial Inoculants:
1. Bio inoculants should be highly virulent to the target organism.
2. Should be stable to the stress of nature.
3. Virulence should be confined to the species of insects for which it is treated (Should be host specific).
4. Should neither damage the crop, nor harm beneficial insects and micro flora and predators and animals.
5. Should be easily mass culturable.
6. Should be stable for a long period of time.
Advantages of Microbial Inoculants:
1. They are environment friendly and leave behind no toxic residues.
2. Most of them target the specific insect and in turn protect beneficial insects.
3. Most inoculants are easily culturable in the lab, with minimum space.
4. Inexpensive to produce large quantities of inoculum.
5. Slowness in developing resistance to microbial pathogens.
6. Can control insect in cavities where chemical insecticides cannot reach.
7. In a way, we try to mimic nature by releasing them into an open environment.
Microbial inoculants can be used in two following ways:
i. Short Term Control:
For a particular season (annual) or by using highly virulent pathogens.
ii. Long Term Control:
Generally used for perennial crops and less virulent pathogens are preferred.
Disadvantages of Microbial Inoculants:
1. Necessity for careful and correct time of application.
2. Host specificity of most pathogens, narrows down its use.
3. Necessity of maintaining a pathogen in a viable condition, until the insect is contacted.
4. Difficulty in producing some obligate and facultative pathogens on a large scale.
5. Requirement of favorable environmental conditions for the pathogen to act, multiply and execute its mode of action.
6. Tendency of dead insects remaining attached to the host.