Viral and Fungal Pesticides

Viral Pesticides

Viruses are strong regulator of insect population in nature and hence they are identified as key agent for use as insecticides. They are parasitically replicating infectious microscopic elements attacking insect and related arthropods. Baculoviruses (BVs) and Cytoplasmic Polyhedrosis Viruses (CPVs) are commercially produced and used as bioinsecticides.

1. Baculoviruses (BVs)

Baculoviruses are the viruses that infect insect mostly of the order Lepidoptera (butterflies, moths, etc), Hymenoptera (saw-flies) and Coleoptera (Beetles) and have also been found to infect Crustaceans.

They are rod-shaped (Baculo means rod-shaped) and have double standard DNA as their genome and replicate in nucleus of infected cell. They are three sub-groups of Baculoviruses:

a. Nucleus Polyhedrosis Virus (NPVs)

An important characteristic of NPVs is that many virus particles are occupied in a single polyhedral inclusion body made of protein. It produces polyhedral bodies in the nucleus of infected cells. Polyhedral protein protects virions from action pf chemicals, drying, sunlight, enzymes and high temperature.

b. Granulosis Virus (GV)

InGranulosis virus, oval inclusion body is present which contains only one viral particle.

Mode of Action

BVs primarily infect insect larvae and adult insects are not susceptible to them . Viral insecticide is applied in the field by spraying virus particles on the surface of the leaves of agricultural crops. Insect larva get infection after ingestion of virus during ingestion of leaves. Under the alkaline condition in the gut of larva, the inclusion body disintegarte or solubilised and free virus particles are released. The released virus particles replicate within the nuclei of epithelial cells lining the midgut to produce more virions which then infect secretory cells of midgut and cells of other tissues causing the death of insect. One limitation of viral insecticide is that they cannot multiply on the surface of leaf. So that they must be sprayed on leaf in higher density.,

Some examples of Baculovirus and teir application in control of various insects are given below:

Lymantia disper NPV: destroy gypsy moth

Heliothis NPV: sprayed ontp cotton plant, maize plant and sunflower plants to control Heliothis.

Neodiprion sertifer NPV: destroys sawflies in the pine forest

Autographa california NPV: control cabbage looper in cabbage

2. Cytoplasmic Polyhedrosis Viruses (CPVs)

CPVs is the genus of viruses in the family Reoviridae. It has RNA genome and replicate in the cytoplasm of the infected cell. They produce polyhedral bodies int he cytoplasm of the cells of the latval gut and so are known as cytoplasmic polyhedrosis virus. They cause disease in over 20 species of insects.

Examples:

Rhynchoporous ferrugineus CPV - sprayed on coconut palm to destroy red palm beetles

Oryctes rhinoceros CPV- sprayed on coconut to control rhinocerous beetles

Method of Application

Some viruses are strong regulator of insect population in nature and hence they are identified as key agent or use as insecticide. Generally, BVs and CPVs are commercially produced and used as bioinsecticide to contol differernt types of pest of plants.

Viral insecticides is applied in field by spraying virus particles on the surface pf leaf of plants, with the sprayed droplet size playing a key factor. Smaller droplets give better surface coverage on the leaf thus increasing the chances of killing the insects encountering a virus containing droplet. Other application techniques are the release of infected insect in the field. An application as Baculovirus dust to stored products to control pest.

Fungal Pesticides

Different fungal biopesticides can be used to control plant diseases caused by other fungi, bacteria or nematodes as well as some insects, pests and weeds. Entomopathogenic fungi are among the first organism to be used for the biological control of the pest. More than 700 species of fungi from around 90 genera are pathogenic to insect. One advantage of fungal pesticide over bacterial and viral pesticide is that they infect the insect with contact and do not need to be consumed by insect to cause infection. Entomopathogenic fungi are commercially produced by cultivating them in solid cultivation media. Their spores are harvested and mixed with stable carriers like charcoal powder and then applied in the field in the form of spray.

The mechanism of killing insect by fungi is different from that of virus and bacteria. The most common mode of actions is through competitive exclusion, mycoparasitism and production of toxic metabolites. Some fungi can exhibit this entire mode of actions.

The infective form (i.e. spore) of fungi first comes in contact with the cuticle of (skin of insect). Then they germinate and penetrate the cuticle by germ tube. Then they reach the haemocoel and multiply and produced mycotoxin which results in death of insect.

Some commonly used fungal pesticides are:

1. Beauveria bassiana

It is filamentous fungus that grows naturally in soil throughout the world and acts as a pathogen on various insect species causing ‘white muscadine’ disease. Therefore belongs to the entomopathogenic fungi.

When a spore of this fungus comes in the contact with cuticle of susceptible insect, they germinate and penetrate directly through the cuticle of the inner body of their host. Here , the fungus proliferates throughout the insect body producing toxins and draining off nutrients and eventually killing it. Therefore, unlike bacterial and viral pathogens of insect, Beauveria and other fungal pathogens infect the insect with contact and do not need to be consumed by their host to cause infection. Once the fungus killed its host, it comes out through the softr portion of the cuticle covering the insect with the layer of white mold hence the disease is called ;white muscadine’.

2. Trichoderma viridae and Teichoderma harzanum

Trichoderma is one of the common fungal bio-control agents and is worldwide for suitable management of various foliar (leaf) and soil borne plant pathogen. They are a potent fungicide that kills pathogenic fungi affecting plant. They have the ability to colonize plant roots without harming the plants. These microbial fungicides can out-compete pathogenic fungi for food and space. In addition, they have the ability to attack and parasitize plant pathogens under environmental conditions.

3. Aspergillus flavus AF 36

It is used as fungicide for cotton plantation. This strain does not produce aflatoxin and it out-competes aflatoxin producing strains in the plantation thereby protecting workers and public.

Commercial production of Biopesticides

Production of Bacterial and Fungal Pesticides

Bacteria and Fungi can grow in non-living cultivation media. They are commercially produced by typicak fermentation process. Bacteria are produced in liquid cultivation media but fungi are usually produced by cultivating them in solid concentration media. It is because most fungi do not produce spores in liquid cultivation media.

At first, cultivation media is prepared by mixing al; essential nutrients. Then media is sterilized by heating. It;s p^H is adjusted and cooled. Then cultivation media is placed in production fermentation tank. Then the inoculum of organism is added in the media and is incubated. During incubation, bacteria multiply and increase in number. After incubation, microbial cells are separated from other debris by various techniques like centrifugation, precipitation, filtration, etc. Bacterial cells are then mixed with some stable carriers like charcoal powder and then packed until the application in field. Biopesticides are preserved by storing them in cold environment. In case of fungi. spores are harvested and then mixed with carriers.

Viruses are obligatory intracellular parasites and require host cell for growth. Furthermore, viruses are host specific therefore, virual insecticides can be cultivated only in insect cells. At first, nutrient medium copmosed of agar, caesin, sucrose, wheat and other growth factors is prepared. Then the cultivation mediium is digested by heating and then is cooled. The media is then placed in plastic trays. Examples of insects are are spread on cultivation media in plastic tray. After 5-7 days, virus particles are inoculated in insects. Viruses multiply in insect cells causing their lysis. After all insect cells have been lysed, the cultivation medium is filtered to separate viruses from other debris. Fyrther processing of virus is done by centrifugation, precipitation, filtration, etc. Separated viruses are then mixed with some carriers like charcoal powder and is packaged. During application in field, the powder is dissolved in water and applied in field in the form of spray.

Biological Control of Weeds (Bioherbicides)

Many fungi, bacteria and viruses are pathogen of various aquatic and terrestrial weeds. Nowadays, such plant pathogens are successfully used in biological control of weeds. Among microorganisms, fungi are leading agent used in biological control of weeds. Some microorganisms used in control of various weeds are given below:

Reference

Atlas, RM and R Bartha. Microbial Ecology:Fundamentals and Applications. The Benjamin Cummins Publication co. Inc., 1998.

Gordis, L. Epidemiology. third edition. 2004.

Maier, RM, IL Pepper and CP Gerba. Environmental Microbiology. Academic press Elsevier Publication, 2006.

Park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.