Viruses in Bio-controls of Crop Pest
Viruses are a non cellular sub microscopic / ultra microscopic infective entily, less than 200 mu in diameter and that multiples only as obligate pathogen. Intra cellularly in host tissues and consist of a protein coat surrounding a core of self replicating nucleic acid either DNA/RNA.
Viruses are the most exciting and promising group of pathogenic micro-organisms under consideration for use in biological insect pest suppression. So far more than 1200 virus-host relations have been described. Majority of them are found in order Lepidoptera, (83%), Hymenoptera (10%), Diptera (4%) and few examples from Coleoptera, Neuroptera, and Orthoptera etc.
The viruses are currently named and classified on the basis of virus particle characteristics such as shape, outline and symmetry, molecular weight of the nucleic acid as well as symptomology of the diseases, site of viral replication and sensitivity to chemicals. There are six groups of viruses according to the criteria and classification approved by the International Committee on Taxonomy of viruses.
a) Baculoviruses (BV):
Main characteristics of Baculoviruses are:
1. Properties virus particle: Nucleic acid is a single molecule of circular super coiled DNA, protein virus particle (virion) structurally complex and contain 10-25 polypeptides of which 4-11 are associated with necleocapsides, and carbohydrate present.
2. Replication is in host cell nuclei.
3. Transmission naturally through food contamination and experimentally by injection, infection or translocation cell cultures.
1) Nuclear polyhedrosis virus (NPVs):
These are the best known and account for 41% of the described arthropod viruses. They show great promises for practical use in pest suppression. These viruses develop in the host cell nuclei and their virions are occluded singly or in groups in polyhedral inclusion bodies. The rod shaped virion contains double stranded DNA and are 230-420 nm long. The POBs are 0.2-15 um in diameter. Virulent obligate pathogen requires living cells for development and multiplicate. The NPV infect all insects cells regardless of their derivation and cause death. Polyhedral protein protects virion from action of chemical, drying, sunlight, enzymes and high temperature. Free virus particle is less stable. It is highly host specific with no effect on beneficial fauna. It is safe to plant birds and higher animals and man. It enters through injection of plant material into insect gut through mouth and cuticle. Infected insect appear dull in colour and inceptive. Feeding rate of insect is reduced. In advanced stage integument fragile and rapture on slight disturbance emitting liquefied content (whitish fluid). Incubation period is 4-5 in 20 days. Earlier instars are more susceptible than 5th of 6th instar. Infected larvae hang invertedly from twigs.
2) Granulosis viruses (GVs):
It has also show considerable promise as agents for insect pest suppression. They develop in either the nucleus or cytoplasm of host fat, tracheal matrix or epithelial cells. The virions are occluded singly in small inclusion bodies called capsules. The rod shaped virion contains DNA and are similar to NPV viruses. They usually oval occlusion bodies about 200×400 nm size. They enter through ingestion. The diseased larvae are less active, flaccid and fragile and period from infection to death is 6 to 20 days.
b) Cytoplasm polyhedrosis Virus (CPV):
It is also a promising group for practical use. They develop only in the cytoplasm of host midgut epithelial cells. The spherical virions are occluded singly in polyhedral inclusion bodies and contain double stranded RNA. Their average diameter is 60 nm. The POBs ranges from 0.5-15 um in diameter. Infection by CPVs is not always lethal but shows larval growth reduces.
c) Entomopox viruses (EPV):
This is a recently discovered group. On the basis of particle size and structure, there are apparently 3 EPV sub groups, each affecting different insect orders. EPV replicate in the cytoplasm of host fat body cells and possible in heamocytes. Virions are ovoid cuboids, 200×300×200 nm and contain double stranded DNA. Two types of IBs may be produced along ovoid to irregular shaped IB, 2-8 um long and smaller spindle shaped bodies, deveoid of occluded virus particles. Because of their close relationship to vertebrate pox viruses conformity for safety of EPV before use.
d) No Occluded indescent Viruses (IV):
There are intensively studied by Smith (1967). The initial site of virus multiplication is in cytoplasm of host fat body’s cells. The viruses crystalise spontaneously within the living insect o give it a characteristic Iridescence by Bragg reflection. The Tipula iridensect virus (TIV) is 130 nm in diameter and has icosahera shape. It contains double stranded DNA and has been artificially transmitted successfully across ordinal lines to non-dipteous hosts in the Lepidoptera and Coleoptera.
General considerations for field application of BVs:
1. Method of application : Foliar spray.
2. Stage of pest : Early in stars are highly susceptible.
3. Dose of virus preparations : HNPV-250LE/ha (1LE=6× PIB),2 to 3 sprays in early stages of pest.
4. Preparation of spray fluid NPV in required strength is mixed with good quality soft water +0.1% teepol or Trition-x-100.
5. Time of application : Preferably in evening hours.
6. Frequency of application : 2-4 application in case of NPV/weekly.
7. Application equipment : High volume applications are more effective than low volume.
8. Integration of chemicals : NPV is compatible with most of pesticides.
9. Use of adjuvants : Skimmed milk, juggary, Teepol, sandovit.