Insects suffering from virus disease show characteristic symptoms, These are: change in body colour, body rupture and oozing out of infected haemolymph and hanging-on posture on pro legs of dead larvae. For identification, infected tissues are smeared. stained and observed under microscope. Other tests such as histological, histochemical. biochemical and serological are also carried out.
Most insect viruses end up with proteinic occlusion bodies of different sizes and shapes in different organs/cells of the infected host insect. The occlusion body comprises of virus particles. single or in groups, rod or spherules.
Identifying characters of Nuclear Polyhedrosis Virus (NPV): are: Virions are rod-shaped. comprising a lipoprotein envelop around a protein capsid containing DNA-protein core. inclusion bodies contain major polypeptides mol. wt. 30,000 dalton. virions contain 11-25 polypeptides. viral genome is a single molecule of double stranded circular supercoiled DNA with mol. “”‘1. 50-100 x 106 dalton. Replication of virus occurs in nuclei ‘of host cells.
The nuclear polyhedrosis viruses are among the most studied and utilized viruses that infect lepidopterans. These are irregular spherical, cubic or hexagonal crystals within blunt or melted edges. These are deposited in the nuclei of the fat body cells. Each polyhedron measures 3 to 10 microns in diameter and contains many particles or virus rods. The virus rods are alkali sensitive.
Each rod measures 20-50 nm thick and 200-400 nm long.The granulosis virus infected insects contain 0.3 to 0.5 micron long oval granules, Each granule contains only one virus rod in occlusions. The haemolymph of the infected caterpillar turns milky-white. The granulosis virus of sugarcane tissue borer and codling moth of apple are being utilized for combating their respective hosts. Non-occlusion viruses are spherical, do not form occlusion or polyhedron and their particles are distributed in infected tissues. Mostly they are cytoplasmic but some are located in the nuclei of the infected insects. Baculovirus of rhinoceros beetle is being utilized on a large scale.
Out of various types of viruses, only two as stated above are pathogenic to insects. Some examples are given below:
1. A nuclear-polyhedrosis virus is pathogenic to the larvae of silkworm.
2. A nuclear-polyhedrosis virus is pathogenic to the larvae of nun-and gypsy moths.
3. A nuclear-polyhedrosis virus is pathogenic to the cabbage worm, lucerne caterpillars. pink bollworm of cotton, etc.
4. A granulosis virus is pathogenic to the cabbage worm. tobacco caterpillars. lucerne caterpillars. Heliothis spp. etc.
Key Reference : Insects and Pests Management by Ali Asghar Hashmi