Viruses As Insect Pests

Viruses As Insect Pests

	Viruses As Insect Pests Insect viruses are encased in protein coats which are insoluble in water. These protein crystals serve to protect the virus particles and due to the presence of these protein crystals, the infective viruses remain active even under normal storage condition for many years. Specificity of the virus-host relationship makes viruses an ideal candidate for use in the control of specific pest population. There are two major groups of insect viruses:
(i) Polyhedrosis virus where many virus particles are embedded in each protein crystal. When they occur in the host cell nucleus, they are called nuclear polyhedrosis virus (NPV), and when they occur in the host cytoplasm they are called cytoplasmic polyhedrosis virus (CPV). (ii) Granulosis virus where only one virus is contained in each protein crystal. They either develop in the nucleus or cytoplasm of the host cells (NGV or CGV). NPV and CGV come under the group of Baculoviruses and they are the most extensively studied insect viruses. Infection is caused by ingestion of contaminated food containing NPV / GV followed by cell invasion beginning in .the midgut. The GV are sprayed over the leaves and they enter the larvae feeding on the foliage. The viruses multiply fast in the larvae and make them lethargic. Affected larvae become sluggish and stop feeding and die. For example, NPV controls the pest Helicoverpa armigera.
The affected larvae hang upside down from the leaves and twigs in a characteristic way and a brownish fluid oozes from them. This is a highly infective fluid and is readily disseminated amongst the healthy insect population. The spread takes place by wind, rain, etc. TAIVIRIDAE is a commercial NPV pesticide used to control cotton pest bollworm and budworm.
Production The specificity and obligate parasitism of insect viruses makes their mass production possible only on live insects, e.g. production of bollworm-budworm NPV (Baculovirus helothis). Bollworm larvae are raised on a semi-synthetic diet containing a water-based mixture of casein, sucrose, wheat germ, growth factors, etc. Agar is used to solidify the diet. Chemicals like formalin, sorbic acid, aureomycin are used to inhibit bacteria, yeasts and fungi. The diet is dispersed hot from a large tank to plastic trays and sealed. These are inoculated by caterpillars from an insectary and incubated under controlled conditions to produce a mass of larvae.
	A known volume of virus is sprayed on the diet which replicates on the caterpillars to the extent of 5000-10000 times in 5-7 days. The infected caterpillars are suctioned and then treated with water, filtered, centrifuged, precipitated and spray dried. After quality control testing, the preparation is packaged and sold.

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