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Virus Insecticides

Virus Insecticides
There are more than 300 viruses with are known to rapidly infect sus­ceptible species of insects. Unlike plant and other animal viruses, insect viruses are encased in protein crystals either singly or in groups. The protein crystals are insoluble in water and they are produced abundantly inside insect tissues and released when insects die. The crystals can re-in­fect live insects and retain their infectivity even after long storage outside the living tissues of insects.

Insect viruses are classified as polyhedroses and granuloses. Polyhedroses are those which contain many virus particles in the protein crystal while granuloses contain only one virus particle in each crystal. Yet another minor group is recognised as nuclear polyhedrosis viruses which develop on the nuclei of host cells. Such viruses are largely used in the biological control of insect pests on a commercial scale. In polyhedrosis type of viruses, the infective unit is embedded in protein polyhedron which protects the viruses from damage during its shelf life. Their host specificity is confined to a species or a group of species.

When artificially introduced in pest-ridden plant populations, the viruses multiply and are dispersed by air currents and rain water thus becoming potent insecticides. For instance, the European spruce sawfly, once recognised as a serious plant pest in Canada and U.S.A. in the early part of this century, was controlled by the chance introduction of a nuclear polyhedrosis virus.

Since then, numerous successful at empts have been made to use insect viruses in biological control of plant pests either by collecting naturally occurring virus-infected insects and applying their ex­tracts or by spraying preparations from laboratory grown virus infested insects.

Many virus insecticides have been developed on an industrial scale in U.S.A. by artificial rearing of infected insects. Among them, the nuclear polyhedrosis virus of cotton bollworm (Heliothis-zea) and that of the cab­bage worm (Trichoplusiani) hold promise as revealed by results of extensive field trials.

The commercial aspects of microbial insecticides have been included in the chapter on Biotechnology in Agriculture.

 
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