Introduction

Today, about 1200 viruses are known to infect insects (Martignoni and Iwai, 1986). These do not form a taxonomic unit but belong to a range of different virus families including the Iridoviridae, Parvoviridae, Poxviridae, Reoviridae and Baculoviridae. Most of these families also have representatives that infect not only insects, but also vertebrates and even plants. There is one exception to this rule: viruses that belong to the family Baculoviridae have so far only been isolated from arthropod hosts. More than 60% of all insect viruses known today belong to this family. Some baculoviruses have been found in shrimps and mites, but most of them have been isolated from insects, in particular lepidopterans.

Baculoviruses are characterized by doublestranded circular DNA, which is included in rodshaped nucleocapsids (Federici, 1986). They are formed mainly in the nucleus of the host cells. In common with many insect viruses from other virus families, the virions of most baculoviruses are contained within proteinacious paracrystalline formations, the so called occlusion bodies, which are often polyhedral (hence the name polyhedrosis virus for one group of the baculoviruses). The thick layers of polyhedral protein provide protection against adverse physical and chemical factors within the environment, allow the viruses to survive outside the host cell, and enable them to kill their host rapidly, without jeopardizing their own existence (Jaques, 1977). It is obvious that protection of the virus particles by the occlusion bodies is a great advantage for the use of these viruses as biological insecticides. The natural infection process is by ingestion of food contaminated with virus.