Baculoviruses

The family Baculoviridae includes the nuclear polyhedrosis viruses (NPV) and granulosis viruses (GV). These are double-stranded DNA viruses (dsDNA) with rod-shaped nucleocapsids. The infectious virus particles or virions are occluded in protein bodies called polyhedra (NPV) or granules (GV). NPV polyhedra are larger than the virions (usually 1-15 µm) and may contain many virions. Infection with baculoviruses occurs when a susceptible host eats the polyhedra or granules, which are dissolved in the basic digestive gut juices. The virions are released when the protein matrices dissolve. The virions enter the nuclei of midgut cells and eventually infect many of the tissues and organs in the insect, primarily the fat body, epidermis, and blood cells. Nonoccluded baculoviruses (NOB), or nudiviruses, are not occluded in polyhedra and have recently been removed from the Baculoviridae. They are currently sedis incertae.

Infection with baculovirus was historically called "wilting disease" because the tissues of the host liquefy and infection of the epidermis causes the host to appear to melt, releasing virus particles into the environment.

Baculoviruses are considered to be the most beneficial of the insect viruses to man, because of their utility in insect control, their specificity to the arthropods, and their more recent use in fundamental biological studies using molecular techniques. Nevertheless, they also cause diseases in beneficial insects and, therefore, use in the environment as biological control agents requires an understanding of host range and the mechanisms that control host specificity (Miller, L.K., 1997). An excellent text for detailed information about baculoviruses is "The Baculoviruses" (1997) edited by Lois Miller.

Nuclear Polyhedrosis Viruses

Biological Control: NPVs must replicate in the nuclei of living host cells. They cannot be produced in culture media without living cells. This means that the production of NPV isolates as microbial insecticides requires a colony of insects or an acceptable insect tissue culture. This is more expensive than producing organisms such as the bacterium Bacillus thuringiensis in an artificial medium. Used as a microbial insecticide, NPVs can provide relatively fast kill and many are very host specific. All are specific to arthropods, most to a narrow range of Lepidoptera. Research on the production of NPVs as microbial insecticides has been primarily aimed at developing more efficient methods of production in insect tissue culture, as well as developing more virulent NPV strains. Most of the research on virulence involves inserting genes that produce toxic substances into the polyhedral gene. For example, genes for insect specific toxins, inserted into the polyhedral gene locus are expressed at the time that the polyhedral gene would have been expressed. The toxins kill the insect at an earlier stage than occurs in a normal infection (Black et al., 1997). One NPV that has been formulated for insect control is the MNPV of the gypsy moth, Lymantria dispar, or LdMNPV as Gypcheck.

Granulosis Viruses

Biological Control: Several GVs have been formulated as microbial insecticides, for example, GV from the codling moth, Cydia pomonella. Like NPV, these viruses are produced in vivo because of difficulties producing them in cell culture. In vivo production costs and narrow host spectrum limits their attractiveness to industry.