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Reoviridae
Superficially resembling nuclear polyhedrosis viruses, cytoplasmic polyhedrosis viruses (CPV) in the genus Cypovirus are occluded (0.1-10 µm), nonenveloped dsRNA viruses. Easily mistaken for NPVs when seen outside the cell, CPV is can be diagnosed from intact host cells because the polyhedra are formed only in the cell cytoplasm, unlike the NPVs which replicate and form polyhedra in the host cell nuclei. Vertebrate and plant reoviruses do not produce polyhedra. CVP viral particles are icosahedral in shape and have 12 spikes or projections at each of the 12 vertices of the particles. CPVs have been isolated from Lepidoptera (80%), Diptera (16%), Hymenoptera (3%), and Coleoptera and Neuroptera (<1%) (Hukuara
and Bonami, 1991). In Lepidoptera, CPVs are primarily midgut pathogens, but some CPV species also infect fat body and other tissues.
Transmission of CPV is oral and, when ingested by a host, CPV polyhedra dissolve in the midgut juices and the virions are released. The virus particles attach to the midgut cells and the infective core is injected through the spikes (Kobayashi,
1971). New virus particles appear in the cell cytoplasm within three hours of infection. CPV tends to be more chronic and less lethal to host larvae than other viruses, nevertheless, destruction of the midgut tissues leads to nutritional deficiencies and physiological consequences (Billoncik
and Mori, 1998). CPVs seem to have little or no effect on dipteran hosts (Tanada
and Kaya, 1993). In Lepidoptera, symptoms occur at about 4 days post infection. Heavily infected larvae become sluggish and may develop diarrhea and vomiting. Larvae die in 7-15 days depending on the species of CPV and host, initial inoculum, etc. (Billoncik
and Mori, 1998).
CPVs can be an important problem in laboratory colonies of insects. Over time, the infected colony declines in productivity and vigor, and may be lost. Since large numbers of polyhedra are shed in the feces, the disease can be easily transmitted within a colony. In addition, there is some evidence for transovum transmission in a few species.
Biological Control:
CPVs are highly infectious, protected by polyhedra, and may synergize other pathogens,
but they are also relatively slow acting and chronic in nature (Billoncik
and Mori, 1998). They probably have more potential in systems where the economic
threshold is high (not unlike most other insect pathogens), or as inoculatively
or augmentatively released natural enemies.
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