These small ssRNA viruses in the family Nodaviridae are represented by two genera, alpha nodaviruses, which primarily infect insects, and beta nodaviruses, which infect fish. One species, Nodamura Virus or NOV, infects both insects and mammals, and mosquito transmission to pigs in Japan was documented. No disease symptoms were recorded in infected pigs, however. Other nodaviruses have been isolated from scarab beetles (Black Beetle Virus), from various grass grub species (Flock House Virus, Boolarra Virus, and Manawatu Virus), cultured Drosophila (New Zealand Virus), and dead Lymantria ninayi in New Guinea. All isolates have been from the South Pacific (Ball and Johnson, 1998).

Only NOV has been studied in vivo. This virus produces localized lesions in the cytoplasms of muscle, nerve, salivary gland, and molting gland cells. Wax moth larvae become paralyzed at 4-6 days post inoculation and die from 7-14 days post inoculation (Garzon, et al., 1978).

Until recently, several small RNA-containing viruses (called SRVs) were placed in Picornaviridae, a family of nonenveloped, nonoccluded, ssRNA viruses that include human viruses such as the common cold virus and polio virus. One previously isolated and described virus, the infectious flacherie virus of the silkworm, may indeed belong in this group; several others will most likely eventually be placed in as yet undescribed families (Christian and Scotti, 1998).

Besides the flacherie virus in silkworms, which has been fairly well established as a picornavirus, the best known of the possible picornaviruses from insects are the cricket paralysis virus (CrPV) and the Drosophila C virus (DCV). CrPV and DCV are the best studied because they can be grown in tissue culture. CrPV has one of the broadest natural host ranges known for insect viruses. It has been detected in 22 species in 5 insect orders (Christian and Scotti, 1998). DCV, on the other hand, seems to be limited to a few Drosophila species. CrPV can be transmitted per os and may also be transmitted on the surface of eggs (transovum transmission). The honey bee (Apis mellifera) viruses such as those that cause sacbrood disease, acute bee paralysis, and chronic bee paralysis are three of seven different picorna-like bee viruses. Picorna-like viruses have also been isolated from mosquitoes and hemipterans.

The Tetraviridae appear to be completely restricted to insects, and they have only been isolated from Lepidoptera. These ssRNA, nonenveloped viruses have been found to be the causative agent of viral epizootics in emperor pine moths, emperor gum moth, and Helicoverpa armigera. The family contains 10 known members and another 8 unassigned possible members. Like the Nodaviruses, this group has been extensively studied because of its small genome, which has been completely sequenced for some isolates (Gordon and Hanzlik, 1998).

Tetravirus disease ranges from inapparent infections to acutely lethal infections. In lethal infections, larvae become moribund, discolored and flaccid at about 7-9 days post infection, and hang from their prolegs much like NPV-infected larvae. Replication appears to occur only in the midgut cells

Because the tetraviruses studied have shown a huge range of effects on hosts depending on larval age and dosage, it is possible that they are successful in controlling populations due to host and, therefore, viral survival, which promotes spread and epizootics. They appear to be transmitted horizontally by oral ingestion, although there is some evidence for vertical transmission. The capsids of the virus appear to be environmentally resistant, sensitive to direct sunlight, but resistant to desiccation and proteases (Hanzlik et al, 1993).