Comparative histopathology of three ascovirus isolates in larval noctuids

The histopathology caused by three ascoviruses isolated respectively from Heliothis zea, Spodoptera frugiperda, and Trichoplusia ni was studied in the host species from which the viruses were isolated originally, or in the case of the H. zea isolate, in H. virescens. In all three isolates, infected cells and virion-containing vesicles in the hemolymph were observed by 3 days postinoculation. The isolates from H. zea and T. ni exhibited relatively broad tissue tropisms infecting the tracheal matrix, epidermis, and connective tissue, and in the latter isolate, the fat body. Although considered variants of the same virus, these isolates varied slightly in their tissue tropism in that the isolate from H. zea replicated more extensively in the epidermis. The isolate from T. ni replicated in the fat body during early stages of disease, but not in advanced stages, and was not as common in the epidermis as the isolate from H. zea. The isolate from S. frugiperda replicated only in the fat body and completely destroyed this tissue by 12 days postinoculation. The progression of disease within the tissues was correlated with a rise in the concentration of virion-containing vesicles in the hemolymph of all three isolates. Vesicle concentrations rose from 10(7) on day 2 to 10(8)/ml of hemolymph on day 9, plateaued for 5-7 days, and then declined gradually through day 20. These results indicate that the chronic nature of the diseases caused by ascoviruses is due to the limited degree to which they infect major tissues (isolates from T. ni and H. zea) or the infection of tissues less essential to the maintenance of larval life (isolate from S. frugiperda).