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A study of the pathogenicity of a cytoplasmic polyhedrosis virus of the gypsy moth, Porthetria dispar, was conducted. Third- and fourth-instar larvae were highly susceptible to the pathogen even when fed low dosages of polyhedra. Only 30–50% of the treated insects died of polyhedrosis, but the debilitating effects of the virus on larval and postlarval stages were remarkable. Pupal weights were consistently reduced. An inverse and, respectively, a direct, relationship was found between the size of diseased specimens and the developmental periods of larvae and pupae surviving to lethal infection. The highest infected male adults showed a lighter wing coloration. Structural abnormalities and melanotic abdominal inclusions, recorded in diseased adults, were more frequent in females than in males. The presence of melanotic inclusions in diseased females obtained from early larval infection experiments was associated with a greater pupal weight. Lower dosages were the most effective in producing larval mortality and reducing pupal weights and adult emergence. An interference phenomenon due to a mixed virus infection is suspected.

The pathogenicity of an American isolate of the nucleopolyhedrosis virus of Porthetria dispar was studied. Laboratory data on third-instar larvae showed that mortality was directly related to virus concentration. The computed LD50 was 1,729 PIBs/larva or 72 PIBs/mg larval body weight. The LT50's for 2.5 × 106, 2.5 × 105, 2.5 × 104, 5 × 103, and 2.5 × 103 PIBs/larva were 8.1, 9.9, 11.3, 12.2, and 13.1 days, respectively. Approximately 37 and 60% of the total larval mortality occurred during the third- and fourth-instar, respectively. The periods to pupation and the pupal weights of survivors apparently were not affected by virus concentration. Moth emergence from surviving pupae was not reduced.