Replication and passage of alfalfa looper nuclear polyhedrosis virus plaque variants in cloned cell cultures and larval stages of four host species

As alfalfa looper nuclear polyhedrosis virus (NPV) was serially passed through TN-368 cell cultures, the percentage of the cell population that developed infection with the MP variant decreased, while cells infected with FP variant increased. The MP variant was more virulent to Trichoplusia ni than the FP variant. Cells from the TN-368 cell line and strains derived from single cells were infected with isolated MP plaques. The viral progeny remained homogenous after one or occasionally two passages in cultured TN-368 cells. However, further serial passes in TN-368 cells or one pass in cell strains resulted in loss of the homogeneity, and the FP variant was detected. Four species of Lepidopterous larvae were also infected with the MP variant. Both variants were detected in viral progeny from diseased larvae.