Abstract
Studies on non-treated plots at Harrow, Ontario, in 1967 and 1968 and at Kentville, Nova Scotia, from 1963 to 1968 inclusive showed that residues of viruses of the cabbage looper, Trichoplusia ni (Hübner), found in soil were related qualitatively and quantitatively to the virus residues on leaves of cruciferous plants grown in the plots and to mortality by viruses in the host populations. At Harrow where the cabbage looper is common and where virus diseases occur naturally in field populations, the viruses that occurred in the populations of the host built up in soil as the season progressed. At Kentville where the host is rare, virus was rarely found in soil or on foliage. The typical nuclear-polyhedrosis virus, an atypical nuclear-polyhedrosis virus that formed abnormally large polyhedra, and a granulosis virus were found at Harrow, with the typical nuclear-polyhedrosis virus predominating. Only the typical nuclear-polyhedrosis virus was detected at Kentville.