The spruce budworm, Choristonerna fumiferrna (Clemens), is susceptible to a nuclear polyhedrosis virus and to a granulosis virus which may occur as single infections (Bergold 1950, 1951) or as double infections (Bird, 1959). Laboratory studies have shown that relatively heavy concentrations of either virus must be injected or fed to budworm larvae to cause infection and death. In one quantitative study Bergold (1951) estimated that the intralymphal LD50 of the polyhedrosis virus for the budworm is about 5000 times that for the silkworm, Bombyx moriL, when each is administered to its natural host.