The European spruce sawfly, Diprion hercyniae(Htg.), was first discovered in Canada in 1930 and in the next few years attained outbreak proportions in spruce forests over large areas in the Gaspé Peninsula and New Brunswick. The accidental introduction of a polyhedrosis in the early 1930’s was responsible for the rapid decline of the infestation, and under the influence of both disease and introduced parasites, densities throughout the range of the sawfly have been fluctuating at very low levels for the past 20 years. Population studies have been conducted on this insect in New Brunswick since 1938 and data are now available for 50 consecutive generations over a 25-year period. Morris’ key-factor approach, using parasites, disease and weather as the key factors, has been used in the analyses of the data. Although disease obscured the effect of other control factors, during high sawfly densities, larval parasites have been largely responsible for maintaining the low densities experienced in New Brunswick for the past 20 years. A pronounced change in the relative importance of the different parasite species and in the amount of cocoon predation attributable to small mammals and wireworms has occurred during the change from high to low host densities.