 In 20th-century eastern Canada, the major outbreaks occurred in periods circa 1910–20, c. 1940–50, and c. 1970–80.These outbreaks impacted, respectively, 10, 25, and 57 million hectares of forest.Longer-term tree-ring studies suggest that spruce budworm outbreaks have been recurring approximately every three decades since the 16th century.