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AbstractA dendrochronological study was performed at six sites dominated by eastern larch, Larix laricinain Québec’s southwestern boreal forest. The objectives were to reconstruct periods of larch sawfly (Pristiphora erichsonii) outbreak in the region and to determine which physical factors (precipitation, temperature, water level or drought) explained the greatest variation in radial growth. From the presence of light latewood rings followed by periods of growth suppression, we identified larch sawfly outbreaks for the years 1895-1912, 1937-1942, and 1955-1962. We suspect that additional outbreaks occurred in the early 1920s, late 1970s and early 1980s as well, but at the same time as spruce budworm outbreaks (Choristoneura fumiferana). Response function analysis demonstrated negative relationships between larch radial growth and May and August precipitation and May and September current year water level, and demonstrated positive relationships with May current year drought index and September previous year drought index. These results suggest that flooding in the early growing season and excessive water levels at the end of the growing season may negatively affect larch radial growth. Our results also indicate an increase in the year-to-year variation in radial growth in larch sites subjected to flooding. This may reflect the increase in the Lake Duparquet water level at spring break up.