More than a decade after its discovery in North America, the European woodwasp, Sirex noctilio Fabricius (Hymenoptera: Siricidae), has not been an aggressive tree killer in northeastern North American pine (Pinus Linnaeus; Pinaceae) forests. Concern that S. noctilio has potential to become a more aggressive pest as it spreads south, or as environmental conditions change, is warranted, because it has caused extensive pine losses on other continents that it has invaded. We observed S. noctilio impact and attack behaviour in eight pine stands throughout Ontario, Canada annually for a five-year period (2012–2016). Sirex noctilio impact was variable in unmanaged pine forests; it killed 3–48% of pine stems, and 3–36% of pine basal area. Most S. noctilio-caused mortality was limited to suppressed and intermediate trees. Of the 17% of pines in the entire study that were affected by S. noctilio, many (44%) were attacked in multiple years. Depending on the year, between 46% and 79% of trees remained alive in the year immediately after attack, which suggests that many study trees were at least initially resistant to attack by S. noctilio. Though its impact appears to be limited in most forests for now, we recommend that observations continue in future years.