The larch sawfly, Pristiphora erichsonii (Hartig) (Hymenoptera: Tenthridinidae) is widely distributed in the Northern Hemisphere. Its population outbreaks tend to continue for many years (sometimes > 5 yrs) in each area. Our final goal is to clarify the reasons why the larch sawfly outbreaks continue so long compared to other forest defoliators. Previous study hypothesized that mammal predation on cocoons prevented numerical response of parasitoids because cocoons parasitized by parasitoids were predated by mammals. We evaluated possible biotic factors influencing its population dynamics, which include parasitic fly, parasitic wasps, entomopathogenic fungi, and mammal predators and test the hypothesis. The percentage mortality during larval period was small compared to other forest defoliators. Few predators on the larvae were observed. Greater mortality was found during pupal period. The mortality by mammal predation showed spatial density dependence. No any consistent patterns on spatial density dependence were found for the parasitic fly, whereas each parasitic wasp showed Holling’s type III response. The percentage parasitism by parasitoids did not decrease during winter so that our results did not support the hypothesis.