Very little is presently known about the natural enemies and mortality factors associated with siricids (Hymenoptera: Siricidae) in the United States of America (USA), especially those that may directly affect the woodwasp, Sirex noctilio Fabricius (Hymenoptera: Siricidae). S. noctilio is an invasive woodwasp, is considered a major economic pest of pine, and has a severe effect on North American pine species planted in the Southern hemisphere. The mortality factors of siricid larvae were determined in three host species (Pinus sylvestris, Pinus resinosa, and Pinus strobus) from naturally infested trees in the northeastern USA. Siricid larvae were classified at the time of sampling as: (1) healthy, (2) parasitized by rhyssines (Hymenoptera: Ichneumonidae), (3) parasitized by Ibalia spp. (Hymenoptera: Ibaliidae), (4) parasitized by nematodes (Tylenchida: Neotylenchidae), and (5) dead from unknown causes. Combining data from the three host species, the average percentage of larvae that were healthy was 66%, 10% of the larvae were parasitized by rhyssines, 18% were parasitized by Ibalia spp., 1% were infected with unidentified nematodes, and about 5% of the larvae were dead in the galleries. Information from this study has important implications for understanding population regulation mechanisms in an invasive species, and will be critical for developing integrated pest management plans for S. noctilio.