The seasonal pattern of parasitism by a parasitoid can be influenced by many factors, such as interspecific competition and host instar preference. We conducted field and laboratory experiments to describe the seasonal pattern of parasitism of spruce budworm Choristoneura fumiferana (Clemens) larvae by Tranosema rostrale (Brischke), and to investigate whether this pattern can be explained by interaction with other parasitoid species, or by host instar preference. Larval survival, developmental time, sex ratio, and adult size of T. rostrale developing in different host instars were also measured to further assess the potential importance of host instar on parasitoid life history. Parasitism by T. rostrale increased over the season, reaching the highest rate during the fourth-instar larva, and then decreased again until the sixth-instar. At the same time, parasitism by another parasitoid, Elachertus cacoeciae (Howard), increased over the season, and multiparasitism with T. rostrale suggests potential competition between these two parasitoids. Tranosema rostrale showed no host instar preference when third- to sixth-instar larvae were exposed simultaneously in a manipulative field experiment. The proportion of females emerging from spruce budworm larvae increased over the season; however, no difference in sex ratio was observed in the manipulative field experiment. Only male pupal development time and adult size were marginally increased in fifth-instar spruce budworm larvae. We conclude that T. rostrale's seasonal phenology or competition with E. cacoeciae, but not host instar preference, were possibly responsible for the observed seasonal pattern of parasitism.