Sex pheromone traps for P. alaskensis (Rohwer), and its ichneumonid parasitoid, Syndipnus rubiginosus Walley, were placed in white spruce, Picea glauca (Moench) Voss, plantations in north central Minnesota (USA). The objective was to study the relationship between catches of the sawfly and its parasitoid and subsequent sawfly-caused defoliation. A significant linear relation was obtained when catches of both the sawfly and parasitoid were included in the model. The best-fit model was log(I) = 1.36 - 0.17 log(S) - 0.25 log(P) + 0.13 log(S cntdot P) - 0.3 log(Ht), where I is a trend index calculated using I = N-(f)/N-(t-1) N is amount of defoliation during year t, S is sawfly catch, P is parasitoid catch, S cntdot P is the interaction between the two, and Ht is tree height. Pheromone trap surveys of a host and parasitoid may provide information of the extent on natural control of the pest. none