Sex pheromone traps for the yellowheaded spruce sawfly,Pikonema alaskensis(Rohwer), and its ichneumonid parasitoid,Syndipnus rubiginosusWalley, were placed in white spruce,Picea glauca(Moench) Voss, plantations in north central Minnesota. 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·P) − 0.3 log(Ht), whereIis a trend index calculated usingI=N(t)/N(t-1),Nis amount of defoliation during yeart, Sis sawfly catch,Pis parasitoid catch,S·Pis the interaction between the two, andHtis tree height. Pheromone trap surveys of a host and parasitoid may provide information of the extent on natural control of the pest.