In two laboratory experiments, adult western conifer seed bugs, Leptoglossus occidentalis Heidemann, did not feed on seed infested by the Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl. When presented with seed lots containing 0, 20, 40, 60, 80, or 100% chalcid-infested seed, seed bugs fed on uninfested seed only when the percent chalcid infestation was ≤ 40% (Exp. 1), ≤ 20% (Exp. 2, females), or ≤ 60% (Exp. 2, males). In a third experiment, nymphs exposed to similar seed lots did not feed on M. spermotrophus when sound seeds were present. However, when exposed to 100% chalcid-infested seed, nymphs in two of 10 replicates fed on M. spermotrophus. Feeding by seed bugs caused seeds to lose 51% of their weight on average, but 18% of 43 test seeds on which seed bugs had fed germinated. In three Douglas-fir orchards surveyed, both species exhibited a clonal preference, but the ranking of clones preferred by each species was not the same. These results suggest that the impacts of L. occidentalis and M. spermotrophus are segregated and additive.