Known numbers of adult codling moths,Laspeyresia pomonella(L.), were released into an isolated 80-acre apple orchard in the Wenas Valley, Wash., (determined to be essentially free of native moths) to test the efficiency of traps baited with females when free-flying females were or were not present. Trap efficiency was reduced as much as 75% when a moderate-sized population of free-flying females was present to compete with the caged virgin females. However, when actual catches were compared with expected catches (based on the assumption that caged and free-flying females were equally attractive), the virgin females in the traps were seen to be more attractive than the free-flying females. Also, when populations grow large, these sex attractant trap catches do not reflect the increase and therefore cannot be used to estimate the size of the population.