Two greenhouses were operated as emergence traps in a stream in southwestern Lower Austria from 1984 to 1989 at a distanceof 28 m from each other. The stream bed under the traps was covered by a metal sheet alternately for one year, thus preventingemergence under the trap with covered bottom so that insects caught in this trap must have come from outside. Theresults have shown that insects from outside constitute a high but un quantifiable proportion of the total catch. In many instancesthere were more insects in the trap with a covered stream bed than in the open one. Therefore the emergence trap method is notadequate for quantitative surface-related studies, including secondary production.