Two greenhouses were operated as emergence traps in a stream in southwestern Lower Austria from 1984 to 1989 at a distance of 28 m from each other. The stream bed under the traps was covered by a metal sheet alternately for one year, thus preventing emergence under the trap with covered bottom so that insects caught in this trap must have come from outside. The results have shown that insects from outside constitute a high but un quantifiable proportion of the total catch. In many instances there were more insects in the trap with a covered stream bed than in the open one. Therefore the emergence trap method is not adequate for quantitative surface-related studies, including secondary production.