Abstract: A casing in the form of cylindrical hollow body is terminated at one end by a base and is optionally provided with a hanger. The other end of the cylindrical hollow body is covered by a cover. The cover has a gauze arranged in a frame. The gauze meshes have a clearly defined cross-sectional surface. Useful organisms are placed in a protected manner in the hollow body and only those useful organisms having a desired size can pass through the gauze. The casing is treated by chemical or physical means so that it will remain intact and the organisms contained therein will be protected for a time period predetermined by such treatment. As the casing is made from non-toxic materials, its decomposition at the end of the intactness period does not adversely effect the environment.