The function of the vestibulum in nests of a solitary stem-nesting bee, Osmia rufa (L.)

Abstract

Nests of the stem- or hole-nesting megachilid bee, Osmia rufa, were analysed to help clar- ify the function of the outermost empty chamber of the nest, the vestibulum. Only nests in an exposed nesting environment had a long vestibulum, whereas nests protected from sun and temperature fluc- tuations (within a honey bee hive body) had short vestibuli or none at all. The rate of parasitism of the first cell from nests with a vestibulum did not differ from nests without a vestibulum in both nesting environments. The short vestibulum built by O. rufa in protected nests probably represents the remaining space of a nest hole that is too short for an additional cell. Nests in exposed environments suffered much higher mortality in all brood cells owing to parasites attacking open cells during nest construction. Additionally, mortality caused by the cleptoparasitic drosophilid, Cacoxenus indaga- tor, one of the main parasites of the Red Mason bee, increased greatly from the rear to the entrance of nests. When nesting bees detect the cleptoparasitic fly, they do not provision the outermost space of nesting holes, thus constructing a vestibulum, to avoid misinvestment due to the high risk of par- asitism. In nests exposed to normal weather factors, vestibular cells probably shelter the brood also from high fluctuating temperatures. © Inra/DIB/AGIB/Elsevier, Paris