Red mason bee Osmia bicornis (L.) is a solitary bee which has been shown to be a successful pollinator of many field crops and greenhouse crops. In favorable environmental conditions, this solitary bee can significantly raise the efficiency of crops. Controlled honeybee farms are invaded by various accompanying fauna. The aim of the study was to find out if the biotope may increase or limit the presence of foreign fauna in the nests of the solitary bee O. bicornis (L.). Four different biotopes were selected: a traditional orchard, the dendrological park of the Polish Academy of Sciences in Kórnik, a mixed forest, and a haymeadow, where artificial aggregations of O. bicornis (L.) were made, with 300 specimens in each. They were given artificial nests of common reed. The nests were then analysed after the end of the brooding period. The number of brood chambers built by females was determined. The number of chambers where the forms of development stopped at the egg or larva stage, the number of parasite-invaded chambers, and the systematic membership of the encountered foreign fauna, was also determined. The analysis of variance and Duncan's test with the significance level of α=0.05 proved that the biotope influenced the development of the O. bicornis (L.) population. The females in the forest biotope built the most brood chambers in each tube on average, and the result was significantly different from the other biotopes. The largest number of larvae died in the nests which had been placed in the dendrological park, and that value was also statistically different from the others. The highest parasite invasion was noted in the nests situated in the forest.
