The research enabled the optimal parameters of reed stalks Phragmites australis (Cav.) Trin. ex Steud, to be determined and then be used for rearing red mason bees (Osmia rufa L.). During research conducted in 2015 and 2016, 3,293 reed stalks were occupied by red mason bees, and 19,461 nest cells were counted in the stalks, of which there were 16,222 non-parasitized larvae. The mean weight of one cocoon was 0.085 g. The mean length of the reed stalk occupied by the red mason bee was 169.45 mm, its mean diameter was 6.11 mm and the mean number of cells in the reed was 5.47. The mean length of the first cell, the vestibulum, was 38.38 mm. The mean degree of parasitism was 0.16. The length of the reed stalk positively correlated with the number of cells in the reed nest (r=0.543), vestibulum length (r=0.392) and cocoon mass (r=0.362) The diameter of the reed positively correlated with the mass of the cocoon (r=0.536), the number of cells in the reed (r=0.406) and vestibulum length (r=0.254). Correlation between degree of parasitatism and length, and diameter of the reed was insignificant. Reed stalks with a length of 150 to 200 mm and a diameter of 5–7 mm are recommended for the choice of red mason bee nesting material.
