It is all about the building materials

Leafcutting bees like to make their nests in cavities or in soft rotting wood provided through hollow plant stems or decaying logs. Once a suitable spot has been found, they will build cells using pieces of leaf as lining, by overlapping segments of leaf to make a cylindrical cavity that looks a little like a cigar. Each cell is sealed up with a little segment of leaf. The bees cut leaves from various trees, shrubs, and wildflowers. In parts of Europe, some species line their egg cells with petals instead of leaves. 

The availability of suitable nesting material may limit the geographical ranges and abundances of particular leafcutter bee species. The problem is that their preferences have rarely investigated. For good reasons as the identification of preferred plant species via morphological characters of the leaf fragments is very challenging and direct observation of bees cutting leaves is more than tedious.

Reason enough for a Canadian researcher to barcode the leaf fragments used for cell construction by three different species of the genus Megachile. Two of those species were introduced (Megachile rotundata and Megachile  centuncularis), the third was a native species (Megachile pugnata).  

Interestingly, there was considerable overlap between the preferred species for both introduced bees while the native species showed the least diverse suite of plant species it uses. In addition, the study shows that antimicrobial properties were present in all but six plants collected; all these were exotic plants and none were collected by the native bee, M. pugnata.