Megachilid bee on cactus--Lithurgus? - Lithurgopsis apicalis
Española, Rio Arriba County, New Mexico, USA May 28, 2012 Size: About 10 mm long
The flower is cane or tree cholla, Cylindropuntia imbricata, though I imagine this is the same bee I see on my Opuntia prickly pears. Two or three of them can share a flower amicably.