Osmia Californica, f, ca, santa barbara co., side_2015-11-20-18.47
Osmia californica, the California Mason Bee, is common in the western U. S. (1). They specialize on composite flowers in the Asteraceae (Asters) family (1). Osmia typically emerge in early Spring, ceasing nesting activity by early Summer, but O. californica emerges in May and is a summertime bee. California Mason bees use a combination of mud and leaves (versus only mud, like other Osmia) to form nest cell partitions (2). They are solitary cavity-nesters, nesting in preexisting holes in wood or in man-made nest blocks (2).
Original public domain image from Flickr