With varroa mites and Africanized honey bees now in the United States, solitary bees are being considered as an alternative and or supplement to European honey bees for crop pollination. Many of these solitary species occupy cavities that can be simulated with holes in artificial substrates placed in domiciles among crop plants. Bees from such structures, however, can establish feral populations. One such highly invasive bee, Megachile apicalis Spinola, recorded in southern California in the early 1980s, has spread throughout the state and into Washington state. The spread of this bee apparently is facilitated by the presence of a European weed, yellow star-thistle (Centaurea solstitialis L.), and the bee’s ability to nest in marginal habitats. M. apicalis has reached density levels that far exceed native bees in sampling units at some California sites, especially late in the season, and it appears to be displacing native bees from some nest sites (particularly within riparian habitats). Behaviorally, M. apicalis exhibits a high level of intraspecific competition for nest cavities, making it potentially unreliable as a commercial pollinator. In areas where feral population levels become high, eventually it may interfere with the ability of its commercial counterpart, Megachile rotundata (F.), to establish in domiciles for alfalfa pollination. For these reasons, M. apicalis exemplifies the potential problems of introducing nonnative pollinators to both natural and agricultural environments.