Technical Abstract: Technical Abstract: The world¿s only intensively managed ground-nesting bee, the alkali bee (Nomia melanderi Cockerell), has been used for >50 years as an effective pollinator of alfalfa (Medicago sativa L.) grown for seed across the western USA. Throughout the 240 km2 Touchet Valley alfalfa farming region of southeastern Washington state (Fig. 1), all 76 past and present soil nesting beds of the alkali bee were surveyed (Fig. 5), and the 17-24 more populous aggregations annually censused for nesting bees from 1999 to 2006 (Fig. 6). The valley¿s 2-4000 ha of alfalfa bloom (1/4 of U.S. production) supported a 9-fold increase in alkali bee numbers during these 8 years (Fig. 4). By 2006, 17 million females were nesting densely (Fig. 2, 3) on just 19 ha valley-wide, the largest metapopulation of non-social bees ever recorded. Several of these sites have remained populous for 50 years, longer than any other known bee nesting aggregation. The most populous nesting bed (1.5 ha) grew to 5.3 million nesting females (Fig. 7), at densities of up to 1000 nests/m2; it is the largest bee nesting aggregation ever recorded. Grower stewardship practices for sustaining their alkali bees include: nest site subirrigation, surface salting and weed removal, provision of adequate nearby alfalfa bloom, some parasitoid control, and insecticide use practices safe for bees. Failure to subirrigate was the sole cause of demise for alkali bee nesting beds during this study. This first-ever exhaustive landscape-level census for any non-social bee reveals that even amid intensive conventional agriculture, a gregarious native bee can be multiplied to vast numbers and its populations sustained for at least half a century.