The study was conducted within the original Lanphere-Christensen Dunes Preserve on the North Spit of Humboldt Bay near Arcata, California between June 1989 and May 1990. The study site included the least disturbed coastal dune system in the Northern Pacific Border Region and has since been incorporated into the Humboldt Bay National Wildlife Refuge Complex.

Megachile wheeleri Mitchell is a ground-nesting solitary bee that constructs a single, distinctive brood cell from mid-June to mid-September. A single generation is produced per year that overwinters as a prepupa in a tough cocoon. It was one of most abundant bees in the Preserve during the study.

Overwintering mortality was examined with multiple decrement life tables. Marked brood cells were baited for two treatments that allowed or eliminated surface access. Five replicates included two early successional sites, one in mid-successional site, one site adjacent to a young shore pine forest and one site in a large clearing within the same forest.

Overall mortality was 56%, caused mainly by mammal predation (39%). A cleptoparasitic bee, Coelioxys rufitarsus (Smith), caused the least mortality (4%), and herbivorous soil arthropods and “other causes” generated low mortality (8% each). Elimination of mammal predation reduced overall mortality to 34%. Total mortality varied among habitats. It was lowest (11-19%) in sparsely-vegetated pioneer succession mainly from “other causes,” and highest (100%) in some mid-successonal forested sites mainly from rodents. The variation in mortality among sites and the impact of combinations of mortality agents is discussed.