Nesting and associated behaviors of Andrena (Plastandrena) prunorum Cockerell were studied at two sites in western Washington: a suburban lawn and a vacant lot. A maximum of five nests, usually well separated from one another, was found at a given site and year. Nest and cell structure at both sites were similar, but cell depth differed markedly between sites, and between years at one site. All nests were multi-cellular. Provision masses were flattened spheres of pollen moistened to a doughy

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... sistency. The curved egg was placed atop the provision mass with both ends contacting it. A brief description of larval feeding is provided. Andrena prunorum and its Nomada parasite overwinter as adults. Nests, cell, provision mass, egg placement, and a feeding larva are illustrated. Information on mating, female foraging behavior, and local pollen sources are given. Adult phenology and the possibility of two generations per year are discussed. Nomada sp. nr. calloxantha Ckll. parasitized A. prunorum at one site and larval feeding by the cleptoparasite is described.