Technical Abstract: During orchard pollination studies in California, we observed dramatic changes in nesting and foraging behavior of the solitary bee with Osmia lignaria following fungicide sprays. We subsequently exposed O. lignaria females nesting in field cages planted with Phacelia tanacetifolia to several sprays and studied potential changes in their behavior. We tested several fungicides and tank mixtures: iprodione (Rovral®), propiconazole (Orbit™), benomyl (Benlate®), captan (Captan 50WP), and neem oil (Trilogy®), the surfactant Dyne-Amic®, alone and in tank mixture with Rovral®; and the tank mixture IDB (Rovral® + Dyne-Amic + the foliar fertilizer Bayfolan® Plus). An additional cage sprayed with an equal volume of water acted as control, and a cage sprayed with the insecticide dimethoate as a toxic standard. For each female, we recorded mean time spent inside the nest depositing its pollen-nectar load (in-nest time), outside the nest foraging (foraging time), the number of brood cells produced per day (cell production rate), and survival throughout the study. In some cases, significant day-effects and significant treatment x day interactions were obtained (in-nest time increased over time in the benomyl-treated and control cages more so than in the iprodione-, propiconazole-, and captan-treated cages; foraging time decreased over time in the control cage more so than in the neem oil-treated cage). However, treatment effects never were significant. Following the application of IDB, most females interrupted their nesting activities for a few hours. Females in the cage treated with dimethoate behaved erratically the morning after the spray, and were all dead in the afternoon, while in none of the other cages, the sprays did affect female survival.