Foraging and Nesting Behavior of Osmia lignaria (Hymenoptera: Megachilidae) in the Presence of Fungicides: Cage Studies

During orchard pollination studies in California, we observed dramatic changes in nesting and foraging behavior of Osmia lignaria Say (Hymenoptera: Megachilidae) after sprays with tank mixtures containing fungicides. A characteristic pattern of postspray events observed includes erratic behavior and interrupted foraging and nesting activity for several days. In an effort to determine whether fungicidal sprays were disruptive to bee foraging and thus to pollination, we exposed O. lignaria females nesting in field cages planted with lacy scorpionweed, Phacelia tanacetifolia Benth (Hydrophyllaceae), to selected spray mixtures normally encountered in California orchard production systems: iprodione (Rovral), propiconazole (Orbit), benomyl (Benlate), and captan (Captan 50 WP); the surfactant Dyne-Amic, alone and mixed 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 O. lignaria, we recorded time spent inside the nest depositing pollen-nectar loads, foraging time, cell production rate, and survival. All females in the dimethoate treatment died postspray + 1 d. Before death, some of these females behaved similarly to our previous orchard observations. A high proportion of females in the IDB cage were inactive for a few hours before resuming normal foraging and nesting activity. No lethal or behavioral effects were found for any of the other compounds or mixtures tested. Our results indicate that the fungicide applications that we tested are compatible with the use of O. lignaria as an orchard pollinator.