Nesting site density and distribution affect Osmia lignaria (Hymenoptera: Megachilidae) reproductive success and almond yield in a commercial orchard

The potential of commercially managed, native blue orchard bees, Osmia lignaria, to augment honey bees in orchard pollination depends on various factors, particularly how to enhance O. lignaria retention while optimising even pollination throughout orchards by varying their stocking density and nesting site distribution. In 2011, we investigated how artificial nest box density and the number of cavities within nest boxes influenced O. lignaria retention and reproduction in a 61ha almond orchard pollinated by a mixture of O. lignaria and Apis mellifera in the southern Central Valley of California. We assessed how localised O. lignaria nesting affected total nut yield. Retention of O. lignaria females was significantly greater in orchard areas with high density nest boxes compared with areas with low density nest boxes. Females preferred to nest in high density (low cavity) nest boxes compared to low density (high cavity) nest boxes. All measures of O. lignaria reproductive success were greater in orchard areas with high density nest boxes than areas with low density nest boxes. Localised O. lignaria foraging and nesting activity influenced nut yield by producing more nuts in orchard areas with high density nest boxes compared with low density nest boxes, although differences in nut yield just failed to meet statistical significance. Results of this study show that the density and distribution of nest boxes for nesting O. lignaria females can strongly influence the reproductive success of an alternative, managed bee pollinator in a large production orchard.