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1. 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.2. 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 61 ha 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.3. 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.4. 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.5. 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.

Intensively managed, commercial orchards offer resources for managed solitary bees within agricultural landscapes and provide a means to study bee dispersal patterns, spatial movement, nest establishment, and reproduction. In 2012, we studied the impact of 1) the color of nest boxes covaried with four nest box density treatments and 2) the number of bee release sites covaried with two nest box density treatments on the reproductive success of Osmia lignaria Say in a California almond orchard pollinated by a mixture of O. lignaria and Apis mellifera L. Nest box color influenced the number of nests, total cells, and cells with male and female brood. More nests and cells were produced in light blue nest boxes than in orange or yellow nest boxes. The covariate nest box density also had a significant effect on brood production. The number of release sites did not affect O. lignaria nesting and reproduction, but the number of cavities in nest boxes influenced reproduction. Overall, the color of nest boxes and their distribution, but not the number of release sites, can greatly affect O. lignaria nest establishment and reproductive success in a commercial almond orchard. The ability to locate nesting sites in a homogenous, large orchard landscape may also be facilitated by the higher frequency of nest boxes with low numbers of cavities, and by the ability to detect certain nest box colors that best contrast with the blooming trees.

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