Multiple daily brood cells define the fecundity of Osmia lignaria bees in a semi-natural setting

Authors: SPENDAL, RONALD - Collaborator; CANE, JAMES - Retired ARS Employee

Submitted to: Apidologie
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2022
Publication Date: 9/9/2022
Citation: Spendal, R.C., Cane, J.H. 2022. Multiple daily brood cells define the fecundity of Osmia lignaria bees in a semi-natural setting. Apidologie. 53. Article 54. https://doi.org/10.1007/s13592-022-00965-y.
DOI: https://doi.org/10.1007/s13592-022-00965-y

Interpretive Summary: Non-social bees reproduce slowly. Many studied species lay just one egg daily on that day's provision of pollen and nectar. For the blue orchard bee and other managed Osmia species, this rate is calculated as an average over the days that a nest is active, usually between 1-2 cells daily. In this study, we used long observation nests to observe and tally daily productivity for each of 185 nesting females foraging in a wildland reserve. We found that females daily completed 2-3 provisioned brood cells with eggs during fair weather, exceeding all expectations for their abilities to mature eggs and collect pollen. Furthermore, using double-depth nesting boards, we attained an unusually large fraction of female offspring. These results show the greater realized reproductive potential of these bees and reveal the apparent shortfalls to be remedied in trying to sustainably manage these bees in orchard settings. One improvement worth considering is switching double-length (1 ft.) grooved wood laminates for nesting.

Technical Abstract: Compared with most insects, female solitary bees invest considerable time and labor into each of their offspring, including huge eggs, protective nests and diets that are more uniform and digestible. This investment strongly constrains their reproductive tempo, which is generally taken to be one offspring per foraging day over several weeks. Such daily and lifetime reproductive rates are best documented among several species of cavity-nesting Osmia bees. In this study, we tracked daily production of new brood cells with eggs by each of nearly 200 female O. lignaria occupying observation nests set in a nature preserve. More than 80% of their 3,630 brood cells and progeny were produced on multi-cell days, when mother bees daily completed 2-3 and sometimes even four provisioned brood cells with eggs during 12 h of fair weather. Thus, in a span of < 4h, females matured an oocyte while building and provisioning their next brood cell. Females averaged 1.84 cells per day, a tempo approached by several other Osmia species. For them, 2- and 3-cell days should also be common albeit not yet directly observed. These findings warrant revised expectations for the faster rates at which female Osmia bees can mature oocytes, build and provision brood cells and multiply as populations, with implications for their sustainable management as crop pollinators and their stewardship in wildlands.