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

Abstract

Unlike most nonsocial insects, female solitary bees lavish time and resources on each offspring, including huge eggs, protective nests, and cached diets. This added investment is thought to constrain their reproductive output to one offspring per foraging day over several weeks of adult life. These reproductive rates are best documented for several species of cavity-nesting Megachile and Osmia bees. In this study, we tracked the daily accumulation of new brood cells with eggs by each of about 200 O. lignaria females occupying observation nests set in a nature preserve. In 2021, multicell days accounted for > 80% of 3630 brood cells and progeny. Nesting females completed 2–3, even four brood cells with eggs during 12 h of fair weather. Thus, females could mature their next oocyte in < 4 h. Females averaged 1.84 cells/day, a rate approached by several other Osmia species. For them, 2- and 3-cell days should also be observable. These findings warrant revised expectations for the faster rates at which Osmia females can mature sequential oocytes, as well as build and provision brood cells, and ultimately increase as populations. These findings have implications for sustainably managing Osmia species as crop pollinators and their stewardship in wildlands.