Solitary bee larvae prioritize carbohydrate over protein in parentally provided pollen

Most organisms must regulate their nutritional intake in an environment
full of complex food choices. While this process is well understood for
self-sufficient organisms, dependent offspring, such as bee larvae, in
practice have limited food choices because food is provided by parents.
Nutrient balancing may therefore be achieved by offspring, by parents on
offspring’s behalf, or by both, whether cooperatively or in conflict. We
used the Geometric Framework to investigate the capacity of dependent
larval mason bees (Osmia bicornis) to regulate their intake of protein and
carbohydrate. Female Osmia seal eggs individually inside cells they have
provisioned with pollen, and have no contact with developing offspring,
allowing offspring choices to be studied in isolation. Herbivorous insect
larvae are typically expected to balance protein and carbohydrate to
maximise growth and reproduction. Contrary to prediction, carbohydrate and
not protein mediated both growth and survival to pupation. Accordingly,
larvae prioritised maintaining a constant intake of carbohydrate and
self-selected a relatively carbohydrate-biased diet compared to other
hymenopterans, while tolerating wide excesses and deficiencies of protein,
rendering them potentially vulnerable to dietary change or manipulation.
Reasons for prioritising carbohydrate may include (1) the relative
abundance of protein in their normal pollen diet, (2) the relative paucity
of nectar in parental provisions making carbohydrate a scarce resource, or
(3) the requirement for diapause for all O. bicornis larvae. Larvae were
intolerant of moderate dietary dilution, likely reflecting an evolutionary
history of nutrient-dense food. Our results demonstrate that dependent
offspring can remain active participants in balancing their own nutrients
even when sedentary, and, moreover, even in mass provisioning systems
where parents and offspring have no physical contact. Research should now
focus on whether and how the evolutionary interests of parent and
dependent offspring coincide or conflict with respect to food composition,
and the implications for species’ resilience to changing environments.