Dietary pollen needs of adult female solitary bees

Solitary bees daily provision nest cells with pollen and nectar for their progeny, but do mother bees eat pollen too? The alimentary canals of nesting female alkali bees (Nomia melanderi) of known age were dissected daily at different hours throughout their nesting season, beginning on the day of emergence.  Females ate pollen daily. Females were increasingly likely to eat pollen over the course of a day, such that by evening, every dissected female’s crop contained pollen.  A full pollen meal comprises the pollen taken from 34 or so tripped alfalfa flowers, obtainable in about 4 mins of foraging when virgin flowers are plentiful.  Is this pollen needed for egg maturation? A greenhouse study was used to compare egg development of adult Osmia montana bees confined to either pollen-less or pollen-bearing sunflowers.  After a week, basal eggs of those with pollen were enlarging to full laying size.  Eggs of those lacking pollen never grew despite access to nectar-bearing conspecific flowers. We conclude that adult females of these solitary bees eat pollen daily, which they need for vitellogenesis. If floral resources are exhausted before evening, nesting females may not be able to replenish the pollen proteins and fats likely needed to mature subsequent eggs.  When nesting females eat pollen, they also could be digestively evaluating pollen nutrient quality, enabling the selection of superior pollens with which to provision later nest cells.

 Nomia melanderi, Osmia californica