Patterns of pollen feeding in adult male and female solitary bees

Adult solitary bees are well known to forage on nectar, visiting flowers to consume nectar only (males and females) or to both consume nectar and gather pollen for nest provisions (females). Evidence of pollen feeding by adult solitary bees is fragmentary, with some studies suggesting that nesting females of certain species make special “feeding” trips to consume pollen. Clear documentation of pollen feeding requires dissection of bees to reveal pollen contents in the different regions of the digestive tract. In the alkali bee, Nomia melanderi (Halictidae), dissection of close to 200 bees indicates that adult females feed daily and extensively on pollen (filling their crop and midgut), and do so across the duration of their flight season. Follow-up parallel studies of pollen feeding by females of three species in Megachilidae, including the alfalfa leafcutting bee (Megachile rotundata) and two oligolectic bees (Chelostoma florisomne and C. rapunculi), suggest that active pollen feeding is a widespread phenomenon in adult solitary bees.  Data reveal variations among bee species in the passage of pollen along the digestive tract: some species accumulate ingested pollen in the crop (foregut), releasing it gradually into the midgut, whereas others (particularly in Megachilidae) pass ingested pollen directly into the midgut. The adaptive significance of this variation may lie in the nesting biology, as well as climatic environment of the bees. Furthermore, pollen consumption by adult bees is not limited to females:  a survey of adult males in seven solitary bee species shows that they also feed regularly on pollen, although they consume less and more irregularly than do females.  The adaptive significance of pollen consumption by males is currently under investigation.

Key words: pollen, solitary bees, pollen feeding