ARS | Publication request: Nectar and pollen sugars constituting larval provisions of the alfalfa leaf-cutting bee (Megachile rotundata) (Hymenoptera: Apiformes: Megachilidae)

Submitted to: Apidologie
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 2, 2010
Publication Date: July 13, 2011
Citation: Cane, J.H., Gardner, D.R., Harrison, P. 2011. Nectar and pollen sugars constituting larval provisions of the alfalfa leaf-cutting bee (Megachile rotundata) (Hymenoptera: Apiformes: Megachilidae). Apidologie. 42:401-408.

Interpretive Summary: Bee larvae receive diets of pollen and nectar. For species such as the alfalfa leaf-cutting bee, the mother bee mixes pollen with sufficient nectar to mold an individual pollen mass, one for each of her progeny. Each provision mass contains all of the nutrients needed to produce one offspring. These provision masses appear to vary in consistency, but the specific proportions of nectar and pollen that mother leaf-cutting bees combine into such provision masses are unknown. We developed reliable methods to isolate, identify and quantify the sugars in larval bee provisions. Each 90 mg provision is 47% sugar, or 42 mg sugar. How much nectar foraging effort is required of a female alfalfa leaf-cutting bee to provision one nest cell? Alfalfa yields paltry nectar rewards, accumulating about 1/3 mg of nectar sugar per flower. It follows, then, that about 125 alfalfa flowers could supply a foraging female with the nectar she needs for one larval provision. We have elsewhere shown that far more floral visits are needed to acquire the matching amount of pollen. We conclude that pollen, not nectar, is likely the floral resource limiting reproduction by alfalfa leaf-cutting bees in alfalfa.

Technical Abstract: As with most solitary bees, larvae of the alfalfa leaf-cutting bee, Megachile rotundata Fab., eat a diet blended from pollen and nectar of unknown proportions. In this study, we developed protocols to isolate and quantify sugars from larval provision masses. The method removed free amino acids that leach from pollen and confound chromatography, but without autohydrolyzing sucrose. Pollen sugars were a negligible fraction of provision mass sugars. Glucose and fructose constituted about half of the provision fresh weight. Sucrose in alfalfa pollen and nectar is absent from the provision, presumably enzymatically hydrolyzed to glucose and fructose in the provision. Provision masses are composed of 2-3x more floral-equivalents in pollen than nectar. Female M. rotundata, and other solitary bees with pasty provisions, gather proportionally more pollen than nectar compared with the resource needs of colonies of social honeybees and bumblebees.