Diet
In North America and throughout much of the world, bees are phytophagous, deriving all of their nutrients from plant sources. As flowering plants diversified and became more abundant, the availability of flower-based nutrients increased. Bees arose from their wasp ancestors and switched to this plant-based diet composed of a combination of pollen, nectar, and/or floral oil.
Pollen
Pollen, the primary component of a bee larval diet, contains protein, lipids, carbohydrates, amino acids, vitamins, minerals, carotenoids, and flavonoids. This floral resource is essential for larval development and an important part of an adult female's diet, providing the essential nutrients required to help her develop eggs. Pollen protein content and pollen/lipid ratios in flowering plants vary. Pollen protein content can range from 10 to 60% and lipids from 2 to 20%. Studies suggest that Bombus (bumble bee) visitation to flowers is influenced by the pollen/lipid ratio of the floral resources with bumble bees seeking out optimal ratios (5:1) to maximize the nutritional quality of larval provisions.
Nectar
The carbohydrates and amino acids in nectar fuel an adult bee's movement from flower to flower, supplement larval pollen provisions, and for adult males, provide sustenance for their mate-searching activities. Nectar contains a complement of different types of sugars (sucrose, glucose, and fructose) with variable sugar concentrations influenced by the growing conditions of the plant, overall plant health, air temperatures, and a plant's nectar production cycle. Nectar can become concentrated on hot days due to water evaporating from the fluid. Extremely concentrated nectar is viscous, and it can be difficult for bees and other insect pollinators to imbibe the too-concentrated nectar through their mouthparts.
Floral Oil
Most bees combine nectar with the pollen provisions they provide for their larvae. In Minnesota, one genus of bees, Macropis, instead mixes floral oil with the pollen provisions. Macropis spp. are pollen and oil specialists of plants in the genus Lysimachia. Lysimachia flowers have oil-secreting glands on their sepals and stamens that Macropis females collect, often while they simultaneously collect the flower's pollen! These floral oils are also used as a waterproofing material in the ground nest to keep moisture from infiltrating brood cells.
Diet Breadth
The diet of an individual bee species varies; some species collecting a wide range of flower pollen while other species only collect pollen from a small number of plants. Approximately one-third of bee species in Minnesota are pollen specialists or oligolectic bees. Specialists collect pollen from a handful of plant genera within a specific plant family or, more narrowly, a single plant genus or species. The majority of bees have a polylectic (broad and varied) diet, dependent upon the forage available within their flight range, their preferences for the floral resources, and their physical ability to access resources from a particular flower.
Eusocial bees such as Bombus (bumble bees) have one of the most varied or diverse diets because the colony of bees remains active for the entire growing season and relies upon a number of plants to provide pollen and nectar. When solitary bees are active as adults, their diet is limited by the availability of the plants in flower. For example, vernal (spring-emerging) solitary bees such as Andrena have fewer forage choices compared with summer-emerging species that have many forage opportunities with more plants in flower.
Pollen-Collecting Structures
Corbiculae (Pollen Baskets)
Bombus
Bombus
Bombus and Apis females (other than parasitic Bombus) collect pollen on their hind leg tibiae. The tibiae are broad, concave, and surrounded by long bristly hairs. Using the forelegs and mid-legs, females groom pollen from their hairs to the hind leg, moisten the pollen with nectar, then pack the mixture into the corbiculae for transport back to the nest.
Abdominal Scopae
Megachile
Heriades
Female bees (other than cuckoo bees) in the family Megachilidae transport pollen on hairs on their sternites (underside of the abdominal segments).
Hind Leg Scopae
Most female bees collect pollen on scopae on their hind legs. The scopae can occur on a particular segment of the hind leg such as the tibia or on multiple segments of the hind leg, for example, a combination of the femur, tibia, and basitarsus. In addition to the scopal hairs on the hind legs, a few genera also pack pollen on the side of the thorax and/or the edge of the abdomen. See the examples below.
Calliopsis
Agapostemon
Andrena
Perdita
Melissodes
Colletes
An Osmia lignaria (orchard mason bee) female provisions a brood cell with pollen.
Nest Provisioning and Egg Laying
Once a female has established and prepared a nest for provisions, she makes multiple foraging trips to flowers, collecting pollen and nectar, then deposits these resources into a brood cell. These provisions (bee bread) range from being dry and cake-like to a liquid slurry of pollen and nectar. For example, Colletes (cellophane bees) provide liquid provisions; females lay their egg attached to the roof of the brood cell. The female bee usually provides more food for female offspring than male offspring as females typically have a longer development period than males.
After mating, a female stores sperm in her spermatheca, an organ in her abdomen. As she lays one egg on, near, or suspended above the provisions, eggs that include the addition of sperm from this reservoir develop into females and eggs left unfertilized develop into males. The egg hatches in one to three days and the larva begins feeding on the provisions in the brood cell. Most solitary female bees provision anywhere from fifteen to twenty-five brood cells in their short adult lives. In contrast, queen bumble bees can lay hundreds of eggs in their annual colony.
An Osmia egg laid on top of a pollen ball.
Larval Development
Larvae undergo five instar stages, molting their skins between each stage until the final instar stage. After consuming all the provisions in four to twenty days, the last larval instar defecates, then usually spins a silken cocoon by producing silken strands from spinerets in their mouthparts to prepare for pupation (prepupa).
Most solitary bees overwinter in this prepupal stage and finish their development (pupation) the following spring or summer. This overwintering prepupal stage is one of suspended development (diapause). Pupation takes approximately two weeks, then the adult bee emerges from its nest. Adult males have an approximate active life span of two weeks, and adult females four to six weeks.
An early instar larva consuming pollen provisions.
A late instar larva preparing to spin a cocoon.
A prepupal silken cocoon containing the larva and later pupa.
An Osmia adult male about to emerge from the nest.
Explore Bee Families
Citations and Further Reading
Cane, J. H., Eickwort, G. C., Wesley, F. R., & Spielholz, J. (1983). Foraging, grooming and mate-seeking behaviors of Macropis nuda (Hymenoptera, Melittidae) and use of Lysimachia ciliata (Primulaceae) oils in larval provisions and cell linings. American Midland Naturalist, 257-264.
Cane, J. H., Griswold, T., & Parker, F. D. (2007). Substrates and materials used for nesting by North American Osmia bees (Hymenoptera: Apiformes: Megachilidae). Annals of the Entomological Society of America, 100(3), 350-358.
Lau, P., Lesne, P., Grebenok, R. J., Rangel, J., & Behmer, S. T. (2022). Assessing pollen nutrient content: a unifying approach for the study of bee nutritional ecology. Philosophical Transactions of the Royal Society B, 377(1853), 20210510.​
Portman, Z. M., Gardner, J., Lane, I. G., Gerjets, N., Petersen, J. D., Ascher, J. S., ... & Cariveau, D. P. (2023). A checklist of the bees (Hymenoptera: Apoidea) of Minnesota. Zootaxa, 5304(1), 1-95.
Vaudo, A. D., Patch, H. M., Mortensen, D. A., Tooker, J. F., & Grozinger, C. M. (2016). Macronutrient ratios in pollen shape bumble bee (Bombus impatiens) foraging strategies and floral preferences. Proceedings of the National Academy of Sciences, 113(28), E4035-E4042.