The Nesting and Development of Native Solitary Bees
Most people are somewhat familiar with the social lifestyle of honeybees and bumblebees, which includes a caste system of an egg-laying queen, male drone bees, and female worker bees (the ones we mostly see at flowers). With the exception of Bumblebees, native wild bees are different in that they lack a caste system and are largely solitary in their lifestyle.
When solitary bees emerge from their nests as adults they quickly groom themselves and set out to find food in the form of nectar. Male bees emerge earlier than female bees and often position themselves near floral resources, waiting to encounter receptive females. Male bees die shortly after mating and female bees go in search of a specific place to build a nest.
There are two main types of nests, ground nests and cavity nests. Ground nesting bees, such as sweat bees (Halictids) and alkali bees (Andrenids) will dig out a subterranean nest in loose, sandy substrate. Cavity nesters such as leaf-cutter bees and orchard bees (Osmia) will look for pre-existing cavities such as hollow stems or holes in wood, that are just the right size to use as a nest.
Once the nest is either excavated or a suitable cavity is found, the female bee constructs a series of cells, each of which will house a food source, usually a mix of pollen and nectar packed into a ball, and a single egg. Depending on the species, she will line the walls of the nest with leave, petals, mud or a secretion from her own body to create the cell. The female bee will make numerous foraging trips to flowers collecting pollen and nectar that she will then deposit and pack into a ball in each cell. Once this food ball is complete for a given cell she will lay a single egg on top of it. She then seals off this individual cell and proceeds to build another. In this way the female bee constructs a series of individual cells that each contain one egg and a food source. It can take anywhere from 20 to 30 trips to line the cell and provision food. It is on these trips to collect pollen and nectar for her nest that the female bee acts as a pollinator for wild plants and food crops. When the entire nest is complete she seals off the end and moves on to creating a new nest.
The eggs are left to hatch and develop without any parental care. When they hatch, the tiny larvae eat the food that the female provisioned. Larvae grow and molt and eventually reach the pupal stage where they transform into an adult bee. When it is time for the adults to emerge, they chew their way out of the of the cell, and exit the nest.
The number of generations that a species of solitary bee will have during a year can vary from just one to two or three, depending on the species. When a bee has a single generation in a year, the larvae or pupae will overwinter in the nest and will emerge the following year as an adult. In species that have multiple generations per year, the first generation will emerge into adults who will mate, forage and build a nest; the final generation of the season will overwinter as either larvae or pupae.
More detailed information on the lifecycle of ground nesting bees in the genus Andrena including photographs of nests and developing larvae can be found at http://www.vernalpools.org/Thorp/