Reproduction and nesting
Nest of red-tailed bumblebee. Bombus lapidarius, showing wax pots full of honey
Further information: haplodiploidy and worker policing

Nest size depends on species of bumblebee. Most form colonies of between 50 and 400 individuals,[44] but colonies have been documented as small as ~20 individuals and as large as 1700.[45] These nests are small compared to honeybee hives, which hold about 50,000 bees. Many species nest underground, choosing old rodent burrows or sheltered places, and avoiding places that receive direct sunlight that could result in overheating. Other species make nests above ground, whether in thick grass or in holes in trees. A bumblebee nest is not organised into hexagonal combs like that of a honeybee; the cells are instead clustered together untidily. The workers remove dead bees or larvae from the nest and deposit them outside the nest entrance, helping to prevent disease. Nests in temperate regions last only for a single season and do not survive the winter.[44]

In the early spring, the queen comes out of diapause and finds a suitable place to create her colony. Then she builds wax cells in which to lay her eggs which were fertilised the previous year. The eggs that hatch develop into female workers, and in time, the queen populates the colony, with workers feeding the young and performing other duties similar to honeybee workers. In temperate zones, young queens (gynes) leave the nest in the autumn and mate, often more than once, with males (drones) that are forcibly driven out of the colony.[46] The drones and workers die as the weather turns colder; the young queens feed intensively to build up stores of fat for the winter. They survive in a resting state (diapause), generally below ground, until the weather warms up in the spring with the early bumblebee being the species that is among the first to emerge.[46][47][48] Many species of bumblebee follow this general trend within the year. Bombus pensylvanicus is a species that follows this type of colony cycle.[49] For this species the cycle begins in February, reproduction starts in July or August, and ends in the winter months. The queen remains in hibernation until spring of the following year in order to optimize conditions to search for a nest.[50]
Bumblebee life-cycle showing adults and larvae in nest of B. terrestris. Engraved in 1840 by William Home Lizars after drawing probably by James Hope Stewart.[51]

In fertilised queens, the ovaries only become active when the queen starts to lay. An egg passes along the oviduct to the vagina where there is a chamber called the spermatheca, in which the sperm from the mating is stored. Depending on need, she may allow her egg to be fertilised. Unfertilised eggs become haploid males; fertilised eggs grow into diploid females and queens.[52] The hormones that stimulate the development of the ovaries are suppressed in female worker bees, while the queen remains dominant.[46]

To develop, the larvae must be fed both nectar for carbohydrates and pollen for protein. Bumblebees feed nectar to the larvae by chewing a small hole in the brood cell into which they regurgitate nectar. Larvae are fed pollen in one of two ways, depending on the bumblebee species. Pocket-making bumblebees create pockets of pollen at the base of the brood-cell clump from which the larvae feed themselves. Pollen-storing bumblebees keep pollen in separate wax pots and feed it to the larvae.[53]
An above-ground nest, hidden in grass and moss, of the common carder bee, Bombus pascuorum. The wax canopy or involucrum has been removed to show winged workers and pupae in irregularly placed wax cells.

After the emergence of the first or second group of offspring, workers take over the task of foraging and the queen spends most of her time laying eggs and caring for larvae. The colony grows progressively larger and eventually begins to produce males and new queens.[46] Bumblebee workers can lay unfertilised haploid eggs (with only a single set of chromosomes) that develop into viable male bumblebees. Only fertilised queens can lay diploid eggs (one set of chromosomes from a drone, one from the queen) that mature into workers and new queens.[54]

In a young colony, the queen minimises reproductive competition from workers by suppressing their egg-laying through physical aggression and pheromones.[55] Worker policing leads to nearly all eggs laid by workers being eaten.[56] Thus, the queen is usually the mother of all of the first males laid. Workers eventually begin to lay male eggs later in the season when the queen's ability to suppress their reproduction diminishes.[57] Because of the reproductive competition between workers and the queen, bumblebees are considered "primitively eusocial".[11][56]

Although a large majority of bumblebees follow such monogynous colony cycles that only involve one queen, some select Bombus species (such as Bombus atratus) will spend part of their life cycle in a polygynous phase (have multiple queens in one nest during these periods of polygyny).[5