Beginning of the Bumble Bee Lifecycle
Queens are the only members of the nest to survive from one season to the next. The queen's life span lasts one year. Queens will spend the winter months hibernating in a protected underground overwintering chamber called a hibernaculum. The queens are able to survive extreme cold by producing glycerol, a chemical anti-freeze. The constructions of these cavities vary slightly from species to species, but all queens favor certain soil conditions and locations. Soil must be well-drained with a proper humidity. In dry soil, less than 50% humidity, a queen's body will dry out. However, the greater the soil humidity the higher the risk for infection by microorganisms. A queen will usually construct a hibernaculum on a northern facing slope, because there is little sun exposure. The queue for queen emergence from a hibernaculum is the ground warming to a certain temperature. When the ground warms, the queen will "know" spring has arrive and will emerge to begin the next stage of her life cycle. If the ground receives too much light exposure and warms prematurely, queens will emerge too early and not have flowers to feed upon. The depth of the chamber, soil conditions, and species influence the exact emergence date of individual queens. By May, most queens have emerged from hibernation and are searching for nesting sites.
After emergence from the hibernaculum, queens will spend a period of time feeding to gain strength and complete the development of their ovaries. They feed on honey conserved in their honey-stomach from the previous fall, but must soon begin to forage for nectar and pollen. Pollen provides protein and stimulates ovary development, preparing the queen to begin egg laying. At night queens in the process of searching for a nest find shelter beneath plant leaves. Once a queen is strong enough and her ovaries have developed she will begin searching for a suitable nesting site. Just as a suitable hibernaculum allows a queen to survive winter, the development of a successful colony is dependent on the selection of an appropriate nest site. Queens spend 2 to 3 weeks searching for a suitable nesting site. Bumble bees do not create their own nest cavities and they do not forage for nesting materials. A suitable nest site will be pre-existing, and have fine insulating nest material. Bumble bees often choose abandoned rodent dens as nesting sites but will not shy away from man-made structures. Once a queen has located a nesting site, she must often fight away another queen that has already taken up residence. Research suggests that one of limiting factors of bumble bee success is the lack of suitable nest sites. The shortage of nest sites results in competition between queens. In the spring, it is not uncommon to find several dead queens laying around the bottom of a nest chamber. Bumble bees are often divided into two groups, surface-nesters and ground-nesters based on where they form their nests. Observing the flight pattern of a searching queen can reveal which group she belongs in. Surface-nesters often nest in tall grasslands and search for a nest by dropping to the ground and flying up again; landing at frequent intervals to evaluate nest sites. Ground-nesters often nest in old rodent cavities underground and have a faster, more looping search pattern. Frequently, they hover over holes in the ground before landing and investigating more thoroughly.