Fertilisation of the egg (or lack of) determines the sex of the resulting bee. If the egg is fertilised (with sperm from a male), then the adult will be female, whereas, if the egg is not fertilised (no sperm), then the adult will be male. In scientific terms, a female is diploid (has two copies of each chromosome, one from each parent), while males are haploid (has only one copy of each chromosome, half the number of chromosomes a female has). In humans, males and females are both diploid.
Female bumblebee = egg + sperm = 2x Chromosome sets (Diploid)
Male bumblebee = egg = 1x Chromosome set (Haploid)
Determination by the Queen
A bumblebee queen mates (usually singly) before entering hibernation. She stores the male’s sperm in a specialised organ called a spermatheca, which maintains favourable conditions for the sperm, ensuring they continue to be viable throughout her life. When her nest is in the development phase, all of her eggs are fertilised to ensure that only female workers are produced. Later in the nest’s life she stops producing an inhibitory pheromone, and starts producing some unfertilised eggs, which develop into males. She continues to produce fertilised eggs as well, which with extra feeding develop into gynes (unmated queens), becoming the next generation of queens.
Competition heats up between dominant workers and the queen when the queen starts to produce reproductives, (new gynes and males). Fights erupt. Some workers start laying unfertilised male eggs (as the workers have not mated), in defiance of the queen. The queen quickly destroys these eggs, which are less related to her than her own eggs. Eventually the competition leads to the death of the queen, and no new fertilised eggs are laid. Worker populations are no longer restocked, and the hive ultimately dies off.