Bumblebee life cycle: males and queens are produced

The 4 stages in the life cycle of a bumblebee colony

• Queen emerges from hibernation and starts new colony by herself.
• Workers (females) are produced and start to forage; the colony develops and grows.
• Unfertilised eggs (males) are laid and worker larva develop into new queens.
• Males and new queens mate, the colony disintegrates, the old queen, workers and males die, and new queens hibernate.

Male (unfertilised) eggs are laid and worker larva develop into queens

The bumblebee queen can lay two types of egg;

• fertilised eggs with chromosomes from the queen and a male she mated with the previous year. These eggs become either workers or queens.
• unfertilised eggs which contain chromosomes from the queen alone. These eggs become males.

The sperm from the mating is stored in a small container called a spermatheca located in the queen's vagina. As the queen lays the egg she decides whether or not to fertilise it with sperm. The fertilised eggs develop into workers (females), and the unfertilised eggs develop into males.

The queen lays unfertilised eggs only towards the end of the colony life. After she has laid male eggs she may lay fertilised eggs that will become queens, but she will not lay any more eggs that will develop into workers, so the laying of male eggs signals the start of the end of a colony.

Bumblebee workers can lay unfertilised eggs

At about the same time as the queen starts laying unfertilised eggs that will produce males, the ovaries of some workers, usually those performing household duties, may develop. It is possible for workers (without mating) to lay unfertilised eggs that will develop into males, however workers cannot produce queens or other workers. Some workers try to lay eggs of their own, and may even attempt to eat eggs laid by the queen. This leads to aggression between worker and worker, and worker and queen. In many cases the more persistent workers will succeed in laying some eggs that will reach maturity.

The photograph on the right shows a Bombus hortorum male who has spent the night inside a flower, and the one above right shows a male resting under a flower head.

Males do not return to the nest once they have left it, so spend their nights either inside or hanging under flower heads. In the morning they are often very lethargic and may appear to be ill, but this is normal. They just need to get up heat by drinking nectar or being warmed by the sun or both.

Have a look at his rear leg - it has no pollen basket. Only males and cuckoo bumblebees have no pollen basket.

The queen maintains dominance

The queen maintains dominance over her daughters by headbutting and pulling the legs of the most aggressive workers, and it is believed that a pheromone which suppresses activity in the glands of the workers which would stimulate their ovaries to grow is also involved in maintaining the dominance of the queen.

Caste (queen or worker) determination in Bombus terrestris

The queen secretes a pheromone, which, if present within the first 5 days of larval life determines that the larva will develop into a worker. If the pheromone in absent, and if the larva receives sufficient food (more than a larva destined to be a worker) during her final instar she will develop into a queen. Consequently a queen larva is larger than a worker larva. It is believed that caste determination works in a similar way for other bumblebee species too.

New queens are just normal worker (female) larvae that develop into queens.

On the left is a Bombus lucorum male. In many species the male has a similar colour pattern to the workers, but has a greater abundance of yellow hairs.