The continuation of bumblebee populations hinges on the reproductive success of a select few individuals each year. Most bumblebee colonies follow an annual cycle, culminating in a reproductive phase that produces new queens and males. The sole purpose of these individuals is to mate and carry the colony’s genetic legacy forward. This process ensures that new colonies can be founded the following spring, starting the cycle anew.
Key Players: New Queens and Drones
Late in the summer, a bumblebee colony shifts its focus from expansion to reproduction. The queen begins to lay eggs that will become new queens, known as gynes, and males, called drones. Gynes develop from fertilized eggs, the same as workers, but receive significantly more food as larvae. This allows them to grow much larger and develop functioning reproductive organs, differentiating them from their sterile worker sisters.
Drones develop from unfertilized eggs in a process known as haplodiploidy, which means they possess only a single set of chromosomes from their mother. Once they reach maturity, the drones’ primary function is to leave their home nest to find gynes from other colonies. They do not forage for the colony or perform other duties, instead spending their days feeding on nectar to fuel their search. The newly emerged gynes also depart their natal nest to begin the mating process.
Locating a Mate
Drones engage in a behavior known as ‘patrolling,’ flying along specific routes, often just a few feet off the ground. Along these paths, they mark prominent objects like leaves or fence posts with a pheromone produced by glands in their head. This pheromone is unique to the species and acts as a chemical signal to attract queens. Different drones of the same species often use the same patrol routes, creating a scented corridor that increases the chances of encountering a receptive gyne.
A new queen undertakes dispersal flights to move away from her relatives, which helps prevent inbreeding. She is drawn to the pheromones left by the males of her species. Mating flights are timed to specific periods of the day, depending on the species and weather, to maximize the probability of drones and queens being active simultaneously. The queen will land near a drone’s patrol route, where her presence may be detected by a passing male.
The Mating Process
Once a drone locates a receptive queen, copulation occurs on the ground or on vegetation. The drone grasps the queen’s thorax and curves his abdomen underneath hers to connect their reproductive organs. The process is lengthy for insects, lasting from 20 to 40 minutes, and the pair remains firmly attached.
Bumblebee queens typically mate only once. During copulation, the male transfers sperm to the queen, who stores it in an internal pouch called the spermatheca. This single mating provides all the sperm she will need for the following year. The drone may then leave a ‘mating plug’ in the queen’s reproductive tract, a temporary barrier to prevent other males from mating with her.
Post-Mating Developments
The fates of the male and female diverge significantly after mating. For the drone, his life’s purpose is complete, and his lifespan is short. Drones do not return to a nest and will perish as autumn progresses and temperatures drop. The old colony, including the founding queen and all her workers, also dies off at the end of the season.
The newly mated queen must prepare for the coming winter. She spends the remaining weeks of autumn building up her fat reserves by feeding on nectar and pollen. Her next task is to find a suitable, sheltered location to hibernate, known as a hibernaculum. This is often a small chamber she excavates in well-drained soil, under leaf litter, or in a log. Here she enters a state of dormancy, or diapause, surviving the cold months alone until spring signals it is time to emerge and found a new colony.