What Happens to Bumble Bees in Autumn?

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A male Bombus ternarius (orange-banded bumble bee) braces himself against a cool autumn morning in Stockton Springs (photo: Kalyn Bickerman-Martens)

A question that tends to come up when discussing bumble bees (and honey bees) is: “Aren’t they all female?” The answer is that YES, most of the bees we see during the summer, such as the queens and the workers that do the foraging, are female. However, bumble bees still need males to contribute genetic material to the next generation – colonies that will be founded by new queens the following year. Now that it’s officially fall, I’ve been seeing quite a few males and even some new queens on my collecting trips, indicating that the colonies have entered “the beginning of the end” of their colony cycles.

A bit of genetic background: a chromosome is a package inside of an organism that contains that individual’s DNA and is passed from parent to offspring. Humans have 23 pairs of chromosomes and one copy of each chromosome is inherited from the female parent and the other is from the male parent. These chromosomes contain all the genetic hereditary information. Because there are 23 pairs, this means humans have 46 total chromosomes within each cell. Because our chromosomes come in pairs, humans are diploid.

Bumble bees (and other members of the order Hymenoptera) have a “haplodiploid” sex-determination system. This means that females are diploid (have two sets of chromosomes) whereas males are haploid and only have one set of chromosomes. How does this happen? Workers (and future queens) develop from fertilized eggs which contain genetic material from both the queen and the male drone with whom the queen mated the previous fall. Males, on the other hand, develop from unfertilized eggs and all of their genetic material is 100% from the queen. Technically, workers are also capable of laying eggs that would develop into males, but the queen uses a pheromone (an excreted chemical factor) to inhibit worker ovary development since it is in the queen’s genetic interest that all of the offspring in the colony are her own.

In the beginning of the colony cycle, the queen lays only fertilized eggs that turn into worker females. It is only toward the end of the season when she begins to lay both unfertilized eggs that turn into males and fertilized eggs that will develop into the new queens. The factors that cause the fertilized eggs to turn into new queens instead of more workers are not fully understood, and could differ between species. However, research suggests that queen larvae in some species may be fed more frequently as compared to worker larvae. Another possible reason is that worker larvae may be exposed to the queen pheromone very early in their development, which acts to suppress their ovary development.

It has been suggested that the density of workers in the colony is what triggers this switch to laying reproductive males and queens, since there needs to be enough workers to feed the larval queens, which require more food and take longer to develop than workers. The new queens do leave the nest during the day to forage and return at night, but usually do not contribute to provisioning the colony. At this point their main priority is building up enough fat reserves for the long winter ahead. Males, on the other hand, leave the colony and focus on finding a new queen from another colony with whom to mate, often by dropping pheromones in an area then patrolling until queens arrive.

Once males and the new queens depart the nest, the colony has reached its end for the season and the remaining workers and founding queen die. This entire cycle will be repeated once the new queens emerge from their winter dormancy the following spring.

 

References:

Chromosomes. 2015. Available from www.genome.gov/26524120 (accessed September 1, 2015).

Goulson, D. 2010. Bumblebees: behaviour, ecology, and conservation, 2nd edition. Oxford University Press, New York, NY.

Haplodiploidy and relatedness in bumblebees. 2015. Available from www. bumblebee.org/Haplodiploidy.htm (accessed September 1, 2015).

 

 

 

 

 

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