Civil wars in bumblebee families

Text Meelis Uustal
Photo Tiit Jürisson
Translation: Liis
Male early bumblebee (Bombus pratorum)
 
The arrival of male bumblebees to flowers means that in the meantime tempestuous events have taken place in the bumblebee family that will end with the death of this year’s family. What then happened all of a sudden?
 
As a background it must first be explained that for bumblebees and many other social membrane-winged insects (Hymenoptera) the life cycle is arranged so that female bumblebees and workers (they too are female) develop from fertilized eggs, male bumblebees however from unfertilized eggs. Male bumblebees thus have a haploid set of chromosomes, that is, they have a single set of chromosomes whereas the female bumblebees and workers have chromosomes in double sets (as we humans). Genetic relationship ties are thus the basis of for whom the worker bumblebees are prepared to care for willingly and for whom not.
 
Female bumblebees have a 50% relationship with their sons and daughters but 75% relationship with their sisters. And herein the key to the social Hymenoptera insects’ success is hidden: in order to spread ones genes most successfully it is much more sensible to care for one’s sisters (be they then workers or future queens) than even for one’s own offspring.
 
Worker bumblebees have only 25% genes in common with the sons of the queen which makes caring for them and raising them a third-rank activity for the workers. The worker bumblebees are in fact capable of laying unfertilised eggs. In such a situation part of the worker bumblebees find that it is better to raise one’s own sons with whom they have 50% common genes than their own half-brothers.
 
So, when the reigning queen of the bumblebee family decides sometime in June-July-August (depending on species and the development rate of the family) that the family has enough workers to help raise new young bumblebee queens and male bumblebees, troubled times arrive in the bumblebee family.
 
When the queen notices egg-laying workers, or that her sons are destroyed she cannot allow that. The queen attacks these workers, bites and even kills them with her sting. The eggs laid by the workers and the larvae she eats. At the same time the workers try to destroy the progeny of the queen. And so a hopeless civil war has started. By mid-summer there may be more than a hundred workers in the nest, many of whom may also start laying eggs. So the queen must take up a fight against all of them and quite often she, fighting singly against all, perishes.
 
During the civil war, new workers will no longer be hatched. The old worker bumblebees who have laboured throughout the summer die gradually from old age or the queen’s sting. So the bumblebee family also comes to an end. But in the nest young female and male bumblebees are hatched who fly out in the wide world to search for a mate..
 
It is interesting to know that almost 95% of the hatched male bumblebees are in fact sons of the queen and only 2,5% the progeny of the workers. But the remaining 2,5%? Incredible as it may seem some worker bumblebees in secret slip to neighbouring nests to lay eggs and by this safeguard the transmission of their genes. A true soap opera!
 
 
Join in observing bumblebees and help to gain new knowledge about the spread of bumblebees in Estonia!
 
Guide to observing bumblebees and observation questionnaire: LINK
 
Web questionnaire for bumblebees: LINK
 
Join the „Meie kimalased – Our bumblebees“ group on Facebook: LINK


 

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