Missing Male Bumblebees – a mystery?

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This is my third winter observing winter active Buff-tailed bumblebees (Bombus terrestris) in Falmouth. One of their favourite flowers for collecting pollen and nectar is Mahonia. With Fox Rosehill Gardens having several Mahonia shrubs it is my favourite place to see the bees.

By January the number of workers in the colony have increased to the point where it is not unusual to see two or three workers on the same bush. Such are their numbers and the frequency in which visits are made – as one leaves it isn’t long before another appears – that I had never considered that their efforts would not result in the production of males and new queens.

This year though it was pointed out to me by Stuart Roberts of the Bee, Wasp and Ant Recording Society (BWARS) that there are very few records of males produced by winter active colonies.  Differentiating whether a queen seen in late winter/early spring is from a winter active colony or one that has emerged from hibernation is nigh on impossible.

The lack of records for males cast doubts on whether these winter active colonies complete their lifecycle. The main possibilities for the demise of a colony would be the destruction of the nest, a sustained period of very cold weather or a lack of flowering plants to provide food. The death of the queen would usually result in the workers laying unfertilised eggs that produce only male bees.

Buff-tailed bumblebee on Mahonia flowers in January 2017

Although I cannot rule out the destruction of a nest – humans and possibly badgers would be likely culprits – the other factors affecting the health of a colony have not occurred in Falmouth. In fact food resources will now continue to increase as we pass through the rest of winter into spring. Despite all the favourable conditions the fact remains that over the previous two winters I have yet to recognise a male - does this imply that the colonies were unsuccessful?

My suspicion is that, absence of proof is not proof of absence and that the lacks of records for males are possibly due to other factors. From personal experience one reason would be a decrease in the time I spend looking for bees in the winter compared to the other seasons. The days are shorter and the weather is often less inviting. I also tend to favour the locations that I know are favoured by foraging workers. Given that males once they leave their nest do not return and will disperse over quite a distance I could easily be looking in the wrong place.

The odds of seeing a male are far less than those of seeing a worker. Firstly the numbers of males produced is far less than that of workers. Secondly while new workers are produced during the several months of a colony’s activity, males are only produced towards the end of its lifecycle. Thirdly as males do not participate in collecting pollen or nectar for the colony they spend far less time on flowers than workers. Lower temperatures during a longer night may also shorten their life expectancy compared to summer males.

Another factor that may be significant is that the markings of the males are similar to workers and the main difference in the field is that males are larger. I do wonder if any of the bees I saw in previous winters/early spring that I thought were large workers may have been males?

Fox Rosehill Gardens is the best place to see bumblebees in

Falmouth during December and January.

I will also admit to suffering Bumblebee/Mahonia fatigue after several months of photographing bumblebees on Mahonia. Last year I began ignoring them in February having moved onto early emerging hoverflies. Even in March I was more interested in newly emerged queens of other bumblebee species than adding yet another buff-tailed record.

This year I am gritting my teeth and trying to be more dedicated, an added incentive is having recorded Bombus hortorum workers in December I have yet to see them this year.

If my list of reasons why the chances of seeing a male (assuming they are produced) are not enough to make the quest seem somewhat quixotic there is another obstacle to add. There is no certainly when they will appear. It really is a case of wait and see.

Given the difficulties in obtaining records of males there is another way to discover how complete the lifecycle of winter active bumblebees are. An examination of a nest once the colony has died should show how many males and queens were produced if any. This line of enquiry would seem to offer the best opportunity to add to our understanding of winter active bumblebees – I just need to find a nest.