Bumblebees - A Winter's Tale

Nature loves variation; evolution can be thought of as playing, let’s try something different and see if it works. Not only has this produced an astonishing variety of species it also results in variations within specie. Nature plays dice and making general statements is often a bit of a gamble. Quite often there are exceptions that complicate matters.

The bumblebee is a good example of this for while most have a life cycle of one generation in a year there are exceptions. Generally new queens emerge in late summer, they will mate and then forage for food to build up their reserves before digging a burrow that they hibernate in.

Upon emerging the queens feed on nectar while their ovaries mature and then begin to search for a suitable nesting site. Once she has established a nest she will lay the first batch of eggs to produce female workers. Bees can determine the sex of their eggs as fertilised eggs produce females and non-fertilised eggs males. The queens sit on their eggs and it is thought that temperature can influence the size of the workers. Warmer eggs producing larger larvae. The amount of food they consume is also likely to determine their adult size.

The queens forage for nectar and pollen to take back to the nest for their larvae to feed on. The larvae eventually spin a cocoon from which they will emerge as adult worker bees. Once enough workers have been produced to bring an adequate amount of pollen and nectar back to nest the Queen no longer leaves it.

 

Eventually the point is reached where the queen starts to lay male eggs and females to become queens not workers. Up to this point the queen has been releasing pheromones to suppress the female workers from becoming fertile and laying their own eggs. As she now wants to produce eggs that will develop into fertile queens she ceases to produce the phoromones. Without them the social order of the nest breaks down and some of the workers will attempt to lay their own unfertilised eggs to produce males. In many cases the unrest within the nest will lead to the workers stinging the queen to death.

The adult males leave the nest in search of queens to mate with. With their own nest in a state of turmoil they are homeless and fend for themselves. If on a rainy late summer’s day you see a wet bedraggled bee it is likely to be a male. In their search for newly emerged queens from other colonies they can travel up to 10km from their original nest.

This brings us back to where we started and the general life cycle of a bumblebee colony is completed - now for some exceptions.

Let us start with the Early Bumblebee (Bombus pratorum) as in this country there is geographical divide in the behaviour of queens. In the North pratorum follows the life outlined above. In the South though some of the new queens produced in a year rather than hibernate will establish nests to produce a second generation in the same year. It would be easy to assume that the difference is due to the climate. That spring arrives earlier in the South and frosts later. It sounds sensible but some bumblebees in Norway challenge that idea.

The Hearth Bumblebee (Bombus jonellus) is found both in this country and Norway. In this country like Bombus pratorum in the south it can produce two generations in a year and the same is true in Norway. Not only that but the subspecies, Bombus jonellus sub borealis occasionally produces two generations in Troms county, Norway that is north of the Artic Circle. Length of season does not of itself seem to be determining whether a queen produces a second generation or hibernates.

In the U.S.A. it seems three species in Oregon can produce a second generation in a single year. These are the yellow faced bumblebee (Bombus vosnesenskii), the fuzzy-horned bumblebee (Bombus mixtus) and the black-tailed bumble (Bombus melanopygus).

Studies in Norway and the U.S.A. suggest mild and dry summer that result in an abundance of suitable flowers species in late summer is a key factor in motivating some bumblebee species to produce a second generation.

Winter active bumblebees are another exception to the general behaviour of bees in this country. These are queens who establish a colony with workers that are foraging through the winter months. For several years winter active Buff tailed Bumblebees (Bombus terrestris) colonies have been recorded mostly in southern urban locations but with outliers as far north as Hull. In locations where this occurs it seems to become a regular occurrence. This is the third year where I have recorded winter active terrestris in Falmouth. This year for the fist time I also found Garden Bumblebee (Bombus hortorum) workers on the same Mahonia flowers as terrestris workers. This mirrors evidence from New Zealand where we exported both species in the 19th Century. There terrestris is the most common winter active species with hortorum less frequently seen. Another exported species Bombus ruderatus has also been recorded as being winter active there, it will be interesting to see if this eventually happens in this country.

It is not known what prompts a new queen to hibernate. Most enter hibernation before the arrival of cold weather, so temperature appears not to be a trigger. It may be that once a queen has built up her reserve being in a burrow is the safest place for her. Combining the time spent in their hibernation burrow with that spent in their nest suggests that queens have a preference not to be out in the open. Most of a queen’s life is spent sheltered and protected.

Generally the instinct to hibernate after mating and building up their reserves is strong in queens of all species. As the exceptions show the strength of the instinct is variable in at least a few species. Going against the norm is a gamble. For example a queen that begins a colony in the late autumn will be reliant on the availability of flowering plants throughout the winter. In the right urban location the odds of survival are favourable elsewhere they are bleaker. 

Another potential problem faced by a bumblebee colony that breaks the widespread timings of their species' lifecycle is finding mates for their males and queens when most of their species are unavailable. A small isolated population can be susceptible to inbreeding.

Variability within a species offers the possibility of adapting to change and taking advantage of new opportunities. If all of life is a gamble then spreading your bets is the safest option.