Queen bumblebees are born in summer; they seek a male to mate with, feed as much as possible to build up fat reserves for the winter and, in the autumn, dig themselves into loose earth to hibernate. As the weather gets colder and flowers become rarer, the colonies they were born into all die until over winter, the hibernating queens are the only bumblebee left alive. In early spring, the queens emerge from hibernation; eventually they will find suitable nesting spots, collect nectar and pollen, and lay a fresh batch of eggs. Their daughters will become a new generation of workers who will take over the foraging and form a new colony.

 

We don’t know very much at all about the lives of queen bumblebees during the critically important period after hibernation and before founding a new colony. In this study we tried to fill that gap in two ways: first, we kept young, mated queens in a fridge over winter, to simulate hibernation in the cold ground; in the spring we released them in a field and used harmonic radar to track their movements. To our surprise, they spent nearly all their time motionless, or nearly so, on the ground, flying only every half an hour or so on average, and then only spending an average of 15 seconds in flight. There was no consistent direction to their flights, which resembled a random walk pattern, but nevertheless, random variation among these short hop flights was enough to gradually disperse the bees from the starting point so that after five days they were all too far away for us to track.

 

Was it simply that the transponders we use to track the bees disturbed them and made them unwilling to fly, or could queen bees really be so lethargic? To find out, and to get a closer look at what they were doing on the ground, we made observations of wild bees in Epping Forest, UK, in the early spring. Having discovered a patch of woodland that was much frequented by bumblebees, we lay in wait for queens, following them as they flew by and observing their behaviour over long periods. What we found corroborated the radar tracks: our bees occasionally fed on catkins, but spent most of their time on the ground, taking only short and infrequent flights. (In fact, their flights were even shorter, but a little more frequent than the radar data had shown. The mostly likely explanation is that this was caused by our sampling methods: it was almost impossible to spot a queen bee on the woodland floor, so we had to wait for bees to fly past in order to follow them; this meant that we were more likely to spot them during relatively active periods and unlikely to find them during very long breaks; meanwhile any bee that flew too far in a single hop quickly got away from us, so we were more likely to observe queens taking short distance flights. Overall, I think the estimates of activity levels from the radar observations are likely to be more accurate).

 

During the long breaks between flights, the bees occasionally walked around a little on the ground but more commonly sat still, frequently hiding their heads under fallen leaves, or even crawling completely beneath the leaf litter. Only at the very end of our observation period did we observe bees starting to search for nesting sites: they do this in a very characteristic way, totally unlike the resting behaviour we had seen before, flying slowly and systematically close to the ground, investigating small holes.

 

When queen bees first emerge from hibernation, they have typically used up most of their fat stores over winter and need to build up their energy reserves before laying a lot of eggs. At this stage, their ovaries are not yet fully developed, so it also takes several weeks before they are able to start producing eggs. Our study has apparently uncovered a previously unknown stage in bumblebees’ life history, one adapted to these needs: for a period, perhaps up to several weeks, immediately after hibernation, it seems that queen bees feed when they can, resting or sleeping for almost all the rest of their time, allowing them to conserve energy and avoid being spotted by predators. Only once they are in the right physiological condition to found a new colony, do they begin search for nesting sites. Genetic studies suggest that bumblebee queens typically establish their colonies between three and five kilometres from where they were born; we made a simple mathematical model which shows that the random walk patterns we observed during this period are enough to account for this dispersal. We cannot be certain that there is no deliberate dispersal behaviour, but it seems unnecessary in order to account for the placement of bumblebee colonies.

 

This work has some important messages for the conservation of bumblebees. Since their dispersal across the landscape seems to be the result of many short flights, corridors between habitat patches are crucial. Fragments of suitable habitat, separated by longer distances, may simply not be colonised by bumblebees if they are unable to reach them by a series of short hops. Leaf litter and long grasses provided crucial shelter to resting or sleeping bees, so it is important that they are allowed to lie undisturbed throughout the early spring.

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*These authors contributed equally to the study.