In some respects, bumblebees are more visible in the autumn because there are many more males about. They are produced towards the end of the season. They are not allowed back into the nest and so spend the nights outside, protected from the cold by their longer coats. They collect no pollen or nectar except as their own food and so laze about on flower heads, waiting for young females to come past. (It sounds familiar doesn’t it?) Some of them fly along specific scent-marked paths to look for “love”, different species flying at different heights.
One exception is the Tree bumblebee, Bombus hypnorum (image above). These males hang around the entrance to a nest competing for the princesses as they emerge. This species arrived in the UK in 2001 and has successfully spread up to Scotland. It is unusual in that it nests above ground, taking advantage of holes in trees, bird boxes, and roof spaces. They have a ginger thorax, a black body, and a white tail. They can be seen from February to October. Their tongues are short and so they feed on shallow flowers such as those of orchard and ornamental fruit trees, Cotoneaster, and roses. Broods of up to 150 workers are produced. Part of the success of this species is that it appears to have no cuckoo bees that might commandeer the nests.
At this time of year, female bumblebees can be identified easily when the pollen sacs on their legs are loaded but not if they are only gathering nectar as this is carried internally. Even so, they never have time to rest. (Sounds familiar?)
The quality of pollen varies greatly from one plant species to the next and workers have to learn which to visit.
The best are the clovers and vetches where the pollen is as high in protein as prime steak, and the protein is of high quality. This matters, because pollen from Dandelion, for example, is so poor that colonies fed exclusively on this produced no offspring.
The importance of members of the pea family was not recognised in the days when clover leys were part of annual crop rotation on farms but since they were replaced by monocultures of cereals, bumblebee species, particularly the long-tongued ones, have declined significantly. Furthermore, chemical fertilisers made the nitrogen-fixing property of clovers redundant. However, there is a wonderful example of a pollinator strip containing vetch and clover next to Park Wood south of Bisham Wood. All members of the pea and bean family are very important in the projects designed to support pollinator species.
Currently, bumblebees are enjoying open-flowered Dahlia, Hollyhock, Cornflower, and Cosmos in our gardens, and Bugloss and Wild marjoram in the fields. When buying plants for pollinators, look to see if there are bees on them in the Garden Centres because some of the showier varieties have been bred to look good without producing pollen or nectar.
Adrian Doble (Bumblebee Conservation Trust) August 2020