One of the main challenges of creating a bumblebee population model that operates in realistic digitised landscapes is how to represent multiple bee species feeding on multiple flower species both with different morphological characteristics.
What determines if a bee can feed on a flower or not? Bumblebees are attracted to some flowers and not others, high quantities of nectar as well as high sugar concentration per flower are important factors, as well as the quality of the pollen that they can collect too. We know that bumblebees have a preference for flowers in the pea family (Fabaceae), this includes White clover (Trifolium repens), Red clover (Trifolium pratense) and Common vetch (Vicia sativa) for example and these are often recommended as in pollinator friendly wild flower seed mixtures. Bumblebees also have a preference for flowers in the daisy family (Asteraceae) like Dandelion (Taraxacum offinale) and Common knapweed (Centurea nigra) and can collect a large amount of nectar and pollen by crawling around the large flower heads of these species without having to fly to and from individual flowers. But some bumblebee species have their particular favourites.
Co-evolution of flowers and bee pollinators means that some bumblebees are better adapted to feed on certain flower shapes than others. The length of the bees tongue (glossa) will determine how far down the nectarine the bumblebee can feed from. Some flowers have a fused corolla tube, this is at the base of the flower and is a tube like structure that will prevent any insects with a tongue shorted than it to feed on all of the nectar.
The widespread buff-tailed bumblebee, Bombus terrestris has a relatively short tongue at about 6.3 mm on average, in contrast to the Garden bumblebee, Bombus hortorum with 11 mm on average. This means that B. hortorum can feed on flowers with longer corolla tubes, and will possibly mean that B. terrestris and B. hortorum are not always in competition with each other for forage. So in the Bumble-BEEHAVE model we needed to factor in these different bumblebee glossa lengths and flower corolla tube lengths.
There is a great deal of literature out there on this, which makes our job a bit easier. We collected these values from the literature and these formed an input table for the model. We were missing some species data though, Foxgloves (Digitalis purpureum) are tricky, they are essentially entirely a fused corolla tube, but bumblebees can get into these as the flowers are large enough to accommodate most bumblebee body sizes, this means that data on how long this was (up to 45mm) was no use, instead we needed to know how long the part at the back of the flower was that may restrict the bumblebees with shorter tongues from feeding on it.
For this we explored the Cornwall campus of the University of Exeter during the summer which was awash with Foxgloves up to 2 metres high after a bumper year. We sampled 3 flowers from 10 different plants and with digital callipers we measured that back part of the corolla. After washing our hands (as Foxgloves are mildly poisonous!) we calculated that on average this was approximately 7 mm, which may restrict some of the short tongued species. It is also perhaps a combination of a small body size combined with a long tongue that makes B. hortorum (and other long tongued bumblebee species) so effective at feeding on Foxgloves.
We collected data for the 35 different wild flower species that bees feed on nectar from and 3 nectar providing crops that ranged in size from 0 mm corolla tube for open flowers such as Bramble (Rubus fructicosus) and Buttercups (Ranunculus acris), through to Foxgloves, Red dead nettle (Lamium purpureum) and Common vetch (V. sativa) at approximately 7 mm right through to 17 mm for the tubular wild flower Bugle (Ajuga reptans) and a huge 19 mm for Broad bean (V. faba) crops, which may only be assessable to the Queens of long tongued species B. hortorum and B. pascuroum.
With the nectar flower volume and sugar concentration data collected by Dr. Ellie Rotheray from the University of Sussex we had all the data we need to represent different bee species feeding on different flower species in our digitised landscapes.
Now we have this data in our model we can explore a range of questions, we can look into the energetics behind bee feeding preferences, we can look at what species provide the majority of nectar for bees, something that is tricky to do in empirical studies. We also aim to use Bumble-BEEHAVE to explore a range of pollinator friendly Countryside Stewardship options in a variety of landscapes in order to make bespoke management recommendations so watch this space for more news on Bumble-BEEHAVE.
Dr Grace Twiston-Davies is a Postdoctoral Research Associate at the University of Exeter, Environment and Sustainability Institute (ESI). Grace has been working with Prof. Juliet Osborne, Dr. Matthias Becher, and Tim Penny on the Bumble-BEEHAVE model as part of the BBSRC funded project “An integrated model for predicting bumblebee population success and pollination services in agro-ecosystems”. She has a PhD in grassland landscape restoration and ecology from the University of Reading.