Lew Hutchison, a Rhododengron breeder of Mosgiel, asks :-
Why do I see only bumble bees pollinating Rhododendron flowers?
Ruud Kleinpaste, whose programme Buggin' with Ruud screened on Sky's Animal Planet, and Barry Donovan, an independent entomologist at the Canterbury Agriculture and Science Centre, Lincoln, responded.
There are over 850 species in the genus Rhododendron, and in the wild they occur from the tropics to high elevations on mountains. Collectively they display a huge range of flower size, shape, colour and nectar and pollen production, so saying anything definitive about their pollinators and pollination is very difficult. To be quite honest: We are not sure as to the answer to the question (we don't know if anybody knows!), but here are a few possible reasons that favour bumble bees over honey bees.
Both honey bees and bumble bees are terrific pollinators; they love to extract nectar and get contaminated with pollen along the way; they also gather pollen to take back to the hive, especially bumble bees!. Bumble bees come in different species with different tongue length: species with long tongues naturally pollinate plants with long flowers, where the nectar is hidden deep down; short tongued species go for shallower flowers (or bite a hole in the back of long flowers and "break-and-enter" to steal the nectar!). Maybe the Rhododendron flower is too long for honey bees to get access to the nectar!
Bumble bees can generate their own heat by vibrating their wing muscles "in neutral" (without actually flapping the wings), so they can operate earlier in the season and in colder conditions than honey bees. Evolutionarily speaking, rhododendrons that originate from cold mountainous regions would probably be more associated with bumble bee type pollinators than honey bee type pollinators.
Honey bees are extreme generalist flower visitors, but they forage most avidly on the flowers that give them the most rewards. For example honey bees are usually able to obtain good yields of pollen and nectar from white clover, but the long deep flowers of red clover often make accessing the nectar more difficult. (Bumble bees can reach it.) Honey bee numbers can alter because white clover can become scarce during droughts and also when the numbers of beehives in an area can change.
Recently throughout the North Island many colonies of honey bees have been killed by the varroa mite. Bumble bees are not affected by varroa. Because of a decrease in competitive foraging pressure from honey bees, it is possible that bumble bee numbers have increased. This alone could account for fewer honey bees on rhododendron flowers, and possibly more bumble bees, in some areas.
And finally, we believe that some rhododendron flowers don't smell too great and some have no discernible smell at all. This could also be an important factor for honey bees. We know they have a good sense of smell for flower aromas and may simply not like Rhododendrons!