THE SUPERIORITY OF BUMBLEBEE TO HONEYBEES AS POLLINATORS - INSECT VISITS TO RASPBERRY FLOWERS

1. The behaviour and activity patterns of Apis mellifera and of five species of Bombus were analysed in relation to climatic variables and nectar quality on three varieties of unsprayed cultivated raspberry (Rubus idaeus) in eastern Scotland. 2. Stages of floral morphology and reward were similar for the three varieties: young flowers offered both nectar and pollen, but medium and old flowers offered nectar only, in diminishing quantities. 3. A wide range of insects visited raspberry flowers, but bees were dominant, bumblebees being responsible for about 60% of all visits and honeybees making up most of the remaining percentage. All bees had substantial pollen deposited on their bodies during visits, though few specifically collected it. 4. Bombus spp. were found to favour young (receptive) flowers strongly, especially early in the morning when pollen was most abundant; whilst Apis visited unselectively. Bumblebees also foraged over substantially longer periods of the day, and in poorer weather, some being present at most times of observation; and they foraged more quickly in terms of flower visits per minute. 5. Bombus carried more pollen on their bodies than Apis, and also deposited more pollen on raspberry stigmas, with B. lapidarius and B. terrestris being particularly effective and also being the most abundant species. All bumblebees also foraged over a longer range, moving between canes and rows more frequently than did honeybees. 6. Bumblebees are therefore likely to be substantially more important as pollinators of raspberries than are honeybees, especially as raspberries though moderately self-fertile may exhibit metaxenia. Reasons why Bombus may be the preferred pollinator in most sites of raspberry cultivation are discussed, together with implications for present and future growers.