BUMBLEBEES ROBBING PENSTEMON FLOWERS

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Suffolk Natural History, Vol. 44

BUMBLEBEES ROBBING PENSTEMON FLOWERS MICHAEL KIRBY My attention was first drawn to a patch of Penstemon (P. heterophyllus ‘Heavenly blue’) by a loud and continuous buzzing of a host of bees visiting the many open flowers. The patch, about 2 m2 in area, had about three or four bees per 0·9 m2, that is, there were in the order of 100 bees at work. The number dropped during cloudy periods but for most of the time, from 10–17 June 2008, the weather was hot and sunny and the bees were working vigorously from about 8 am until 7 pm giving a very rough estimate of 10,000 bee hours of nectar gathering. The majority of bee visitors were bumblebees and five common species were seen (Bombus lapidarius, B. lucorum, B. pascuorum, B. hortorum and B. terrestris) with the common white-tailed bumblebee and the garden bumblebee the most common. The gypsy cuckoo bee (Psithyrus bohemicus, see Plate 3) and honey bees also visited the flowers. Penstemons are members of the Scrophulariaceae and the flower is similar in form to that of the foxglove with a long fused corolla tube with three upper and two lower lobes, four stamens held close to the roof of the corolla, a staminode (absent in the foxglove) which appears to play no part in pollination and superior ovary with a long arching stigma. Like the foxglove it appears well adapted to pollination by large bees On close observation, however, it was seen that almost all the bees were ‘robbing’ the flowers, that is, getting the nectar through holes cut by them in the base of the upper part of the corolla adjacent to the ovary, rather than ‘legitimately’ by entering the tube and crawling under the stamens to get to the nectar. The holes were probably made by a short-tongued species such as B. lucorum.

Figure 1. Sagittal section of a Penstemon flower; the stamens and staminode are not shown. Nectar is secreted around the ovary and the arrow shows where the bees bite holes to rob the flower. The diameter of the corolla behind the free petals is 7 mm, compared to 9 mm for a foxglove corolla.

Trans. Suffolk Nat. Soc. 44 (2008)


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