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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.

49 BEE WATCHING

Trans. Suffolk Nat. Soc. 44 (2008)

The large bumblebees usually alighted on or walked on to the corolla with their head towards the calyx and inserted their tongue through the holes to suck nectar, while smaller bees generally oriented themselves the other way round, standing on the calyx to insert their tongues to collect nectar (Plate 4).

Robbing, however, was not the universal behaviour and some bees adopted the legitimate method of alighting at the mouth of the corolla and crawling into the tube to find nectar (Plate 5). Bees, which did this, did not change their technique and, were seen visiting several flowers in succession, always entering from the mouth of the corolla. Most bees that adopted this method had empty pollen baskets.

Most robber bees had empty pollen baskets. A few were seen with filled pollen baskets but were using the robbing method to collect nectar rather than entering the corolla where they would also collect more pollen.

Bees usually spent only about three seconds at a flower before moving on, and often another bee would arrive and insert its tongue into the hole within a few minutes. In the prevailing hot weather (c. 24 °C) the patch was a scene of frenetic activity, sometimes with two bees competing for the same hole.

Occasionally a bee would depart to visit Tradesecantia flowers (Plate 6). adjacent to the Penstemons where solitary bees (Sphecodes spp.) were feeding on pollen. They did not stay long, visiting only one or two flowers before returning to the Penstemons.

P. heterophyllus, the Foothills or Chapparal Penstemon is a native of California where it is pollinated by native bees, including several species of bumblebee. Its attraction for British bumblebees is perhaps due to its colour; bees are known to be attracted by blue, and scent which also attracts bees, although none cold be detected by a human nose. The importance of colour may be borne out by the fact that an adjacent, large patch of Penstemon, cv. Garnet was visited by far fewer bees, mostly carder bumblebees and honey bees, all of which visited in the legitimate way. The flowers of this cultivar are slightly larger than cv. Heavenly blue, and have a copious production of nectar but are a liver-red colour.

There is little information about Penstemon as a bee plant, The species is not mentioned in the flower lists in Benton (2000) or in Prŷs-Jones and Corbett (1991), perhaps because the species was not widely grown as it was thought not to be winter-hardy, Recently it has become more popular as it ability to survive current climatic conditions has become apparent.

A feature of the behaviour of the bees working cv. Heavenly blue was the high incidence of robbing. The short tongued species, B. lucorum and B. terrestris are well known as robbers (Alford, 1975). The species responsible for biting into the corolla tube of the Penstemon flowers was not identified because some of the bees seen probing the base of the corolla may have been first making entry holes. Robbing of many species of wild and cultivated species of flowers is a widespread trait (Alford, 1975; Goulson, 2003), and on Westleton Common a majority of mature flowers of the widespread bell heather (Erica carnea) are robbed by bumblebees and solitary bees and wasps.

Some bumblebees used legitimate methods to gather nectar from the flowers and often other bees of the same species, mostly white-tailed bumblebees, were seen working in close proximity sucking nectar from holes

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in the corolla base. The tendency to rob appears to be an inherited trait more common in some species (Alford, 1975) and those bees using the legitimate method may have been naïve, young bees or, as some had full pollen baskets, may have been gathering pollen as well as nectar.

Robbing is generally considered from the point of view of its effect on pollination and seed set of the robbed flower. The energetic costs of bumblebee foraging have been examined by Prŷs-Jones and Corbett (1991), but they did not consider the energetics of robbing. Bees using this method generally walked from flower to flower and appeared to gather nectar with less effort than those who flew from flower to flower, necessary to have the correct approach to get into the corolla tube. Also the time to gather nectar from the perforated corolla was less than the time taken to push into the corolla tube to reach the nectarys.

Penstemon (P. heterophyllus) Heavenly Blue was a new plant in my garden, grown for the first time, and one which was quickly found and fully exploited by several species of bees. These observations show that cultivated penstemons, particularly this species, may be valuable garden plants for bumblebees.

References

Alford, D. V. (1975). Bumblebees. Davis-Poynter Ltd, London. Benton, T. (2000). Bumblebees of Essex. Lopinga Books, Saffron Walden. Goulson, D. (2003). Bumblebees; Behaviour and Ecology. O.U.P., Oxford. Prŷs-Jones, O. E. and Corbett S. A. (1991). Bumblebees. Richmond Publishing

Co., Slough.

Michael Kirby The Studio Blythburgh Road Westleton Saxmundham IP17 3AS

Plate 4: Carder bumblebee visiting flower legitimately (p. 49).

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Plate 3: Cuckoo bumblebee, robbing while standing on corolla (p. 48).

Plate 5: Carder bumblebee robbing from calyx (p. 49).

Plate 6: Solitary bee feeding collecting pollen from Tradescantia (p. 49).

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