Forces and Flower Visitors of Salvia

In 13 Salvia species, the forces necessary to release the staminal lever have been measured, and for 10 of these species, quantitative data are presented for the first time. The data range between 0.5 mN for S. nilotica and 10 mN for S. cf. microphylla (Table 6.2).

A schematic drawing of a S. glutinosa flower is shown in Figure 6.7A. The mean force necessary to trigger the lever is 1.6 ± 0.8 mN (n = 10). In this species the corolla tube is narrowed by a broad ring of hairs and the force necessary to pass the hairy ring with the sensor is 2.5 ± 0.7 mN (n = 8) (Figure 6.7C). In S. glutinosa, it was not possible to measure the release of the staminal lever and the forces necessary to pass the hairy ring within one continuous measurement because the lever and the flower tube were not arranged linearly in the flowers. As a result, the measurements cannot be depicted in the continuous force-distance diagram. We observed B. hor-torum visiting S. glutinosa flowers at both study sites and B. pasquorum and one species of leaf cutter bee Megachile sp. visiting at the Freiburg Botanical Garden. Several pollen and/or nectar thieves were observed that did not trigger the lever, including a small sweat bee and a yellow-faced bee (Lasioglossum sp. and Hylaeus sp.).

FIGURE 6.6 Forces exerted by individual bumblebees (B. terrestris) to gain access to the food source in the artificial flower testing device. (A) Worker, and (B) queen.

A schematic drawing of a S. sclarea flower is shown in Figure 6.7B. The force necessary to release the staminal lever of S. sclarea is 5.2 ± 3.0 mN (n = 43) (Figure 6.7D). The triggering of the lever often occurred during a steep increase in force. The peak value of this force increase is relatively variable and shows a mean value of 14.0 ± 9.9 mN (n = 33). An internal barrier narrows the entrance to the basal region of the flower tube. The force necessary to pass this additional internal barrier is 5.3 ± 1.7 mN (n = 17). At all three localities, the carpenter bee X. violacea was observed visiting S. sclarea flowers. In Boetzingen, we also observed a leaf cutter bee Megachile sp. and in Schwanau a wool carder bee Anthidium manicatum visiting this sage.

To quantify the forces an insect has to produce to reach the nectar in S. sclarea, which is located at the base of the corolla tubes, measurements with the modified sensor were carried out (cf., Figure 6.5). On the metal sensor, the head of a honeybee (A. mellifera) was fixed at various distances from the sensor tip to mimic the influence of different proboscis lengths. Proboscis lengths of 5, 6, 7, 8, and 11 mm have been simulated for the measurements (Figure 6.8). To account for different flower sizes found in S. sclarea, we used a large (internal flower-tube length, 9.7 mm) and a small flower (internal flower-tube length, 9.4 mm) for our measurements.

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