Comparing Insect Forces to the Barriers in Flowers

The force needed to insert a proboscis and trigger the lever in all measured Salvia flowers did not exceed 5.2 ± 3.0 mN (S. sclarea) in bee-pollinated species. This is clearly below the maximum value of 29 mN that even the weakest measured bee species (honeybee) can exert. This means that honeybees as well as bumblebees are strong enough to insert their proboscis in the respective flower, and that these bees are not excluded by the force that is needed to trigger the staminal lever of any measured sage flowers. This even holds true for the additional internal barriers of sage flowers under investigation. Neither the protrusion narrowing the corolla tube in Clary sage (S. sclarea) nor the hairy ring in S. glutinosa flowers forms a barrier that mechanically blocks the access to the nectar for honeybees or bumblebees.

The hypothesis that honeybees and bumblebees are not excluded by force from visits of a given Salvia flower is not new. Meadow sage (S. pratensis) is less specialized than other sages because it is pollinated by many species of insects including bumblebees and honeybees (Table 6.1). Therefore, the lever of meadow sage evidently does not exclude these insects by force [35]. Our results indicate that even sages that are pollinated by big bees (Table 6.1) do not exclude smaller bee species by the magnitude of the force necessary to release the staminal lever. This implies that the forces needed to trigger the staminal lever are of minor significance in the specialization of these Salvia species for specific flower visitors.

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