Bumblebees (Bombus spp.) play an important role in providing pollinator services in both natural and agricultural systems (Goulson 2003; Goulson et al. 2008). The genus Bombus (Apidae) comprises approximately 250 species of medium-sized to large bees, distributed around the world (Michener 2007). These species rely on pollen and nectar as food sources, collecting these floral rewards from a diverse range of plant species (Sladen 1912; Goulson 2010). Previous studies have shown that bumblebees are able to learn to manipulate flowers with different morphologies (Heinrich 1979; Laverty 1980; Laverty and Plowright 1988; Laverty 1994; Raine and Chittka 2007, 2008) and across different contexts (Biernaskie et al. 2009), and that they can even learn from other individuals of the same (Leadbeater and Chittka 2008) or different species (Goulson et al. 2013). The ability to adjust their behavioural repertoire to manipulate flowers and collect resources from diverse plant species allows bumblebees to maximise the exploitation of floral resources over time, across different environments, and in distinct plant communities.

Bumblebees can learn in order to exploit floral resources more proficiently, and at a faster rate, with increasing experience in handling flowers (Heinrich 1979; Laverty 1980; Laverty and Plowright 1988; Laverty 1994; Raine and Chittka 2007, 2008). Previous work has shown that the amount of experience required to achieve maximum proficiency increases with increasing floral complexity (Laverty 1980, 1994). To date, most research on bumblebee learning has focused on behaviours displayed with relatively simple natural and artificial flowers in which nectar is used as the main reward (but see Raine and Chittka 2007; Kitaoka and Nieh 2009; Konzmann and Lunau 2014; Lunau et al. 2015; Muth et al. 2016). In contrast, less is known about whether and how bumblebees learn to manipulate complex flowers in which pollen is the main or only reward. The optimal method for pollen removal varies among plant species as a result of differing floral structure, anther morphology, and pollen properties (Buchmann 1983; Thorp 2000).

In most flowering plants, anthers split lengthwise to passively release pollen, which is then available for collection by pollinators (Buchmann 1983). However, in a sizeable number (around 20,000 species across a diverse suite of plant taxa), pollen is concealed within poricidal anthers (Buchmann 1983). These non-dehiscent, conical or tubular anthers have an apical pore through which pollen is expelled only when the anthers are vibrated (Buchmann 1983; Harder and Barclay 1994; De Luca and Vallejo-Marín 2013). Some floral visitors, including Bombus, can release pollen from these structures using a process known as sonication (Buchmann 1983). During sonication, the bee wraps its body around the anthers, while often holding them with its mandibles, and vibrates its thoracic muscles (the same muscles used in flight) at high frequencies. The vibrations are transmitted from the bee’s body to the anther, thereby releasing pollen.

The process of sonication requires the integration of a complex set of behaviours that involve the production of vibrations of high frequency. Previous studies have provided some evidence that the ability to produce sonication buzzes in the context of pollen extraction is innate in bumblebees (King 1993), and that learning plays an important role in allowing bumblebees to handle buzz-pollinated species effectively (Laverty 1980). However, whether and to what extent bumblebees can modify the characteristics of their buzzes, particularly the frequency and amplitude, in response to their experience in visiting flowers with poricidal anthers is unknown. Sonication buzzes generated during feeding differ in acoustic properties to vibrations produced under other contexts, such as defence buzzes (De Luca et al. 2014) and vibrations produced during flight (King et al. 1996). This raises the possibility that the characteristics of sonication buzzes can, to some extent, be actively modified by bees. The solitary bee species Xylocopa frontalis buzzes at slightly different frequencies when removing pollen from two different buzz-pollinated Solanum species, suggesting that such adjustments may be possible (Burkart et al. 2011). The ability to adjust the type of vibrations produced during pollen collection could allow bees to maximise the amount of pollen collected, e.g., by increasing the number of pollen grains released per buzz (Harder and Barclay 1994; De Luca et al. 2013).

Here we investigated whether bumblebees (Bombus terrestris) modify the characteristics of their sonication buzzes as they gain experience in sonicating on flowers. We used naïve bees that had never visited a flower and exposed them to flowers of Solanum rostratum (Solanaceae), a buzz-pollinated species with poricidal anthers. We recorded sonication buzzes during floral visitation and flight in consecutive visits to flowers over multiple days. By analysing the behaviour of these naïve bees, and the acoustic properties (frequency and amplitude) of their flight and feeding buzzes, we addressed two specific questions: (1) Is the ability to sonicate to elicit pollen release innate? (2) Do bees modify the characteristics of their feeding buzzes (frequency and amplitude) as they gain experience in visiting flowers with poricidal anthers?