3.1 Bee behaviour and Visitation Rates
As expected, two bumblebee species were the only pollinator visitors to our experimental plants, with Bombus hortorum visiting flowers significantly more often (mean = 0.6 ± 1.6 SD visits per flower per hour) than B. pascuorum (mean = 0.2 ± 0.9 SD; P<0.001; N = 1288 3-minute surveys). Overall, robbed flowers received visits at a significantly lower rate (mean = 0.7 ± 1.7 SD visits per flower per hour) than flowers in the control treatment (mean = 1.0 ± 2.0; P<0.001; Fig. 2a), and this was consisted for both bumblebee species.
We found that the average visit length to flowers was also different when comparing robbing treatments. Visits to robbed flowers were significantly shorter (mean = 6.6 ± 5.3 SD seconds; N = 120) than on the control flowers (mean = 10.3 ± 9.5 SD; N = 210; P < 0.001), with visits being on average 3.7 seconds shorter (Fig. 2b). This reduction in visit length was consistent across bumblebee species (Fig. 3); however, visits by B. hortorum were overall of shorter duration (mean = 7.3 ± 6.3 SD seconds; N = 232) than B. pascuorum(mean = 12.9 ± 11.2 SD; N = 98; P < 0.001; Fig. 3).
Bumblebees visited on average 50% of a control plant’s flowers on an inflorescence (N = 52) and 42% of a robbed plant’s flowers (N = 40), and this difference was statistically significant (P = 0.037). When recording the number of times that plants in each treatment were rejected by foragers, robbed plants were rejected 32 times compared with 25 times for the control; this was not significantly different (P=0.090)