The invasive alien bumble bee Bombus terrestris may hinder the reproduction of native plants that have established specialized pollination systems with native bumble bees. To test this hypothesis, we examined the visitation frequency and behavior of native and alien bumble bee species and resultant seed production in Corydalis ambigua, a native plant in Hokkaido, Japan. This species is self-incompatible: the flower has a spur and requires visitation by bumble bees for effective seed production. We compared visitation frequency as well as fruit and seed set after cross- and open pollination at five sites of C. ambigua. Four of these sites occurred near a residential district and included naturalized populations of B. terrestris, and the fifth site was located in a forested habitat with no B. terrestris. The native species B. ardens and B. hypocrita and the alien B. terrestris frequently visited C. ambigua. Bombus ardens legitimately consumed nectar, whereas B. hypocrita and B. terrestris rob nectar by perforating spurs. The legitimate pollinator B. ardens produced fruits and seeds more efficiently than the nectar robbers. At three sites, the proportion of robbed flowers per inflorescence gradually increased through the flowering period, which may be caused by the intrusion of alien B. terrestris into the native plant–pollinator interactions. At these sites, C. ambigua suffered from pollen limitation, as seed production from open pollination was lower than from cross-pollination, despite the fact that the total abundance of three bumble bees was higher than in the other two sites. Legitimate B. ardens visited fewer flowers within inflorescences with more robbed flowers, suggesting that nectar robbing may reduce the frequency of visitations by B. ardens within inflorescences, and resulting in decreased fruit set. Furthermore, reduced seed set implies a reduction in the pollination quality by B. ardens, probably due to decreases in visiting time per flower. Thus, introduction of alien B. terrestris may alter the native plant–pollinator mutualism: C. ambigua could establish a novel pollination relationship with B. terrestris because of its nonzero pollination efficiency, similar to the native robber B. hypocrita.
