Nectar robbing, forager efficiency and seed set: Bumblebees foraging on the self incompatible plant Linaria vulgaris (Scrophulariaceae)

In southern England, Linaria vulgaris (common yellow toadflax) suffers from high rates of nectar robbery by bumblebees. In a wild population of L. vul garis we found that 96 % of open flowers were robbed. Five species of bumbl ebee were observed foraging on these flowers, although short-tongued specie s (Bambus lapidarius, B. lucorum and B, terrestris) robbed nectar whilst lo nger-tongued ones behaved as legitimate pollinators (B. hortorum and B. pas cuorum). Nectar rewards were highly variable; on average there was less nec tar in robbed than in unrobbed flowers, but this difference was not statist ically significant. The proportion of flowers containing no nectar was sign ificantly higher for robbed flowers compared with unrobbed flowers. Seconda ry robbers and legitimate pollinators had similar handling times on flowers and, assuming they select flowers at random to forage on, received approxi mately the same nectar profit per minute, largely because most flowers had been robbed. There was no significant difference in the number of seeds in pods of robbed flowers and in pods of flowers that were artificially protec ted against robbing. However, more of the robbed dowers set at least some s eed than the unrobbed flowers, possibly as a consequence of the experimenta l manipulation. We suggest that nectar robbing has little effect on plant f ecundity because legitimate foragers are present in the population, and tha t seed predation and seed abortion after fertilization may be more importan t factors in limiting seed production in this species. (C) 2000 Editions sc ientifiques et medicales Elsevier SAS.