Nectar robbing in bellflower (Sesamum radiatum) benefited pollinators but unaffected maternal function of plant reproduction

Abstract

Nectar robbing - foraging nectar illegitimately - has negative, neutral, or positive effects on maternal function of plant reproduction and/or on pollinators. It has been suggested that nectar robbing has a non-negative effect on maternal function of plant reproduction in autogamous and mixed breeding plants; however this hypothesis requires deeper understanding with more studies. We investigated the impact of natural nectar robbing on maternal function of plant reproduction and visitation characteristics of pollinators in Sesamum radiatum, an autogamous plant. Pollinators were observed on unrobbed open flowers and robbed open flowers. In robbed flowers, pollinators' visit type and foraging time were examined. The seed sets of these flower types were examined. Xylocopa latipes was both a primary robber and a legitimate pollinator, X. bryorum was an exclusive primary robber, and Megachile disjuncta was a cosmopolitan pollinator. In robbed flowers, most of the pollinators foraged mostly as secondary nectar robbers. The foraging time shortened considerably when pollinators robbed nectar - a positive effect on pollinators' foraging efficiency. Robbing did not negatively affect seed set - a neutral effect on the plant's reproduction. Our study agrees that nectar robbing might have a non-negative effect on reproduction in autogamous and mixed breeding plants.

Figure 1

A flower of Sesamum radiatum.

Figure 2

Nectar robbing had no effect on visitation rate of overall flower visitors of Sesamum radiatum (N_obs = 199).

Figure 3

Nectar robbing had a negative effect on legitimate visits of (A) overall visitors (N_obs = 144), (B) pollinators (N_obs = 97), (C) Megachile disjuncta – a cosmopolitan pollinator of Sesamum radiatum (N_obs = 59), and (D) Xylocopa latipes – a primary nectar robber cum pollinator of Sesamum radiatum (N_obs = 39).

Figure 4

Nectar foraging time of (A) overall pollinators (N_obs = 140), (B) Megachile disjuncta (N_obs = 83), and (C) Xylocopa latipes –the robber cum pollinator (N_obs = 27) – decreased drastically when they robbed nectar.

Figure 5

Nectar robbing had no significant effect on seed set, but caging had a negative effect on seed set (N_obs = 165).