Perception, regulation, and fitness effects of pollen phytosterols in the bumble bee, Bombus terrestris.

PREMISE: Many flowering plants depend on insects for pollination and thus attract pollinators by offering rewards, mostly nectar and pollen. Bee pollinators rely on pollen as their main nutrient source. Pollen provides all essential micro- and macronutrients including substances that cannot be synthesized by bees themselves, such as sterols, which bees need for processes such as hormone production. Variations in sterol concentrations may consequently affect bee health and reproductive fitness. We therefore hypothesized that (1) these variations in pollen sterols affect longevity and reproduction in bumble bees and (2) can thus be perceived via the bees' antennae before consumption. METHODS: We studied the effect of sterols on longevity and reproduction of Bombus terrestris workers in feeding experiments and investigated sterol perception using chemotactile proboscis extension response (PER) conditioning. RESULTS: Workers could perceive several sterols (cholesterol, cholestenone, desmosterol, stigmasterol, ß-sitosterol) via their antennae but not differentiate between them. However, when sterols were presented in pollen, and not as a single compound, the bees were unable to differentiate between pollen differing in sterol content. Additionally, different sterol concentrations in pollen neither affected pollen consumption nor brood development or worker longevity. CONCLUSIONS: Since we used both natural concentrations and concentrations higher than those found in pollen, our results indicate that bumble bees may not need to pay specific attention to pollen sterol content beyond a specific threshold. Naturally encountered concentrations might fully support their sterol requirements and higher concentrations do not seem to have negative effects.

Local Insect Availability Partly Explains Geographical Differences in Floral Visitor Assemblages of Arum maculatum L. (Araceae).

Geographical variation in abundance and composition of pollinator assemblages may result in variable selection pressures among plant populations and drive plant diversification. However, there is limited knowledge on whether differences in local visitor and pollinator assemblages are the result of site-specific strategies of plants to interact with their pollinators and/or merely reflect the pollinator availability at a given locality. To address this question, we compared locally available insect communities obtained by light-trapping with assemblages of floral visitors in populations of Arum maculatum (Araceae) from north vs. south of the Alps. We further investigated whether and how the abundance of different visitors affects plants' female reproductive success and examined the pollen loads of abundant visitors. Local insect availability explained inter-regional differences in total visitor abundance, but only partly the composition of visitor assemblages. Northern populations predominantly attracted females of Psychoda phalaenoides (Psychodidae, Diptera), reflecting the high availability of this moth fly in this region. More generalized visitor assemblages, including other psychodid and non-psychodid groups, were observed in the south, where the availability of P. phalaenoides/Psychodidae was limited. Fruit set was higher in the north than in the south but correlated positively in both regions with the abundance of total visitors and psychodids; in the north, however, this relationship disappeared when visitor abundances were too high. High pollen loads were recorded on both psychodids and other Diptera. We demonstrate for the first time that the quantitative assessment of floral visitor assemblages in relation to locally available insect communities is helpful to understand patterns of geographical variation in plant-pollinator interactions. This combined approach revealed that geographical differences in floral visitors of A. maculatum are only partly shaped by the local insect availability. Potential other factors that may contribute to the geographical pattern of visitor assemblages include the region-specific attractiveness of this plant species to flower visitors and the population-specific behavior of pollinators.