Abstract
Host‐plant selection is a key factor driving the ecology and evolution of insects. While the majority of phytophagous insects is highly host specific, generalist behavior is quite widespread among bees and presumably involves physiological adaptations that remain largely unexplored. However, floral visitation patterns suggest that generalist bees do not forage randomly on all available resources. While resource availability and accessibility as well as nectar composition have been widely explored, pollen chemistry could also have an impact on the range of suitable host‐plants. This study focuses on particular pollen nutrients that cannot be synthesized de novo by insects but are key compounds of cell membranes and the precursor for molting process: the sterols. We compared the sterol composition of pollen from the main host‐plants of three generalist bees: Anthophora plumipes, Colletes cunicularius, and Osmia cornuta, as well as one specialist bee Andrena vaga. We also analyzed the sterols of their brood cell provisions, the tissues of larvae and nonemerged females to determine which sterols are used by the different species. Our results show that sterols are not used accordingly to foraging strategy: Both the specialist species A. vaga and the generalist species C. cunicularius might metabolize a rare C27 sterol, while the two generalist species A. plumipes and O. cornuta might rather use a very common C28 sterol. Our results suggest that shared sterolic compounds among plant species could facilitate the exploitation of multiple host‐plants by A. plumipes and O. cornuta whereas the generalist C. cunicularius might be more constrained due to its physiological requirements of a more uncommon dietary sterol. Our findings suggest that a bee displaying a generalist foraging behavior may sometimes hide a sterol‐specialized species. This evidence challenges the hypothesis that all generalist free‐living bee species are all able to develop on a wide range of different pollen types.
Highlights
Plant–insect interactions range from antagonism to mutualism and from specialization to generalization (Mayhew, 1997; Wcislo & Cane, 1996, and references therein; Lengyel, Gove, Latimer, Majer, & Dunn, 2009)
Regarding O. cornuta, tissues from both larvae and nonemerged females contained the same major phytosterols, namely 24-methylenecholesterol, β-sitosterol, and δ5-avenasterol, but in slightly different ratios (Table 3). For both A. vaga and C. cunicularius, the most abundant sterols identified in larval tissues and nonemerged females were β-sitosterol and δ5-avenasterol followed by 24-methylenecholesterol for nonemerged females and cholestenone for larvae (Table 3)
The desmosterol was more abundant in larvae compared to the nonemerged females but was too variable to be involved in key metabolic pathway such as molting
Summary
Plant–insect interactions range from antagonism to mutualism and from specialization to generalization (Mayhew, 1997; Wcislo & Cane, 1996, and references therein; Lengyel, Gove, Latimer, Majer, & Dunn, 2009). Whereas specialization offers obvious evolutionary advantages such as physiological efficiency, optimal foraging, and efficient host discrimination (Janz & Nylin, 2008, and references therein), a minority of herbivorous insects has taken a different evolutionary route and exploits numerous host plants. Such generalist behavior involves processing multiple sensory and chemical signals (Bernays, 2001; Riffell, 2011) and potentially requires adaptations related to host recognition, foraging, and digestion (Finlay-Doney & Walter, 2012). The specialist species could forage on host-pollen that has a peculiar sterol profile and could be constrained to metabolize just one specific sterol
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