Closely related pollen generalist bees differ in their ability to develop on the same pollen diet: evidence for physiological adaptations to digest pollen

Abstract

Summary 1.Given the enormous quantitative pollen requirements of bees and their high efficiency in pollen removal, flowers should balance the need to attract bees for pollination on the one hand and to restrict pollen loss to bees on the other hand. Although various morphological flower traits have been identified that reduce excessive pollen losses to bees, the question of whether pollen might also be chemically protected remains largely unexplored. 2. In this study we compared the larval performance of the two very closely related and highly pollen generalist solitary bee species Osmia bicornis and Osmia cornuta on four different pollen diets. 3. Despite their very large pollen diet breadth, the two bee species showed striking differences in their ability to develop on pollen of the same plant species. Osmia bicornis developed well on Ranunculus pollen but failed to do so on Echium pollen, whereas the reverse held true for O. cornuta with the exception of two larvae grown on Ranunculus pollen that developed into dwarfish adults. Both bee species performed well on Sinapis pollen, while neither of the two species managed to develop on Tanacetum pollen. 4. The observed differences in larval survival of these two Osmia species when reared on the same pollen diet as well as their failure to develop on Tanacetum pollen clearly demonstrate that bees require physiological adaptations to cope with the unfavourable chemical properties of certain pollen. 5. Our results show a remarkable analogy of bee-flower relationships with herbivore-plant interactions and possibly indicate that the pollen of certain plant taxa might be chemically protected.

Museum specimens reveal loss of pollen host plants as key factor driving wild bee decline in The Netherlands

“…Even the most generalist bee species are restricted in their range of host plants and have distinct preferences for certain pollen taxa (37). Bees may produce lowerquality offspring when larvae are reared on pollen of less-preferred host plants (38) or may fail to produce offspring altogether (39). Declining bee species therefore most likely suffered from reduced fitness as a consequence of the loss of their preferred host plant species.…”

Pollen and nectar quality drive the major and minor floral choices of bumble bees

“…Cirsium palustre contained a high relative proportion of δ7-stigmasterol, which is a common sterol to several Asteraceae (Janson et al 2009). This phytosterol might represent a chemical protection of pollen, as it has been reported to be detrimental to herbivorous insects lacking the enzymes (specifically isomerases) necessary to completely convert δ7-sterols to δ5-sterols (Janson et al 2009;Sedivy et al 2011). …”

Chemical composition of the ‘low quality’ pollen of sunflower (Helianthus annuus, Asteraceae)

“…However, the pollen protein content of Asteraceae is at the low end of the spectrum for beepollinated plants , and beekeepers consider these pollens to be a poor resource (Schmidt et al, 1987). Very poor development of two species of Osmia, generalist solitary bees, was recorded on the pollen of dandelion Tanacetum vulgare (Asteraceae) (Sedivy et al, 2011), and the inability of honeybees to rear brood on dandelion pollen has been attributed to deficiencies in several amino acids (Loper and Cohen, 1987). Bumblebees (Bombus terrestris) in micro-colonies performed poorly and reared very small larvae on sunflower pollen, compared to other pollen sources (Tasei and Aupinel, 2008).…”