Pollen hosts of western palaearctic bees of the genus Colletes (Hymenoptera: Colletidae): the Asteraceae paradox

Abstract

To assess the pollen hosts of 60 western palaearctic bee species of the genus Colletes (Colletidae), we microscopically analysed 1336 pollen loads of collected females. Twenty-six species (43.3%) were found to be specialized at the level of plant family, subfamily or genus. Thirty-four species (56.7%) proved to be pollen generalists to varying degrees, visiting the flowers of up to 15 different plant families. Flowers of the subfamily Asteroideae (Asteraceae) are by far the most important pollen source, contributing 23.6% to the pollen-plant spectrum of the whole bee genus. The high significance of Asteroideae pollen is due to the large number of specialists: 14 Colletes species belonging to four different taxonomic groups harvest pollen exclusively or predominantly on flowers of the Asteroideae. By striking contrast, Asteroideae pollen plays only a marginal role in the diets of the pollen generalists: it was recorded in only 2.7% of the pollen loads and in seven out of the 34 pollen generalists. Among the few generalists exploiting Asteroideae for pollen, three closely related species have ancestors which were possibly specialized on Asteraceae. The pattern of use of Asteroideae pollen by the Colletes bees supports recent findings that this pollen possesses unfavourable or protective properties, which render its digestion difficult, and suggests that bees need physiological adaptations to successfully utilize it.

Macronutrient ratios in pollen shape bumble bee ( Bombus impatiens ) foraging strategies and floral preferences

“…Furthermore, as generalist foragers, bumble bees may avoid or dilute the negative effects of toxic phytochemicals by collecting pollen from multiple host-plant species (50,51). It is unclear, however, whether micronutrient variation in pollen or secondary plant metabolites can alter bee foraging for macronutrients or whether the concentrations of these compounds are somehow associated with the macronutrient levels we measured.…”

Diet characterisation of solitary bees on farmland: dietary specialisation predicts rarity

“…Characterisation of floral preferences in pollen host plants was carried out for species with a minimum of three pollen load samples following Müller and Kuhlmann (2008). A small sample size may result in mischaracterisation of bee diets as certain plant families may be under or overrepresented.…”

“…Consequently, for species with small sample sizes the results presented here should be viewed within this broader context. In characterising diet we used the categories laid out by Müller and Kuhlmann (2008) who modified the work of Cane and Sipes (2006) to include additional subcategories of oligolecty and polylecty (Appendix II in supplementary material). This modification added the category 'polylectic with a strong preference' as this pattern of host use exists in many species (Müller 1996).…”

“…In establishing more definitive pollen preferences and diet breadth ranges, microscopic analysis of pollen loads has been favoured as the total volume of pollen collected from different sources can be quantified. The level of specialisation, as either oligolectic (collecting pollen from one plant family or genus) or polylectic (collecting pollen from many plant families) can then be made with greater confidence (Westrich 1989;Müller 1996;Müller and Kuhlmann 2008). The use of pollen load analysis is becoming more widely used in field studies to assess pollen utilisation of both semi-natural and agri-environment scheme plants (Carvell et al 2006;Kremen and Williams 2007;Beil et al 2008) and to quantify historical bee diets through the analysis of remaining pollen loads present on museum specimens (Kleijn and Raemaekers 2008;Scheper et al 2014).…”

Key environmental determinants of global and regional richness and endemism patterns for a wild bee subfamily

“…S1.1; Michener 2007), Colletes is the seventh most species-rich bee genus, with currently 494 described species (Ascher and Pickering 2016) and an estimated total of about 700 species (Kuhlmann and Proshchalykin 2014). Colletes are ground-nesting solitary bees that have been extensively studied by one of us (MK, comprehensive revisionary studies of the Old World fauna) over more than 20 years and involve species that differ in seasonality, soil preference and degree of floral specialization (Müller and Kuhlmann 2008) making it an ideal model representative of the vast majority of bees. Anecdotal evidence suggests that Colletes diversity, like that of many other bee genera and of bees in aggregate (Michener 1979(Michener , 2007, follows a reverse latitudinal diversity gradient with amphitropical peaks at mid-latitudes in Africa and Eurasia (Kuhlmann 2005(Kuhlmann , 2009) as well as North America and South America (e.g., many species in Chile and Argentina but few in equatorial South America, Moldenke 1976) but quantitative analyses are required to test and more precisely characterize this observation.…”