Microbes make the meal: oligolectic bees require microbes within their host pollen to thrive

1. For solitary bees that specialise on select pollen types (oligoleges), larval development depends on the availability of forage pollen from appropriate host plants and the naturally occurring microbiota present therein. While access to host pollen may be critical for the development of oligolectic bees, the extent to which pollen microbiota contribute to their brood success is unknown. 2. To investigate, we used a diet manipulation experiment to rear larvae of the oligolege, Osmia ribifloris, under in-vitro conditions. Larvae were reared either on host pollen provisioned by their mother or on non-host pollen collected by honey bees, in the presence or absence of the respective pollen-associated microbiota. We assessed impacts on components of larval fitness: developmental time, biomass, and survivorship. 3. Our results revealed a significant interaction between pollen type and pollen-associated microbes. The relative effect of microbes on larval performance was substantially greater than that of pollen type. Host pollen substrate produced the fittest larvae but only when combined with its full complement of naturally occurring microbiota. In contrast, host pollen without microbes resulted in a marked decline in fitness components. Larvae consuming non-host pollen showed intermediate fitness, regardless of whether microbes were present or not. 4. These findings imply that the microbiota associated with maternally provisioned host pollen perform critical functions in larval nutrition and survival. For oligoleges in particular, the ability to develop on poorer quality host pollen likely derives from this sustained symbiosis with their microbial exosymbionts, rather than the biochemical characteristics of pollen type alone.     

Evidence for mutualism between a flower-piercing carpenter bee and ocotillo: use of pollen and nectar by nesting bees

Abstract.

• 1 Carpenter bees (Xylocopa californica arizonensis) in west Texas, U.S.A., gather pollen and ‘rob’ nectar from flowers of ocotillo (Fouquieria splendens). When common, carpenter bees are an effective pollen vector for ocotillo. We examined ocotillo's importance as a food source for carpenter bees.
• 2 The visitation rate of carpenter bees to ocotillo flowers in 1988 averaged 0.51 visits/flower/h and was 4 times greater than that of queen bumble bees (Bombus pennsylvanicus sonorus), the next most common visitor. Nectar was harvested thoroughly and pollen was removed from the majority of flowers. Hummingbird visits were rare.
• 3 Pollen grains from larval food provisions were identified from sixteen carpenter bee nests. On average, 53% of pollen grains sampled were ocotillo, 39% were mesquite (Prosopis glandulosa), and 8% were Zygophyllaceae (Larrea tridentata or Guaiacum angustifolium). Carpenter bee brood size averaged 5.8 per nest.
• 4 We measured the number of flowers, and production of pollen and nectar per flower by mature ocotillo plants, as well as the quantity of pollen and sugar in larval provisions. An average plant produced enough pollen and nectar sugar to support the growth of eight to thirteen bee larvae. Ocotillo thus has the potential to contribute significantly to population growth of one of its key pollinators.
• 5 Although this carpenter bee species, like others, is a nectar parasite of many plant species, it appears to be engaged in a strong mutualism with a plant that serves as both a pollen and as a nectar source during carpenter bee breeding periods.

     

Individual bee foragers are less-efficient transporters of pollen for plants from which they collect the most pollen in their scopae

Premise

Bees provision most of the pollen removed from anthers to their larvae and transport only a small proportion to stigmas, which can negatively affect plant fitness. Though most bee species collect pollen from multiple plant species, we know little about how the efficiency of bees' pollen transport varies among host plant species or how it relates to other aspects of generalist bee foraging behavior that benefit plant fitness, such as specialization on individual foraging bouts.

Methods

We compared the pollen collected and transported by three bee species for 46 co-occurring plant species. Specifically, we compared the relative abundance of pollen taxa in the individual bees' scopae, structures where bees store pollen to provision larvae, with the relative abundance of pollen taxa on the rest of bees' bodies, which is more likely to be transferred to stigmas.

Results

Bees carried five times more pollen grains in their scopae than elsewhere on their bodies. Within foraging bouts, bees were relatively specialized in their pollen collection, but transported proportionally less pollen for the host plants on which they specialized. Across foraging bouts, two bee species transported proportionally less pollen for some of their host plants than for others, though differences didn't consistently follow the same trend as at the foraging bout scale.

Conclusions

Our results suggest that foraging-bout specialization, which is known to reduce heterospecific pollen transfer, also results in less-efficient pollen transport. Thus, bee foragers that visit predominantly one plant species may have contrasting effects on that plant's fitness.

     

Use of novel pollen species by specialist and generalist solitary bees (Hymenoptera: Megachilidae)

If trade-offs between flexibility to use a range of host species and efficiency on a limited set underlie the evolution of diet breadth, one resulting prediction is that specialists ought to be more restricted than generalists in their ability to use novel resource species. I used foraging tests and feeding trials to compare the ability of a generalist and a specialist solitary mason bee species to collect and develop on two pollen species that are not normally used in natural populations (novel pollens). Osmia lignaria (Hymenoptera: Megachilidae) is a generalist pollen feeder; O. californica, is more specialized. Adults of the specialist were more limited in use of novel hosts, but only in some contexts. Both bee species refused to collect one novel pollen. The specialist accepted a second novel pollen only when it was presented along with its normal pollen, whereas the generalist collected novel pollen whether presented alone or with normal pollen. Surprisingly, larvae of the specialist were more flexible than were generalists. The specialist grew well on mixtures of normal and novel pollen species, in some cases better than on its normal host alone. Larvae of the generalist grew more poorly on all diets containing novel pollens than on their normal host. Data on these two species of bees suggest that specialization by itself need not reduce flexibility on novel hosts. The findings also provide information about mechanisms of specialization in bees. Similar to some folivores, specific cues of the pollen host and the bee's interpretation of these contribute, along with foraging economics, to pollen choice by adults. The ability of the larvae to cope with specific components of one pollen species need not interfere with its ability to use others.     

Unique growth paths of heterospecific pollen tubes result in late entry into ovules in the gynoecium of Sagittaria (Alismataceae)

• Pollen‐pistil interactions are a fundamental process in the reproductive biology of angiosperms and play a particularly important role in maintaining incipient species that exist in sympatry. However, the majority of previous studies have focused on species with syncarpous gynoecia (fused carpels) and not those with apocarpous gynoecia (unfused carpels).
• In the present study, we investigated the growth of conspecific pollen tubes compared to heterospecific pollen tubes in Sagittaria species, which have apocarpous gynoecia. We conducted controlled pollinations between S. pygmaea and S. trifolia and observed the growth of conspecific and heterospecific pollen tubes under a fluorescence microscope.
• Heterospecific and conspecific pollen tubes arrived at locules within the ovaries near simultaneously. However, conspecific pollen tubes entered into the ovules directly, whereas heterospecific tubes passed through the carpel base and adjacent receptacle tissue, to ultimately fertilize other unfertilized ovules. This longer route taken by heterospecific pollen tubes therefore caused a delay in the time required to enter into the ovules. Furthermore, heterospecific pollen tubes displayed similar growth patterns at early and peak pollination. The growth pattern of heterospecific pollen tubes at late pollination was similar to that of conspecific pollen tubes at peak pollination.
• Heterospecific and conspecific pollen tubes took different routes to fertilize ovules. A delayed entry of heterospecific pollen into ovules may be a novel mechanism of conspecific pollen advantage (CPA) for apocarpous species.

     

The effect of pollen protein concentration on body size in the sweat bee Lasioglossum zephyrum (Hymenoptera: Apiformes)

Adult bees and wasps provide all the food their offspring require to grow from egg to adult. For a given diet, offspring body size generally increases with an increase in the amount of food consumed as a larva, but the extent to which body size is influenced by the type of food consumed is poorly known. Pollen ranges from 2–60% protein among plant species, and bees are extremely efficient at assimilating nitrogen; therefore, it seems likely that either parent bees adjust the size of larval provisions to compensate for differences in pollen protein concentration or bee offspring attain different body size depending on the pollen type(s) consumed as a larva. We presented the generalist sweat bee Lasioglossum zephyrum with pollen diets that differed in protein content and monitored offspring body size during two experiments. In a protein supplementation experiment, diets ranged from 20–66% protein and consisted of Typha pollen amended with soy protein. On a pollen/soy diet, offspring body size increased 25% with a shift from 20–37% protein, but did not increase further at greater protein concentrations. In a multiple pollen experiment, pollen diets ranged from 20–39% protein and consisted of eight pollens that differed naturally in protein concentration. The largest offspring arose from the most protein-rich pollens, whereas much smaller bees developed on protein-poor pollens. Provision size only predicted offspring size when pollen type, and therefore protein quantity, was considered. Adult foragers did not adjust provision size to compensate for pollen protein. Therefore, offspring body size appears to result from a combination of controlled (provision size) and uncontrolled (pollen quality) factors that arise out of bee foraging decisions.     

Hunting for healthy microbiomes: determining the core microbiomes of <Emphasis Type="Italic">Ceratina,Megalopta</Emphasis>, and <Emphasis Type="Italic">Apis</Emphasis> bees and how they associate with microbes in bee collected pollen

Social corbiculate bees such as honey bees and bumble bees maintain a specific beneficial core microbiome which is absent in wild bees. It has been suggested that maintaining this microbiome can prevent disease and keep bees healthy. The main aim of our study was to identify if there are any core bacterial groups in the non-corbiculate bees Ceratina and Megalopta that have been previously overlooked. We additionally test for associations between the core bee microbes and pollen provisions to look for potential transmission between the two. We identify three enterotypes in Ceratina samples, with thirteen core bacterial phylotypes in Ceratina females: Rosenbergiella, Pseudomonas, Gilliamella, Lactobacillus, Caulobacter, Snodgrassella, Acinetobacter, Corynebacterium, Sphingomonas, Commensalibacter, Methylobacterium, Massilia, and Stenotrophomonas, plus 19 in pollen (6 of which are shared by bees). Unlike Apis bees, whose gut microbial community differs compared to their pollen, Ceratina adults and pollen largely share a similar microbial composition and enterotype difference was largely explained by pollen age. Megalopta displays a highly diverse composition of microbes throughout all adults, yet Lactobacillus and Saccharibacter were prevalent in 90% of adults as core bacteria. Only Lactobacillus was both a core bee and pollen provision microbe in all three species. The consequences of such diversity in core microbiota between bee genera and their associations with pollen are discussed in relation to identifying potentially beneficial microbial taxa in wild bees to aid the conservation of wild, understudied, non-model bee species.     

Invasive Africanized honey bee impact on native solitary bees: a pollen resource and trap nest analysis

Little is known of the potential coevolution of flowers and bees in changing, biodiverse environments. Female solitary bees, megachilids and Centris , and their nest pollen provisions were monitored with trap nests over a 17-year period in a tropical Mexican biosphere reserve. Invasion by feral Apis (i.e. Africanized honey bees) occurred after the study began, and major droughts and hurricanes occurred throughout. Honey bee competition, and ostensibly pollination of native plants, caused changes in local pollination ecology. Shifts in floral hosts by native bees were common and driven by plant phylogenetics, whereby plants of the same families or higher taxa were substituted for those dominated by honey bees or lost as a result of natural processes. Two important plant families, Anacardiaceae and Euphorbiaceae, were lost to competing honey bees, but compensated for by greater use of Fabaceae, Rubiaceae, and Sapotaceae among native bees. Natural disasters made a large negative impact on native bee populations, but the sustained presence of Africanized honey bees did not. Over 171 plant species comprised the pollen diets of the honey bees, including those most important to Centris and megachilids (72 and 28 species, respectively). Honey bee pollination of Pouteria (Sapotaceae) plausibly augmented the native bees' primary pollen resource and prevented their decline. Invasive generalist pollinators may, however, cause specialized competitors to fail, especially in less biodiverse environments.  No claim to original US government works. Journal compilation © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 152–160.     

Spatial predictability and resource specialization of bees (Hymenoptera: Apoidea) at a superabundant, widespread resource

For reciprocal specialization (coevolution) to occur among floral visitors and their host plants the interactions must be temporally and spatially persistent. However, studies repeatedly have shown that species composition and relative abundance of floral visitors vary dramatically at all spatial and temporal scales. We test the hypothesis that, on average, pollen specialist bee species occur more predictably at their floral hosts than pollen generalist bee species. Taxonomic floral specialization reaches its extreme among species of solitary, pollen-collecting bees, yet few studies have considered how pollen specialization by floral visitors influences their spatial constancy. We test this hypothesis using an unusually diverse bee guild that visits creosote bush (Larrea tridentatd), the most widespread, dominant plant of the warm deserts of North America. Twenty-two strict pollen specialist and 80 + generalist bee species visit Larrea for its floral resources. The sites we sampled were separated by 0.5 to > 1450 km, and spanned three distinct deserts and four vegetation zones. We found that species of Larrea pollen specialist bees occurred at more sites and tended to be more abundant than generalists. Surprisingly, spatial turnover was high for both pollen specialist and generalist bee species at all distances, and species composition of samples from sites 1–5 km apart varied as much as repeat samples made at single sites. Nevertheless, the pattern of bee species turnover was not haphazard. As distance among sites increased faunal similarity of sites decreased. Faunal similarities among sites within 250 km of each other were generally greater than if randomly distributed over all sites (the null model). No single ecological category of species (widespread, localized, Larrea pollen specialist, floral generalist) accounted for this spatial predictability. Evidently, concordant local distribution patterns of many ecologically diverse species contribute to the non-random spatial pattern. The ecological dominance of creosote bush does not confer obvious ecological advantages to its specialist floral visitors. Spatial turnover is comparable to that found for bee guilds from other biogeographic regions of the world and is not therefore limited to those bee species that inhabit highly seasonal climates, such as deserts. Philopatry and differences in bloom predictability among sites are probably more important causes for spatial turnover of bee species than are interspecific competition for nest sites or floral resources.     

Diverse Microbiota Identified in Whole Intact Nest Chambers of the Red Mason Bee Osmia bicornis (Linnaeus 1758)

Microbial activity is known to have profound impact on bee ecology and physiology, both by beneficial and pathogenic effects. Most information about such associations is available for colony-building organisms, and especially the honey bee. There, active manipulations through worker bees result in a restricted diversity of microbes present within the colony environment. Microbial diversity in solitary bee nests remains unstudied, although their larvae face a very different situation compared with social bees by growing up in isolated compartments. Here, we assessed the microbiota present in nests and pre-adults of Osmia bicornis, the red mason bee, by culture-independent pyrosequencing. We found high bacterial diversity not comparable with honey bee colonies. We identified a variety of bacteria potentially with positive or negative interactions for bee larvae. However, most of the other diverse bacteria present in the nests seem to originate from environmental sources through incorporated nest building material and stored pollen. This diversity of microorganisms may cause severe larval mortality and require specific physiological or symbiotic adaptations against microbial threats. They may however also profit from such a diverse environment through gain of mutualistic partners. We conclude that further studies of microbiota interaction in solitary bees will improve the understanding of fitness components and populations dynamics.     

Importance of pollen grain volumes for calculating bee diets

Pollen grains harvested by bees differ greatly in volume, thus pollen grains contribute differentially to larval and imaginal nutritional ecology, and ultimately to bee fitness. Simple proportions are inadequate when disentangling the importance of various pollen taxa found on foraging bees, their scopal loads or in their nest provisions. Disparate volumes of pollen grains are an essential feature to be considered in any foraging or dietary study. To document the importance of pollen volume on diet we mixed equal amounts (by weight) of ten morphologically diverse pollen taxa commonly collected by honey bees in the Sonoran desert of Arizona. These taxa were: Cereus giganteus, Ephedra trifurca, Fouquieria splendens, Helianthus annuus, Prosopis juliflora, and Simmondsia chinensis. Additionally, a small grain, Solanum rostratum, and two large grains, Cucurbita foetidissima and Opuntia phaeacantha, rarely harvested by honey bees, were included in the mixture. The mixture was inoculated with calibrated spore tablets, acetolyzed, counted, and % volumes calculated. Pollen grain numbers obtained from the middle of the coverslip were compared with those along the coverslip edge (x²) with no apparent statistically significant difference. The percent by pollen grain number was compared with percentage by pollen grain volume using a Chi-square test.     

The endangered Iris atropurpurea (Iridaceae) in Israel: honey-bees,night-sheltering male bees and female solitary bees as pollinators

Background and Aims

The coastal plain of Israel hosts the last few remaining populations of the endemic Iris atropurpurea (Iridaceae), a Red List species of high conservation priority. The flowers offer no nectar reward. Here the role of night-sheltering male solitary bees, honey-bees and female solitary bees as pollinators of I. atropurpurea is documented.

Methods

Breeding system, floral longevity, stigma receptivity, visitation rates, pollen loads, pollen deposition and removal and fruit- and seed-set were investigated.

Key Results

The main wild pollinators of this plant are male eucerine bees, and to a lesser extent, but with the potential to transfer pollen, female solitary bees. Honey-bees were found to be frequent diurnal visitors; they removed large quantities of pollen and were as effective as male sheltering bees at pollinating this species. The low density of pollen carried by male solitary bees was attributed to grooming activities, pollen displacement when bees aggregated together in flowers and pollen depletion by honey-bees. In the population free of honey-bee hives, male bees carried significantly more pollen grains on their bodies. Results from pollen analysis and pollen deposited on stigmas suggest that inadequate pollination may be an important factor limiting fruit-set. In the presence of honey-bees, eucerine bees were low removal–low deposition pollinators, whereas honey-bees were high removal–low deposition pollinators, because they removed large amounts into corbiculae and deposited relatively little onto receptive stigmas.

Conclusions

Even though overall, both bee taxa were equally effective pollinators, we suggest that honey-bees have the potential to reduce the amount of pollen available for plant reproduction, and to reduce the amount of resources available to solitary bee communities. The results of this study have potential implications for the conservation of this highly endangered plant species if hives are permitted inside reserves, where the bulk of Oncocyclus iris species are protected.     

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

Changes in agricultural practice across Europe and North America have been associated with declines in wild bee populations. Bee diet breadth has been associated with sensitivity to agricultural intensification, but much of this analysis has been conducted at the categorical level of generalist or specialist, and it is not clear to what extent the level of generalisation within generalist species is also associated with species persistence. We used pollen load analysis to quantify the pollen diets of wild solitary bees on 19 farms across southern England, UK. A total of 72 species of solitary bees were recorded, but only 31 species were abundant enough to allow for formal diet characterisation. The results broadly conformed to existing literature with the majority of species polylectic and collecting pollen from a wide range of plants. Pollen load analysis consistently identified pollens from more plant species and families from each bee species than direct observation of their foraging behaviour. After rarefaction to standardise pollen load sample sizes, diet breadth was significantly associated with frequency of occurrence, with more generalist bees present on more farms than less generalist bees. Our results show that the majority of bee species present on farmland in reasonable numbers are widely variable in their pollen choices, but that those with the broadest diet were present on the greatest number of farms. Increasing the diversity of plants included in agri-environment schemes may be necessary to provide a wider range of pollen resources in order to support a diverse bee community on farmland.     

Pollen competition between two sympatric Orchis species (Orchidaceae): the overtaking of conspecific of heterospecific pollen as a reproductive barrier

The frequency of hybrid formation in angiosperms depends on how and when heterospecific pollen is transferred to the stigma, and on the success of that heterospecific pollen at fertilising ovules. We applied pollen mixtures to stigmas to determine how pollen interactions affect siring success and the frequency of hybrid formation between two species of Mediterranean deceptive orchid. Plants of Orchis italica and O. anthropophora were pollinated with conspecific and heterospecific pollen (first conspecific pollen then heterospecific pollen and vice versa) and molecular analysis was used to check the paternity of the seeds produced. In this pair of Mediterranean orchids, competition between conspecific and heterospecific pollen functions as a post‐pollination pre‐zygotic barrier limiting the frequency of the formation of hybrids in nature. Flowers pollinated with heterospecific pollen can remain receptive for the arrival of conspecific pollen for a long time. There is always an advantage of conspecific pollen for fruit formation, whether it comes before or after heterospecific pollen, because it overtakes the heterospecific pollen. The conspecific pollen advantage exhibited in O. italica and O. anthropophora is likely to result from the reduced germination of heterospecific pollen or retarded growth of heterospecific pollen tubes in the stigma and ovary. Overall, the results indicate that our hybrid zone represents a phenomenon of little evolutionary consequence, and the conspecific pollen advantage maintains the genetic integrity of the parental species.     

Pollen specialisation is associated with later phenology in Osmia bees (Hymenoptera: Megachilidae)

1. Species exhibit a range of specialisation in diet and other niche axes, with specialists typically thought to be more efficient in resource use but more vulnerable to extinction than generalists. Among herbivorous insects, dietary specialists seem more likely to lack acceptable host plants during the insect's feeding stage, owing to fluctuations in host-plant abundance or phenology. Like other herbivores, bee species vary in host breadth from pollen specialisation (oligolecty) to generalisation (polylecty).
2. Several studies have shown greater interannual variation in flowering phenology for earlier-flowering plants than later-flowering plants, suggesting that early-season bees may experience substantial year-to-year variation in the floral taxa available to them.
3. It was therefore reasoned that, among bees, early phenology could be a more viable strategy for generalists, which can use resources from multiple floral taxa, than for specialists. Consequently, it was expected that the median dates of collection of adult specimens to be earlier for generalist species than for specialists. To test this, phenology data and pollen diet information on 67 North American species of the bee genus Osmia was obtained.
4. Controlling for latitude and phylogeny, it was found that dietary generalisation is associated with significantly earlier phenology, with generalists active, on average, 11–14 days earlier than specialists.
5. This result is consistent with the generalist strategy being more viable than the specialist strategy for species active in early spring, suggesting that dietary specialisation may constrain the evolution of bee phenology—or vice versa.

     

Asymmetric larval competition between two species of solitary parasitoid wasps: the influence of superparasitism

Abstract.

• 1 We test the hypothesis that a solitary parasitoid wasp may gain in fitness if she lays more eggs in a host.
• 2 Using heterospecific superparasitism (=multiparasitism) between the solitary aphid parasitoids, Aphidius smithi Sharma & Subba Rao and Ephedrus californicus Baker, we show that (i) a superparasitizing female's chance that her offspring will survive competition is an increasing function of egg density, and (ii) survival among same-aged larvae is independent of the oviposition sequence.
• 3 These findings on asymmetric larval competition provide indirect evidence that supports two fundamental, but untested, assumptions underlying models of adaptive superparasitism between conspecific wasps.

     

Insights from measuring pollen deposition: quantifying the pre-eminence of bees as flower visitors and effective pollinators

Using our accumulated datasets from Kenyan savanna, Mediterranean garigue, UK gardens and heathland, involving 76 plants from 30 families, we present detailed data to quantify the superiority of bees as pollinators of most flowering plants when compared with other flower visitors. Bees provided the majority of visits to study species at all sites, and 33 of the 76 plants received more than 90% of their visits from bees. Furthermore, pollen deposition onto stigmas from single-visit events (SVD, a measure of pollination effectiveness) was significantly higher for bees than non-bees at all the four sites where a major proportion of the flora was sampled. Solitary bees, and also bumblebees in temperate habitats, were the best potential pollinators for most plants in this respect, and significantly out-performed honeybees. Only a few plants were well served by bombyliid flies, and fewer again by larger hoverflies, butterflies, or solitary wasps. Bees also achieved better matches of their visit timing to peak pollen availability (measured indirectly as peak SVD), and made much shorter visits to flowers than did non-bees, permitting a substantially greater visit frequency. Additionally, they deposited significantly lower levels of potentially deleterious heterospecific pollen on stigmas in heathland and Mediterranean garigue, though not in the UK garden with densely clustered high-diversity flowering, or in the Kenyan savanna site with particularly dispersed flowering patches and some specialist non-bee flowers. Our data provide a novel and quantified characterisation of the specific advantages of bees as flower visitors, and underline the need to conserve diverse bee communities.     

Origins and ecological consequences of pollen specialization among desert bees

An understanding of the evolutionary origins of insect foraging specialization is often hindered by a poor biogeographical and palaeoecological record. The historical biogeography (20,000 years before present to the present) of the desert-limited plant, creosote bush (Larrea tridentata), is remarkably complete. This history coupled with the distribution pattern of its bee fauna suggests pollen specialization for creosote bush pollen has evolved repeatedly among bees in the Lower Sonoran and Mojave deserts. In these highly xeric, floristically depauperate environments, species of specialist bees surpass generalist bees in diversity, biomass and abundance. The ability of specialist bees to facultatively remain in diapause through resource-poor years and to emerge synchronously with host plant bloom in resource-rich years probably explains their ecological dominance and persistence in these areas. Repeated origins of pollen specialization to one host plant where bloom occurs least predictably is a counter-example to prevailing theories that postulate such traits originate where the plant grows best and blooms most reliably Host-plant synchronization, a paucity of alternative floral hosts, or flowering attributes of creosote bush alone or in concert may account for the diversity of bee specialists that depend on this plant instead of nutritional factors or chemical coevolution between floral rewards and the pollinators they have evolved to attract.     

Generalized host‐plant feeding can hide sterol‐specialized foraging behaviors in bee–plant interactions

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.     

Better safe than sorry? A Fabaceae species exhibits unfavourable pollen properties for developing bee larvae despite its hidden anthers

Empirical evidence suggests that pollen chemistry plays an important role in shaping the pollen host spectra of many bee species. Although the underlying mechanisms are poorly understood, pollen diets of several plant taxa have experimentally been found to impede larval development of unspecialized bees. The pollen of all plant taxa, for which such a detrimental effect on bee larval development has been observed so far, is freely accessible in the flowers and thus easily harvestable for flower visitors, suggesting that this pollen might be chemically protected in order to reduce its loss to pollen-feeding animals. In the present study, we compared larval performance of five solitary bee species on pollen diets of the two Fabaceae species Onobrychis viciifolia and Lotus corniculatus, which have their anthers concealed inside the flowers, with that on control diets composed of host pollen provisions. As the complex flower morphology of the two Fabaceae species already considerably narrows the spectrum of pollen harvesting bee taxa, which might supersede costly chemical protection of the pollen, we expected bees that usually do not exploit Fabaceae to develop well on Onobrychis and Lotus pollen diets. Larval survival on the Onobrychis pollen diet was successful for all five bee species tested. In contrast, larval survival on the Lotus pollen diet was reduced in three species despite the fact that Lotus flowers are more difficult to exploit for pollen than Onobrychis flowers. We conclude that there is no trade-off between pollen concealment and pollen defence in Lotus and that pollen of morphologically complex flowers with a restricted visitor spectrum is not necessarily an easy-to-use nutritional source.