Is the caste-ratio of the oligolectic bumblebee Bombus gerstaeckeri Morawitz (Hymenoptera: Apidae) biased to queens?

Three bumblebee species, foraging on Aconitum spp. have been commonly observed in Eyne (France, East Pyrénées): Bombus gerstaeckeri, B. hortorum and B. wurflenii. We estimated the population of these three species. For B. hortorum and B. wurflenii, the total workers populations foraging on Aconitum spp. ranged from 101 to 523 and 156 to 270, respectively. These two species also forage on other plants while B. gerstaeckeri visits only Aconitum spp. The population of B. gerstaeckeri observed was extremely small, founded by 33 queens only in 2001. With a total number of workers estimated from 40 to 102, the observed workers/queens ratio, 1 to 3 workers for each queen, is very unusual for a eusocial species. Also we observed queens foraging during the whole life of the colony. This kind of social organisation is similar to that of some high arctic species. It could be interpreted as the result of an insularity syndrome.     

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

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. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 719–733.     

Patterns of host-plant choice in bees of the genus Chelostoma: the constraint hypothesis of host-range evolution in bees

To trace the evolution of host-plant choice in bees of the genus Chelostoma (Megachilidae), we assessed the host plants of 35 Palearctic, North American and Indomalayan species by microscopically analyzing the pollen loads of 634 females and reconstructed their phylogenetic history based on four genes and a morphological dataset, applying both parsimony and Bayesian methods. All species except two were found to be strict pollen specialists at the level of plant family or genus. These oligolectic species together exploit the flowers of eight different plant orders that are distributed among all major angiosperm lineages. Based on ancestral state reconstruction, we found that oligolecty is the ancestral state in Chelostoma and that the two pollen generalists evolved from oligolectic ancestors. The distinct pattern of host broadening in these two polylectic species, the highly conserved floral specializations within the different clades, the exploitation of unrelated hosts with a striking floral similarity as well as a recent report on larval performance on nonhost pollen in two Chelostoma species clearly suggest that floral host choice is physiologically or neurologically constrained in bees of the genus Chelostoma. Based on this finding, we propose a new hypothesis on the evolution of host range in bees.     

Estimating the population size of specialised solitary bees

Abstract.  1. Reliable methods for quantifying population size are crucial for strategies to conserve endangered wild-bee species. Estimates of population size obtained through survey walks were compared with estimates obtained through mark–recapture studies in 10 populations of the red-listed solitary bee Andrena hattorfiana in southern Sweden.
2. The mean number of bees observed during survey walks was strongly correlated with estimates of population size obtained with mark–recapture. It was found that 5.5–23.4% (mean 13.4%) of the total population was observed during an average survey walk.
3. One component in mark–recapture analysis is the measure of survival of individuals. In the largest bee population, females of A. hattorfiana that emerged in early season were found to forage for pollen on average 18.4 days.
4. The findings suggest that during large-scale surveys, for example re-inventories for red-listed species, the population size of solitary bees can be quantified reliably and effectively by performing survey walks in a two-step process. The first step consists of survey walks to establish the relationship between number of bee observations per survey walk and mark–recapture population size for a small set of populations. In the second, simple observation survey walks can be performed for a large set of populations. In each population of A. hattorfiana , it is recommended that at least six survey walks are performed.     

Osmia species (Hymenoptera, Megachilidae) from the southeastern United States with modified facial hairs: taxonomy, host plants, and conservation status

We describe females and males of Osmia (Melanosmia) calaminthaesp. n., an apparent floral specialist on Calamintha ashei (Lamiaceae), and provide observations on the behavior of female bees on flowers of this plant. We also provide diagnostic information for Osmia (Diceratosmia) conjunctoides Robertson, stat. n., and synonymize Osmia (Diceratosmia) subfasciata miamiensis Mitchell with Osmia conjunctoidessyn. n. Females of both Osmia calaminthae and Osmia conjunctoides are unique among North American Osmia for having short, erect, simple facial hairs, which are apparent adaptations for collecting pollen from nototribic flowers. Osmia calaminthae is currently only known from sandy scrub at four nearby sites in the southern Lake Wales Ridge in Highlands County, Florida, USA, while Osmia conjunctoides is known from limited but widespread sites in the southeastern USA. We discuss the conservation status of both species based on known or speculated floral associates and distributions.     

Pollen-host specificity and evolutionary patterns of host switching in a clade of specialist bees (Apoidea: Diadasia)

We examined the levels of pollen-host specificity in North American Diadasia (Hymenoptera: Apoidea), a clade of specialist bees. We analysed the scopal pollen loads of 409 individuals representing 25 of the 30 species of Diadasia that occur in North America. Each Diadasia species showed a preference for one of five plant families. However, the 25 species varied in their level of host specificity: the average percentage by volume of the preferred host in pollen loads ranged from > 99% to < 75%. In 17 of the 25 species, all or most individuals examined contained pure loads of one host taxon, while in eight species individuals were less specialized and carried mixtures of several unrelated host taxa. Mapping these host preferences onto a phylogenetic tree indicated that Malvaceae is the most likely ancestral host for the genus, and use of other hosts can be explained by a single switch to each of the other four host-plant families. Thus, most speciation events were not associated with a host switch; this pattern does not support host switching as a niche partitioning strategy to avoid competition. Diadasia species are more likely to use host-plant families that are used by other Diadasia and Emphorine bees; however, there was no evidence of residual adaptation to ancestral hosts. Diet breadth appears to be a labile trait: transitions from narrower to broader host use, as well as vice versa, were observed. The observed patterns of host-use evolution may be driven, in part, by host morphology and/or chemistry.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 487–505.     

Stamen movements in flowers ofOpuntia (Cactaceae) favour oligolectic pollinators

Opuntia brunneogemmia andO. viridirubra occur sympatrically in the Serra do Sudeste, Rio Grande do Sul, Brazil. Their flowers have 450–600 thigmonastic stamens and provide large amounts of pollen and nectar for bees. Bees of 41 species were registered at the flowers ofO. brunneogemmia and 30 at the flowers ofO. viridirubra. Females of three oligolectic species are the only effective pollinators:Ptilothrix fructifera (Anthophoridae),Lithurgus rufiventris (Megachilidae), andCephalocolletes rugata (Colletidae). During their visits inOpuntia-flowers, bees touch the filaments and stimulate the movement of the stamens to the centre of the flower. At the end of this movement, the anthers are densely packed around the style. As a consequence the pollen is presented in an easily accessible upper layer of anthers and various, nearly inaccessible lower layers. The lower layers contain about 80% of the pollen reward. Only females of the three oligolectic pollinators exploit the pollen from the lower layers and reach the nectar furrow. Therefore, through their stamen movements,Opuntia flowers hide most of their pollen from flower visitors but favour effectively pollinating, oligolectic bees.     

Micro-Foraging Routes of Bicolletes pampeana (Colletidae) and Bee-Induced Pollen Presentation in Cajophora arechavaletae (Loasaceae)

In flowers of Cajophora arechavaletae Urb. the stamens are hidden from flower visitors in naviculate petals. In the male phase the stamens successively migrate at irregular intervals to the centre of the flower where they present pollen. Therefore, non-specialised pollinators cannot predict the time of pollen presentation. The oligolectic females of Bicolletes pampeana are effective pollinators of Cajophora arechavaletae. Females and males can elicit stamen movements by pressing the scales of the nectaries outwards with their head while taking up nectar. If this stimulus is responded to, up to 3 stamens move and reach the centre of the flower on average after 2.4 min. Experiments showed that the stimulus of nectar scale pressing was responded to maximally when the inter-stimulus interval was at least 14 min. B. pampeana females have evolved a foraging strategy which is adapted to the unique pollen presentation of C. arechavalete flowers. On microforaging routes they trapline 30–60 flowers and, therefore, most frequently come back to the same flower after intervals of about 3 min. This is exactly the period after which a female can expect pollen in the centre of the flower if her previous stimulus was responded to. Competition between the females causes them to return to the flowers at such short intervals.