Pollen Host Research Articles

Species of the Western Palaearctic Genus Tetralonia Spinola, 1838 (Hymenoptera, Apidae) with Atypical Pollen Hosts, with a Key to the pollinosa-Group, Description of New Species, and Neotype Designation for Apis malvae Rossi, 1790

The long-horn bee genus Tetralonia consists of 35 Western Palaearctic species that are associated mostly with the family Asteraceae as host plants. A minority of the species are, however, exclusively associated with other host plants that have particularly large pollen grains, such as those in the plant families Caprifoliaceae, Malvaceae, and Onagraceae. This work presents a taxonomic account and morphological description of the assemblages of Tetralonia species with atypical (non-Asteraceae) host plants. It includes a key to the pollinosa-group, which contains most of the species, a description of three regionally restricted new species, namely T. eoacinctella Dorchin sp. nov., T. epilobii Dorchin sp. nov., and T. stellipilis Dorchin sp. nov., a lectotype designation for Eucera cinctella Saunders, 1908 [=Tetralonia cinctella (Saunders, 1908)], and a neotype designation for Apis malvae Rossi, 1790 [=Tetralonia malvae (Rossi, 1790)]. In addition, the name Eucera macroglossa Illiger, 1806 is confirmed as a synonym of Apis malvae Rossi, 1790; Tetralonia macroglossa ssp. xanthopyga Alfken, 1936 is officially placed in synonymy with Apis malvae Rossi, 1790; and Macrocera confusa Pérez, 1902 is listed as a doubtful synonym of Tetralonia scabiosae Mocsàry, 1879 (syn. nov.).

Host-switching by a bee where its usual pollen host is not present: Diadasia diminuta (Cresson, 1878) (Apidae: Eucerinae: Emphorini) uses the rare mallow, Iliamna latibracteata Wiggins (Malvaceae), as its pollen host in northwestern California and southwestern Oregon

Host-switching by a bee where its usual pollen host is not present: Diadasia diminuta (Cresson, 1878) (Apidae: Eucerinae: Emphorini) uses the rare mallow, Iliamna latibracteata Wiggins (Malvaceae), as its pollen host in northwestern California and southwestern Oregon

The hidden diet – examination of crop content reveals distinct patterns of pollen host use by Central European bees of the genus Hylaeus (Hymenoptera, Colletidae)

Masked or yellow-faced bees of the genus Hylaeus (Colletidae) differ in their mode of pollen transportation from most other bees in that they ingest the pollen directly on the flowers and carry it back to the nest inside the crop located in the anterior half of the metasoma. Due to this hidden mode of pollen transportation, the examination of pollen collected by Hylaeus females requires the dissection of the metasoma. Although this method has never been applied in Europe, the great majority of the Central European Hylaeus species were supposed to be pollen generalists based on observations of flower visits. The microscopical analysis of pollen removed from 30 crops each of 36 Central European Hylaeus species revealed that the proportion of species exhibiting an exclusive or strong preference for pollen from a single plant taxon is much higher than hitherto assumed and that the current assumption of the genus Hylaeus to largely consist of pollen generalists is wrong. Nineteen of the 36 species examined are strictly or largely dependent on a single plant taxon for collecting pollen, such as Apiaceae (n = 11 species), Rosaceae (n = 3), Reseda (Resedaceae) (n = 2), Allium (Amaryllidaceae) (n = 1), Asteraceae (n = 1) and Melilotus (Fabaceae) (n = 1). The 36 Hylaeus species examined collected pollen from the flowers of 31 plant families, of which the Apiaceae and Rosaceae (particularly Potentilla and Rubus) were by far the most important contributing almost 60% to the pollen host spectrum of the entire genus. The comparison between pollen host spectrum and flower visiting records showed that the pollen generalists use the flowers of the Asteraceae as nectar rather than pollen sources, corroborating earlier findings that the digestion of Asteraceae pollen requires physiological adaptations to cope with its unfavourable or protective properties. In summary, the patterns of pollen host use by bees of the genus Hylaeus do not substantially differ from those of other Palaearctic bee taxa despite the masked bees’ unusual habit to ingest the pollen directly on the flowers and to transport it inside their body back to the nest.

How wild bees find a way in European cities: Pollen metabarcoding unravels multiple feeding strategies and their effects on distribution patterns in four wild bee species

Abstract Urban ecosystems can sustain populations of wild bees, partly because of their rich native and exotic floral resources. A better understanding of the urban bee diet, particularly at the larval stage, is necessary to understand biotic interactions and feeding behaviour in urban ecosystems, and to promote bees by improving the management of urban floral resources. We investigated the larval diet and distribution patterns of four solitary wild bee species with different diet specialization (i.e. Chelostoma florisomne, Osmia bicornis, Osmia cornuta and Hylaeus communis) along urban intensity gradients in five European cities (Antwerp, Paris, Poznan, Tartu and Zurich) using two complementary analyses. Specifically, using trap‐nests and pollen metabarcoding techniques, we characterized the species' larval diet, assessed diet consistency across cities and modelled the distribution of wild bees using species distribution models (SDMs). Our results demonstrate that urban wild bees display different successful strategies to exploit existing urban floral resources: not only broad generalism (i.e. H. communis) but also intermediate generalism, with some degree of diet conservatism at the plant family or genus level (i.e. O. cornuta and O. bicornis), or even strict specialization on widely available urban pollen hosts (i.e. C. florisomne). Furthermore, we detected important diet variation in H. communis, with a switch from an herbaceous pollen diet to a tree pollen diet with increasing urban intensity. Species distribution modelling indicated that wild bee distribution ranges inside urban ecosystems ultimately depend on their degree of specialization, and that broader diets result in less sensitivity to urban intensity. Policy implications. Satisfying larval dietary requirements is critical to preserving and enhancing wild bee distributions within urban gradients. For high to intermediate levels of feeding specialization, we found considerable consistency in the preferred plant families or genera across the studied cities, which could be generalized to other cities where these bees occur. Identifying larval floral preferences (e.g. using pollen metabarcoding) could be helpful for identifying key plant taxa and traits for bee survival and for improving strategies to develop bee‐friendly cities.

Dufourea ( Dentirophites) in Iberia, with first records of a pollen host for Dufourea lusitanica Ebmer, 1999 (Hymenoptera: Apoidea, Halictidae)

Recent study of the two Iberian species of Dentirophites has revealed a likely pollen host for the Iberian endemic bee, Dufourea lusitanica Ebmer. This suggests that both species are oligoleges of Caryophyllaceae. Photographs are provided for both species: those for D. lusitanica are believed to be the first images of the living insect.

Introduced bees (Osmia cornifrons) collect pollen from both coevolved and novel host-plant species within their family-level phylogenetic preferences.

Studying the pollen preferences of introduced bees allows us to investigate how species use host-plants when establishing in new environments. Osmia cornifrons is a solitary bee introduced into North America from East Asia for pollination of Rosaceae crops such as apples and cherries. We investigated whether O. cornifrons (i) more frequently collected pollen from host-plant species they coevolved with from their geographic origin, or (ii) prefer host-plant species of specific plant taxa independent of origin. To address this question, using pollen metabarcoding, we examined the identity and relative abundance of pollen in larval provisions from nests located in different landscapes with varying abundance of East-Asian and non-Asian plant species. Our results show that O. cornifrons collected more pollen from plant species from their native range. Plants in the family Rosaceae were their most preferred pollen hosts, but they differentially collected species native to East Asia, Europe, or North America depending on the landscape. Our results suggest that while O. cornifrons frequently collect pollen of East-Asian origin, the collection of pollen from novel species within their phylogenetic familial affinities is common and can facilitate pollinator establishment. This phylogenetic preference highlights the effectiveness of O. cornifrons as crop pollinators of a variety of Rosaceae crops from different geographic origins. Our results imply that globalization of non-native plant species may ease the naturalization of their coevolved pollinators outside of their native range.

Palaearctic Osmia bees of the subgenera Hemiosmia, Tergosmia and Erythrosmia (Megachilidae, Osmiini): biology, taxonomy and key to species

Osmia (Hemiosmia), O. (Tergosmia) and O. (Erythrosmia) are three related Palaearctic osmiine bee subgenera (Megachilidae, Megachilinae, Osmiini) comprising eight, seven and four species, respectively. Analysis of female pollen loads and field observations revealed that the species of both O. (Hemiosmia) and O. (Tergosmia) have a distinct or even exclusive preference for Fabaceae as pollen hosts. Species of O. (Erythrosmia) also collect pollen on Fabaceae, but additionally frequently visit nototribic flowers of Lamiaceae and Antirrhineae (Plantaginaceae), which they exploit by rubbing a specialized facial pilosity over the raised anthers. The representatives of O. (Hemiosmia) and the basal members of O. (Tergosmia) build their brood cells in self-excavated burrows in the ground, whereas the more derived O. (Tergosmia) species as well as the O. (Erythrosmia) species place them in preexisting cavities above-ground, which are empty snail shells hidden under prominences of rocks or under stones in the latter subgenus. In O. (Hemiosmia) and O. (Tergosmia), the brood cells are entirely constructed from foreign material and either consist of chewed leaves ("leaf pulp") in the former subgenus or are distinctly three-layered with a thin layer of mud sandwiched between two layers of large pieces of petals in the latter subgenus. In O. (Erythrosmia), the brood cells within the shell whorls are delimited by one-layered partitions of leaf pulp and the nests are closed by a barricade of densely packed small particles followed by a double wall of leaf pulp at the shell opening. The taxonomic revision of the three subgenera revealed the existence of an undescribed species, O. (Hemiosmia) spinicoxa spec. nov., which occurs in southwestern Morocco. The subgenus O. (Ozbekosmia) Zanden, 1994 syn. nov. is newly put into synonymy with O. (Tergosmia) Warncke, 1988. Osmia (Tergosmia) rhodoensis arquata Warncke, 1988 syn. nov. and O. (Tergosmia) tergestensis ononidis Ferton, 1897 syn. nov. are no longer accepted as valid subspecies of O. (Tergosmia) rhodoensis (Zanden, 1983) and O. (Tergosmia) tergestensis Ducke, 1897, respectively. Identification keys for all species of the three subgenera are given.

Taxonomic revision of the aequata-group of the subgenus Eucera s. str(Hymenoptera, Apidae, Eucerini).

The aequata-group of the subgenus Eucera s. str. Scopoli from the Eastern Mediterranean region, Bulgaria, and Iran is described, and the three species included are revised. The little-known E. aequata Vachal 1907 known from Turkey, Cyprus, Syria and Israel, is redescribed and a Lectotype is designated. Two species are described as new: E. dafnii sp. nov. from Iran, Israel, Syria, Turkey, Bulgaria, and Greece, and E. wattsi sp. nov. from Israel and Lebanon. An identification key is provided, and natural history information including assessment of preferred pollen host plants is presented.

Key pollen host plants provide balanced diets for wild bee larvae: A lesson for planting flower strips and hedgerows

Abstract Bee‐friendly plants are defined by the quantity of food they produce and the visitation rates of adult insects foraging for nectar. However, it is pollen nutritional quality that enables proper larval development of bees, affecting their populations. Not all plants produce pollen that satisfies the nutritional requirements of bee larvae, and we lack an understanding of how different plant pollens impact bee nutritional demands. This study examined whether nutritionally desirable key plant species may promote wild bee larval development, which is essential if the population is to thrive. The generalist solitary mason bee Osmia bicornis L. was used as a model species to examine differences between bee larva nutritional demand and host plant nutrient supply; an ecological stoichiometry framework was applied. The stoichiometric ratios of 12 elements were investigated in bee bodies and cocoons (reflecting nutritional demand) and in the pollen supplied by the mother (nutritional supply; N = 15 × 2 sexes). Similarly, the stoichiometry of 62 pollen taxa, including native, alien, and garden plants and crops, was compared with the bee demand based on the literature. Compared to males, females had higher demands for P, Cu and Zn and were supplied with pollen richer in these elements. Therefore, when collecting pollen for their progeny, Osmia provides daughters and sons with different pollen mixtures, reflecting sex‐specific nutritional demand. Bees may be limited by the availability of P, Na, Mn, Mg, K, Fe, Ca, Zn and Cu, with high taxonomic variability in their concentrations in pollen. Female fitness may be particularly related to a high proportion of P in the diet. Synthesis and applications. Access to key plant species that allow nutritionally balanced larval diets may be essential for bee development, whether food is gathered intentionally or randomly. Such plant species—and not only those rich in nectar and pollen—should be promoted in wild bee conservation efforts, including planting flower strips and hedgerows. Bee‐friendly plants should not be defined and planted solely based on the quantities of food they produce and on the visitation rates of adult insects foraging for energy.

Pollen foraging behaviour of honey bee (Apis mellifera L.) in southern West Bengal, India

Pollen loads of Apis mellifera were analysed to assess their daily pollen collection, polleniferous flora and their pollen biomass contribution in southern West Bengal, India. After that, the pollen host selection and pollen foraging activity of the bee species were also determined. The average daily harvest of pollen loads per honey bee colony was 8.5 g to 127.2 g. The pollen flow through the colonies was the highest during spring, followed by winter and summer, whereas during the monsoon it was the lowest. The weight of a single pollen load ranged from 3.22 mg to 11.73 mg and greatly varied within each season, but seasonal average values do not statistically differ. Families that contributed major amount of biomass were Arecaceae, Myrtaceae, Brassicaceae, Fabaceae, Pedaliaceae and Apiaceae. Taxa providing large amounts of biomass were Brassica nigra, Eucalyptus globulus, Borassus flabellifer, Sesamum indicum, Acacia auriculiformis, Coriandrum sativum, Cocos nucifera, Phoenix sylvestris and Mikania scandens (Asteraceae). As the bee species foraged on 26.32% (80 plant taxa) of the local angiosperm flora, the broad polylecty category was inferred. The pollen host selection by the bee species significantly depends on the flower availability index (FAi) and pollen availability index (PAi) rather than on an individual flower’s pollen quality (in term of pollen morphological characteristics and crude protein content) and pollen quantity (number of pollen grains/flower and amount of pollen biomass/flower). The pollen foraging activity of the bee species varied according to time as well as season. The activity was higher between 9.00 and 10.00 am. Seasonally, the highest activity was observed in spring and the lowest during the monsoon. Furthermore, the ratio of pollen foragers to total incoming foragers also varied according to the time of day as well as the season, and was largely dependent of resource availability.

Hidden diversity in European bees: Andrena amieti sp. n., a new Alpine bee species related to Andrena bicolor (Fabricius, 1775) (Hymenoptera, Apoidea, Andrenidae)

We revise the Alpine bee taxa related toAndrenabicolor(Fabricius, 1775), includingA.montanaWarncke, 1973 andA.allosaWarncke, 1975, the status of which has remained contentious. Phylogenetic analyses of one mitochondrial gene and one nuclear gene, as well as morphological examination reveal the presence of four Alpine species in this complex, one of which is new to science,A.amietisp. n.This new species is widely distributed in the Alps from southern France throughout Switzerland, northern Italy and southern Germany to Austria; a single record is known from the Apennines. The type locality is located within the Unesco World Heritage site “Swiss Alps Jungfrau-Aletsch”. Two widely divergent mitochondrial lineages are found in sympatry inA.amietisp. n.; the status of these lineages, which together form a paraphyletic unit from whichA.allosaarose, is briefly discussed. We show thatA.allosa,A.amietisp. n.andA.montanaare polylectic but that each species exhibits a distinct spectrum of pollen hosts: the univoltineA.allosashows affinities for pollen of the early-blooming Alpine plant genusCrocus.A.amietisp. n.is bivoltine and, as inA.bicolor, the summer generation exhibits a distinct preference for Campanulaceae, while the spring generation is widely polylectic.A.montanahas a single generation in the summer and forages on a diversity of flowers such as Campanulaceae, Cistaceae and Caryophyllaceae. An identification key is presented for central European members of the subgenus Euandrena Hedicke, 1932. Lastly, the new Alpine species appears to represent the tip of the iceberg of substantial cryptic diversity in southern European Andrena (Euandrena):A.croaticaFriese, 1887 is resurrected from synonymy withA.bicolorand treated as a valid species (stat. rev.),A.pileataWarncke, 1875, described as a subspecies ofA.allosa, is elevated to species rank (stat. n.), and three additional unclear taxa are briefly described.

Pollen host selection by predominantly alpine bee species of the genera Andrena, Panurginus, Dufourea, Megachile, Hoplitis and Osmia (Hymenoptera, Apoidea)

The pollen host selection by 19 bee species, which have their main Central European distribution in the Alps, was assessed by microscopical analysis of the scopal contents of about 900 females from museum and private collections. The results of the pollen analyses were complemented by a literature survey as well as by field observations. The examined species widely vary in pollen host spectrum and specialization, revealing a fascinating diversity in bee host plant use. Observed patterns of host plant choice range from narrow specialists, which exclusively collect pollen from the flowers of a single plant genus, to pronounced generalists, which harvest pollen from the flowers of up to 17 different plant families. A quantitative character is given to separate the morphologically very similar females ofPanurginusherziandP.montanus.

Biology of Palaearctic Wainia bees of the subgenus Caposmia including a short review on snail shell nesting in osmiine bees (Hymenoptera, Megachilidae)

Wainia, a species-poor genus of osmiine bees, contains two Palaearctic species, whose biologies are unknown. In the present publication, we describe the nesting site and nest architecture ofW.sexsignataand analyse the pollen host spectra ofW.sexsignataandW.eremoplanaby microscopical analysis of larval faeces and female scopal contents.W.sexsignatanests in empty snail shells. It uses large petal pieces of yellow flowered Asteroideae to separate the linearly arranged brood cells and to construct a series of partitions between the outermost cell and the final nest closure, which consists of masticated green leaves.W.sexsignatais recorded as a new host of the meloid beetleSitarobrachysthoracica. Both PalaearcticWainiaspecies have a strong preference for Asteraceae as pollen hosts, but occasionally also collect pollen on other plant taxa. A literature survey revealed that nesting in empty snail shells is widespread among osmiine bees: obligate snail shell nesting is assumed to have independently evolved at least twelve times in their evolutionary history and to occur in at least 56 species belonging to 4 genera and 14 subgenera.

Pollen preferences among the bee species visiting apple (Malus pumila) in New York

Maintaining a diverse and abundant wild bee community is essential for sustainable agricultural pollination, especially in crops in which wild bees are effective pollinators. Many land managers have an economic interest in encouraging healthy wild bee populations, but it is not always clear how to accomplish this. In apple orchards, wild bees play a critical role as pollinators, but are active before most forbs begin to flower. To investigate which flowering plant species might serve as alternative pollen hosts for the most abundant wild bee species collected in eastern apple orchards, we analyzed the pollen carried by 15 wild bee species and 1 managed bee species (Apis mellifera) collected during apple bloom. We identified the pollen grains carried by these bees and found that the majority of alternative pollen hosts for apple visitors are early blooming tree species, including six other tree genera. This may partly explain why the presence of forest fragments is an important predictor of bee abundance and species richness in New York apple orchards.

Crop domestication facilitated rapid geographical expansion of a specialist pollinator, the squash bee Peponapis pruinosa.

Squash was first domesticated in Mexico and is now found throughout North America (NA) along with Peponapis pruinosa, a pollen specialist bee species of the squash genus Cucurbita The origin and spread of squash cultivation is well-studied archaeologically and phylogenetically; however, no study has documented how cultivation of this or any other crop has influenced species in mutualistic interactions. We used molecular markers to reconstruct the demographic range expansion and colonization routes of P. pruinosa from its native range into temperate NA. Populations east of the Rocky Mountains expanded from the wild host plant's range in Mexico and were established by a series of founder events. Eastern North America was most likely colonized from squash bee populations in the present-day continental Midwest USA and not from routes that followed the Gulf and Atlantic coasts from Mexico. Populations of P. pruinosa west of the Rockies spread north from the warm deserts much more recently, showing two genetically differentiated populations with no admixture: one in California and the other one in eastern Great Basin. These bees have repeatedly endured severe bottlenecks as they colonized NA, following human spread of their Cucurbita pollen hosts during the Holocene.

Palaearctic Hoplitis bees of the subgenera Formicapis and Tkalcua (Megachilidae, Osmiini): biology, taxonomy and key to species.

Formicapis and Tkalcua are related subgenera of the osmiine bee genus Hoplitis (Megachilidae) containing four and three species, respectively. H. (Formicapis) species inhabit open forests and windfalls mainly in the boreal zone with one species distributed across the Holarctic and three species occurring in eastern Asia. H. (Tkalcua) species live in deserts and semideserts from the Canary Island and northern Africa eastwards to the Arabian Peninsula. Analysis of female pollen loads, field observations and literature data reveal that H. (Formicapis) species have a distinct or even exclusive preference for the flowers of herbaceous Rosaceae as pollen hosts, while H. (Tkalcua) species appear to be pollen generalists. Nesting sites of H. (Formicapis) are insect burrows in dead wood, while the only H. (Tkalcua) species, whose nesting behaviour is known, builds its brood cells inside empty snail shells. Species of both subgenera construct brood cell partitions and nest plug with masticated leaves, which, however, are mixed with sand grains in H. (Tkalcua) zandeni (Teunissen & van Achterberg 1992). The taxonomic revision of H. (Formicapis) and H. (Tkalcua) revealed the existence of two undescribed species, H. (Formicapis) coreensis spec. nov. from South Korea and H. (Tkalcua) emiratensis spec. nov. from the United Arab Emirates. Hoplitis excisa (Morawitz 1880) is newly assigned to H. (Formicapis), whereas H. latifemoralis (Wu 1985), formerly considered to belong to H. (Formicapis), is removed from this subgenus due to morphological and biological differences and newly assigned to Hoplitis subgenus incertum. Identification keys for all H. (Formicapis) and H. (Tkalcua) species are given.

Nest architecture and pollen hosts of the boreoalpine osmiine bee species Hoplitis (Alcidamea) tuberculata (Hymenoptera, Megachilidae)

Although Hoplitis tuberculata is a rather common bee species in the upper montane and subalpine zone of the Alps, its biology is only fragmentarily known. In the present publication, both nest architecture and pollen host spectrum are described. H. tuberculata nests in insect borings in dead wood, where one to several brood cells are built in a linear series. Examination of four nests obtained from trap nests revealed three peculiar characteristics of its nest architecture: i) the 0.3-0.5 cm thick partitions between the brood cells are three-layered consisting of two walls built from masticated leaves which enclose an interlayer that is densely packed with pebbles, earth crumbs and other small particles; ii) in the majority of the nests, a vestibule varying in length from 2.2-8.9 cm and loosely filled with small particles is present between the outermost cell partition and the nest plug; iii) the nest is sealed by a 1.2-1.9 cm long plug consisting of two walls of masticated leaves which enclose a space that is densely packed with small particles and divided up by one to three additional walls. The nest architecture of H. tuberculata is unique among Palaearctic osmiine bees; however, it corresponds to that of three North American species closely related to H. tuberculata. Microscopical analysis of female pollen loads and brood cell provisions revealed that H. tuberculata is polylectic with a strong preference for Fabaceae. Among the Fabaceae, Lotus and Hippocrepis were by far the most important pollen hosts. Non-Fabaceae taxa represented by substantial proportions in pollen loads or cell provisions were Helianthemum (Cistaceae), Vaccinium (Ericaceae) and Rubus (Rosaceae).

Pollen host selection by a broadly polylectic halictid bee in relation to resource availability

The relationship between utilized and available resources in the same environment has not yet been studied in most wild bees. In a riparian forest, a total of 15 pollen types from 10 plant families were identified from pollen provisions of 17 nests of the wood-dwelling Augochlora amphitrite (Hymenoptera: Halictidae) for which only floral records have been known. During the foraging period, only five out of 108 available flowering species were visited intensively (more than 10 %) for pollen. As more than three families and 32.2 % of the available plant families were visited for pollen, the broad polylecty category was inferred. Like in other Augochlora species, a variety of floral types was visited by A. amphitrite, in agreement with its broadly polylectic foraging habit. The selection index was positive for the highly abundant Ludwigia (Onagraceae) and the rare Gymnocoronis spilanthoides (Asteraceae). The intensive foraging on Ludwigia could indicate temporary specialization for this abundant pollen resource. The tendency of this bee to visit intensively one flower type for pollen suggests that bee individuals had to learn to handle this abundant flower type. Pollen analysis complemented with field observations on floral resource availability proved to be helpful tools to provide information on the foraging behaviour of a wild bee in its natural environment.

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

Evidence for declining populations of both wild and managed bees has raised concern about a potential global pollination crisis. Strategies to mitigate bee loss generally aim to enhance floral resources. However, we do not really know whether loss of preferred floral resources is the key driver of bee decline because accurate assessment of host plant preferences is difficult, particularly for species that have become rare. Here we examine whether population trends of wild bees in The Netherlands can be explained by trends in host plants, and how this relates to other factors such as climate change. We determined host plant preference of bee species using pollen loads on specimens in entomological collections that were collected before the onset of their decline, and used atlas data to quantify population trends of bee species and their host plants. We show that decline of preferred host plant species was one of two main factors associated with bee decline. Bee body size, the other main factor, was negatively related to population trend, which, because larger bee species have larger pollen requirements than smaller species, may also point toward food limitation as a key factor driving wild bee loss. Diet breadth and other potential factors such as length of flight period or climate change sensitivity were not important in explaining twentieth century bee population trends. These results highlight the species-specific nature of wild bee decline and indicate that mitigation strategies will only be effective if they target the specific host plants of declining species.

The oligolectic bee Osmia brevis sonicates Penstemon flowers for pollen: a newly documented behavior for the Megachilidae

Flowers with poricidally dehiscent anthers are typically nectarless but are avidly visited and often solely pollinated by bees that sonicate the flowers to harvest pollen. Sonication results from shivering the thoracic flight muscles. Honey bees (Apis) and the 4,000+ species of Megachilidae are enigmatic in their seeming inability to sonicate flowers. The oligolectic megachilid bee Osmia brevis was found audibly sonicating two of its beardtongue pollen hosts, Penstemon radicosus and P. cyananthus. The bees’ high-pitched sonication sequences are readily distinguishable from flight sounds in audiospectrograms, as well as sounds that result from anther rasping. Instead, floral sonication by O. brevis resembles the familiar sounds of bumblebees buzzing, in this case while visiting P. strictus flowers.