Pollen Specialist Bees of the United States

Jarrod Fowler (2020)


This website compiles associations among native pollen specialist bees and native host plants recorded across the Western, Central, and Eastern United States. First, pollen specialist bees are defined and methods are described. Next, a table composed of pollen specialist bees and associated host plants is presented. Last, advice about conserving native pollen specialist bees is provided. Please note that this compilation is incomplete and in progress. Researchers are invited to add information to this website. Email records to: j@jarrodfowler.com

Fowler, J. (2020). Pollen Specialist Bees of the Western United States
Fowler, J. (2020). Pollen Specialist Bees of the Central United States
Fowler, J. & Droege, S. (2020). Pollen Specialist Bees of the Eastern United States

Pollen Specialist Bees

Locally, ~15% to 50% of the ~3,600 species of bees native to the contiguous United States are pollen specialists (Moldenke 1979). Pollen specialist bees evolved a continuum of facultative or obligate associations with flowering host plants (Cane & Sipes 2006; Hurd et al. 1980; Linsley & MacSwain 1958; Robertson 1925). Oligolectic bees or oligoleges specifically associate with one host plant family [a.k.a. mesolectic bees or mesoleges] or a few related genera or species and monolectic bees or monoleges specifically associate with a single host plant genus or species, while polylectic bees or polyleges generically forage unrelated plants (Cane & Sipes 2006; Robertson 1925).

Pollen specialist associations can mutually benefit both bees and flowers from improved foraging effectiveness and efficiency, pollen digestibility, and pollination rates, but foraging restrictions may create greater susceptibility to harm from pollination or pollinization shortages due to habitat degradation, fragmentation, and loss or phenological mismatch (Minckley et al. 1994; Packer et al. 2005; Rafferty et al. 2015). Therefore, contemporary anthropogenic threats in the United States potentially imperil native pollen specialist bee species and their endemic, indigenous host plants with population declines and extinctions through loss of species diversity.


Records of native pollen specialist bees captured or observed on flowers of host plants were compiled from Discover Life (Ascher & Pickering 2020), peer reviewed articles (Bouseman & LaBerge 1978; Brooks & Griswold 1988; Cockerell 1916, 1919; Cresson 1878; Daly 1973; Danforth 1994; Donovan 1977; Griswold 1993; Griswold & Miller 2010; Hurd et al. 1980; LaBerge 1963, 1967, 1969, 1971a, 1971b, 1973, 1977, 1980, 1985, 1986, 1989; LaBerge & Bouseman 1970; LaBerge & Ribble 1972, 1975; Lanham, 1981; Linsley & MacSwain 1958; McGinley 2003; Michener 1939; Minckley et al. 1994; Moldenke 1976, 1979; Parys et al. 2018; Portman, Neff, & Griswold 2016; Provancher, 1895; Ribble 1974; Robertson 1926, 1928, 1929; Rozen 1958; Snelling 1983; Thorp 1969; Timberlake 1954, 1956, 1958, 1960, 1962, 1964, 1968, 1975, 1980), technical bulletins (Danforth 1996; Grigarick & Stange 1968; Hurd & Michener 1955; Krombein et al. 1979; LaBerge 1967; Mitchell 1960, 1962; Ribble 1968; Stephen 1954; Thorp & LaBerge 2005; Timberlake 1953), and personal communications. Observations of social behaviors and nesting biologies were compiled from peer reviewed articles (Bohart 1964; Cross & Bohart 1960; Cane 1994; Daly 1961; Eickwort 1977; Eickwort et al. 1981; Grundel et al. 2011; Hicks 1926; Hobbs & Lilly 1954; Kerfoot 1964; LaBerge & Ribble 1966; Mathewson 1968; Michener 1975; Michener & Ordway 1963; Miliczsky 1988; Neff & Simpson 1997; Neff et al. 1982; Parker et al. 1981; Parker et al. 1986; Rozen 1964, 1973; Rozen & Favreau 1968; Rozen & Hall 2011; Rozen & Jacobson 1980; Wcislo & Engel 1996) and personal communications.

If bees and plants were indigenous to the United States without human intervention, then bees and plants were defined "native". Definitions of bee conservation status were averaged across ranges. If there were on average: between one and 100 records, then bees were defined "Rare"; between one and 500 records, then bees were defined "Uncommon-Rare"; around 500 records, then bees were defined "Uncommon"; between 500 and 1000 records, then bees were defined "Common-Uncommon"; around 1000 or more records, then bees were defined "Common". Conservation status may vary according to species-specific distributions and dispersions. Bee-flower records were compared with state-level plant distributions from the USDA Plants Database (USDA NRCS 2020) and county-level distributions from The Biota of North America Program (Kartesz, BONAP 2015). Our study includes only host plant genera that are native and documented as present in at least one county of at least one of the 48 contiguous states. Because plant taxonomy has undergone major recent changes, many names have been updated compared to those published in the original literature of bee-flower associations.


Pollen specialist bee species recorded across the contiguous United States are alphabetically tabulated with associated host plant families in Table 1. Six families, eight subfamilies, 11 tribes, two subtribes, 24 genera, 38 subgenera, and 83 species of pollen specialist bees were listed. The bee family with the most pollen specialists was Andrenidae (46 spp.), while the most speciose subfamily was Andreninae (33 spp.), tribe was Eucerini (11 spp.), subtribe was Perditina (seven spp.), genus was Andrena (33 spp.), and subgenus was Callandrena s.l.. The most recurrent authorities (n = 15 authors) were Charles Robertson (1858–1935, 21 spp.), Ezra Townsend Cresson (1838–1926, 17 spp.), and Frederick Smith (1805–1879, 10 spp.). Most species were first described during years 1891 (12 spp.), 1878 (nine spp.), and 1853 (eight spp.).

Overall state-specific percentages of pollen specialist bees are presented in Figure 1. One bee species was recorded in all 48 contiguous states: Melissodes (Eumelissodes) agilis Cresson, 1878. Overall, 33 spp. were “Uncommon-Rare", while 26 bee species were considered "Rare", 18 spp. were "Uncommon", six spp. were “Common-Uncommon", and zero species were "Common". Generally, 100% of species showed solitary behaviors: 48 species were "Solitary", while 32 spp. were "Solitary-Gregarious" and three spp. were "Solitary-Communal". Over 90% of species nested in burrows: 51 species nested in "Soil Burrows", while nine spp. nested in "Sand Burrows", five spp. nested in "Compacted Soil Burrows", five spp. nested in "Preexisting Wood Cavities”, four spp. nested in "Deep Sand Burrows", three spp. nested in “Compacted Sand Burrows”, and the remaining six species nested in clay, marl, or loam burrows, sloping sand or soil burrows, or preexisting cavities.

The months of activity for pollen specialist bees relative to associated host plant flowering phenology were June (54 spp.), August (52 spp.), July (51 spp.), May (50 spp.), September (43 spp.), April (36 spp.), October (17 spp.), March (16 spp.), November (seven spp.), February (two spp.) December (one spp.), and January (one spp.). Oftentimes, pollen specialist bees had only four month (24 spp.) or three month (21 spp.) flight periods. The most recurrent host plant family among pollen specialist bee species was Asteraceae (34 bee spp.). The most recurrent host plant genera (N = ~100) associated with pollen specialist bee species were Helianthus L. (24 spp.), Grindelia Willd. (19 spp.), Solidago L. (17 spp.), Rudbeckia L. (14 spp.), Salix L. (12 spp.), Symphyotrichum Nees (12 spp.), Coreopsis L. (11 spp.), Chrysothamnus Nutt. (10 spp.), and Verbesina L. (10 spp.).


Table 1. Pollen specialist bee families (n = 6), genera (n = 24), and species (n = 83) with taxonomic authorities, distributions according to records from the 48 contiguous states, regional conservation status (Common, Uncommon, or Rare), social behaviors, nesting biologies, phenology according to records from 12 months, and host plant families, tribes, and genera. Bee families and member genera and species are presented in ascending alphabetical order; families, subfamilies, tribes, and subtribes link to BugGuide profiles; species link to Discover Life species profiles. Month headings link to month-specific lists. Host plants link to USDA PLANTS Database genera profiles: view 'Subordinate Taxa' tabs for regional host plant species. Arthropoda: Hexapoda: Insecta: Hymenoptera: Aculeata: Anthophila (Apoidea):


Figure 1. State-specific percentages of pollen specialist bees (R2 = 0.259, r = 0.591, N = 48, p-value = < 0.00001). Percentages appear correlated with ecoregion or state area, latitude, and longitude.


Pollen specialist bees recorded across the contiguous United States were most often mesolectic on Asteraceae (34 spp.) and monolectic on Salix L. (12 spp.), Physalis L. (4 spp.), Cornus (Swida) L. (3 spp.), Cucurbita L. (3 spp.), Potentilla L. (3 spp.), and Vaccinium L. (3 spp.). Six unique relationships occured between individual monoleges and single host plants: Callirhoe Nutt., Cuscuta L., Geranium L., Hydrophyllum L., Lysimachia L., and Zizia W.D.J. Koch. Host plants were oftentimes either community dominant species or had early/late season blooms, crepuscular blooms, and odd floral morphologies. Pollen specialist bees were generally solitary species with gregarious aggregations of soil burrows or nests in preexisting wood cavities. Burrows were commonly excavated in sandy or compacted soil with sparse vegetation or leaf litter near host plants, which were often crops including sunflower, ground cherry, cucurbit, or blueberry. Preexisting cavity nests frequently contained leaf and soil partitions or pebbles and plant resins. Nest sites were often located on active roadsides, footpaths, and farms or in abandoned sand quarries, gravel pits, baseball fields, or animal nests. Unique bee species nested in sand dunes (Perdita swenki and Trachusa zebrata) or sites that were seasonally puddled by rainwater (Andrena accepta and Andrena rudbeckiae), flooded by saltwater (Svastra obliqua), or snow-covered by snowplows (Macropis nuda). Megachilidae are commonly known to nest in cavities, but 50% of listed pollen specialist Megachilidae nested in burrows. Therefore, pollen specialist bees prefer both particular native host plants and specific nest sites.

Between 30% and 70% of the pollen specialist bees listed were rare. Pollinator habitat conservation and enhancement projects should prioritize practices that foster diverse communities of rare native pollen specialist bees and associated host plants (Fowler 2016a, 2016b, 2020a, 2020b; Fowler & Droege 2020). Site-specific practices including beetle banks, companion plants, cover crops, field borders, hedgerows, insectaries, meadows, pastures, prairies, and riparian buffers should be composed primarily of host plants for pollen specialist bees and secondarily of pollen and nectar plants for generalist pollinators. Regional native plant horticulturists and nurseries should prioritize the propagation of ecoregional host plants for pollen specialist bees. Conservation practices can protect native pollen specialist bees from population declines and extinctions, while identically supporting other managed and wild beneficial insects, providing wildlife habitat, reducing weeds and erosion, improving soil health and water quality, regulating pests, offering harvestable products, and bettering aesthetics (Garibaldi et al. 2014; Kleijn et al. 2015; Wratten et al. 2012). Therefore, habitat conservation and enhancement for native pollen specialist bees works synergistically to promote strong environmental, social, and economic sustainability across the United States.


Thank you for comments, Sam Droege (USGS BIML)... Thanks, BugGuide, BONAP, Discover Life, and USDA-NRCS PLANTS. Thank you for taxonomy and state attributions, John Ascher (National University of Singapore). Thanks for decriptions, authors: Cockerell, Cresson, Timberlake, Robertson, LaBerge, Viereck, Smith, Mitchell, Swenk, Say, Crawford, Viereck & Cockerell, Michener, Provancher, Swenk & Cockerell, Neff, Patton, Ribble, Cockerell & Porter, Stevens, Kirby, Dalla Torre, Linsley & MacSwain, Casad, Ashmead, Shinn, Shinn & Engel, Fabricius, Neff & Larkin, Gusenleitner & Schwartz, Donovan, Lanham, LaBerge & Ribble, Casad & Cockerell, Rozen, Portman & Griswold, Portman & Neff, Griswold, Lovell & Cockerell, Hurd & Linsley, Bohart, Schwarz, Sandhouse, Snelling, Snelling & Stage...

Thank you for sharing records, collectors: American Museum of Natural History, John Ascher, AMNH, Bee Biology and Systematics Laboratory, California State Collection of Arthropods, Computarización de la colección de abejas (Hymenoptera: Apoidea) del Museo de Zoología Alfonso L. Herrera, de la Facultad de Ciencias de la UNAM, CNIN/Abejas de México/Apoidea, Actualización de la base de datos del proyecto H278 Apoidea (Hymenoptera) del Valle de Zapotitlán de las Salinas, Puebla, Dartmouth University Bee Collection, Essig Museum Entomology Collection, Global Biodiversity Information Facility, Illinois Natural History Survey, iNaturalist, Los Angeles County Museum, Lund Museum of Zoology, Bob Minckley, NMNH Entomology Collections, Rocky Mountain Biological Laboratory Bee Collection, Rutgers University Arthropod Collection, Jessica Rykken, UAM Entomology Observations (Arctos), Snow Entomology Collection, University of Kansas, University of California Riverside Entomology Research Museum, United States Geological Survey, Sam Droege...


Records or requests? Email: j@jarrodfowler.com

Photos by Baker, Gould, and Pennsylvania Department of Agriculture

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