Leafcutter and Mason Bees of the Genus Megachile Latreille (Hymenoptera: Megachilidae) in Canada and Alaska1
CJAI 18 November 29, 2011

Cory S. Sheffield*2, Claudia Ratti*, Laurence Packer*, Terry Griswold**

*Department of Biology, York University 4700 Keele St., Toronto, ON M3J 1P3 coryshefield@yahoo.ca; cratti@yorku.ca; xeromelissa@mail.com. **USDA-ARS Bee Laboratory, Utah State Univ., Natural Resources Biology Bldg. Logan, UT 84322 Terry.Griswold@ars.usda.gov.

1This paper is contribution #12 from the Canadian Pollination Initiative. 2Corresponding Author.

IntroductionFigure A
           The Megachilidae is one of two families of long-tongued bees, the other being the Apidae (Michener 2007).  In North America, it is the only family in which females of non-cleptoparasitic taxa carry pollen entirely on the underside of the metasoma (Figure A).  Most megachilid bees have robust bodies with a head about as wide as the thorax, or wider (Banaszak & Romasenko 1998).  Megachilidae is found on every continent except Antarctica, and is one of the largest families in terms of the number of species (4037) and recognized genera (77) (Ascher & Pickering 2011; http://www.discoverlife.org; accessed May 16, 2011).  Species estimates in Michener (2007) are much lower (3198 in 77 genera).
The tribe Megachilini (Megachilinae) is represented in North America by two of the three genera; Megachile Latreille, the leafcutter bees,and their main cleptoparasite, Coelioxys Latreille; the third member, Radoszkowskiana Popov, is also a cleptoparasite of Megachile but is restricted to the Eastern Hemisphere (Rozen & Kamel 2007).  Megachile is one of the most common and diverse genera of bees (Mitchell 1980; O’Toole & Raw 1991; Michener et al. 1994; Baker & Engel 2006; Michener 2007).  Michener (2007) recognizes 56 extant subgenera; Engel & Baker (2006) describe an additional subgenus from Thailand known only from the male.  In the Western Hemisphere, 31 subgenera are known (Raw 2006), though Durante & Abrahamovich (2006) recognize Chaetochile Mitchell as a distinct monotypic subgenus and not a synonym of Dasymegachile Mitchell.  In North America, thirteen subgenera are indigenous, but species belonging to an additional three subgenera have been introduced (Cane 2003; Michener 2007), and also occur in Canada (Richards 1984; Magnum & Sumner 2003; Paiero & Buck 2003; Sheffield et al. 2010).  Hurd (in Krombein et al. 1979) listed 134 species of Megachile in America north of Mexico, including the genus Chalicodoma Lepeletier (recognized here as the subgenus Chelostomoides Robertson); an additional five species were reported by Michener et al. (1994).  A new North American species was described recently (Gonzalez & Griswold 2007).  However, taxonomic knowledge of the genus Megachile in North America remains far from complete as almost a third of the species are known from one sex only, primarily within the subgenus Megachiloides Mitchell (Sheffield & Westby 2007).  Interestingly, several species of Megachile that occur in Canada have been found as gynandromorphs, including M. angelarum Cockerell, M. gemula Cresson, M. latimanus Say, M. onobrychidis Cockerell, M. parallela Smith, M. perihirta Cockerell, and M. rotundata (Fabricius) (see Wcislo et al. 2004).

Biology of MegachileFigure B
           Much is known about the biology of many leafcutter bees due to their importance in crop pollination (Hobbs & Lilly 1954; Pengelly 1955; Osgood 1974; Peterson et al. 1992; Richards 1993; Raw 2002) and the fact that many species accept trap-nests (Medler 1959, 1964; 1965; Fye 1965; Krombein 1967; Frolich & Parker 1983; O’Toole & Raw 1991; Sheffield et al. 2008).  Most species nest above ground in pre-existing cavities (Figure B) or excavate into pithy stems or decomposing wood (Stephen 1956; Ivanochko 1979).  Trap-nesting of bees has allowed detailed study of life-history, nest building, provisioning and egg laying behaviours (Medler 1959, 1964; Klostermeyer & Gerber 1969; Frolich & Parker 1983; Kim 1992 ), and documentation of incidences of cleptoparasitism (Scott et al. 2000; Sheffield et al. 2008).  Trap-nest surveys also allow association of males and females of the same species (Sheffield & Westby 2007).  However, several North American species within the subgenera Argyropile Mitchell, Litomegachile Mitchell, Megachiloides, and Xanthosarus Robertson are ground-nesters (Hobbs & Lilly 1954; Eickwort et al. 1981; Williams et al. 1986; Neff & Simpson 1991; Krombein & Norden 1995; Gordon 2000).  Table 1 summarizes nesting site information for species of Megachile in Canada and Alaska.
Leafcutter bees require suitable nesting sites (see Table 1), nest building materials, and sufficient/suitable food plants for nectar and pollen.  These three factors not only influence the diversity of bees within a given habitat by meeting their specific needs (i.e., pollen specialists), they also strongly affect the abundance and/or fecundity of certain species through the quantities at which they occur (Müller et al. 2006; Williams & Kremen 2007; Sheffield et al. 2008).  Hobbs & Lilly (1954) noted that nesting site availability dictated which species of Megachile were found within specific habitats in southern Alberta; the absence of trees and/or logs (i.e., nesting sites with pre-existing or easily excavated cavities) limited the number of cavity-nesting species, whereas ground-nesting species were more prevalent.  Leafcutters and other cavity-nesting bee species can often be encouraged to nest within natural and highly managed systems simply by placing trap-nests within these habitats (Sheffield et al. 2008).  Using artificial nesting material, M. rotundata, which nests gregariously within shelters, was developed as a commercial pollinator of alfalfa in western North America (Free 1993; Richards 1993).  Thus, seed production in this important industry owes much of its success to this species.
           Cut leaf pieces are the main nesting material used by leafcutter bees for nest cell construction.  Hobbs & Lilly (1954) noted that ground-nesting leafcutter bees were common in the prairies within flying distance of leaf sources (e.g., shrubs, etc.), and declined greatly in open prairies without broad leaved plants.  However, not all Megachile in Canada use leaf pieces as nesting material; members of the subgenera Callomegachile (M. sculpturalis Smith) and Chelostomoides (M. angelarum and M. campanulae (Robertson)) are masons, and collect plant resins, pebbles and mud for cell partitioning and closure (Michener 2007), as does Megachile (Pseudomegachile) ericetorum Lepeletier (Banaszak & Romasenko 1998), a species only recently reported in North America (Sheffield et al. 2010). Megachile (Megachile) montivaga Cresson collects flower petals (Hobbs & Lilly 1954; Ivanochko 1979).
           From a standpoint of encouraging leafcutter bees, all three of the requirements mentioned above can be found in most natural habitats.  Trap-nests can be provided to accommodate cavity nesting species, supporting increases in species richness and abundance in agricultural systems (Sheffield et al. 2008).  Although ground nesting Megachile species are also important alfalfa pollinators (Hobbs & Lilly 1954; Rank & Goerzen 1981), very few ground nesting bees have been developed for pollination (see Richards 1993).

           Many of the Megachile species in Canada and Alaska are common, though no comprehensive account has been published.  Mitchell (1962) published keys to the species found in eastern North America; his descriptions (Mitchell 1927; 1934) and subgeneric revisions (Mitchell 1935a, b, 1936, 1937a-d) cover most of the remaining species found in Canada.  Sheffield & Westby (2007) provided a review of the subgenus Megachile s. str. in the western Hemisphere, which included the previously undescribed male of M. nivalis Friese (considered here as a synonymy of M. lapponica Thomson).  Ivanochko (1979) provided the first comprehensive account of leafcutter bees in Canada, though he omitted the subgenus Chelostomoides (which was later covered by Snelling (1990)), and summarized their biology with respect to alfalfa pollinating potential, but remains unpublished and virtually unknown.
           The purpose of this work is to provide a revision of the species of Megachile occurring in Canada and Alaska, including and interactive and web-based key.  The keys make use of morphological features, but molecular techniques (i.e., divergence levels in a 658 bp segment of the COI mitochondrial gene) have been used to verify male-female associations, and to clarify species designations.  A single middle leg was removed from pinned specimens; whenever possible multiple individuals of a species were analyzed to quantify the extent of intra-specific sequence divergence.  Specimens were primarily collected in Canada, but material was also obtained from several localities in North America.  DNA extracts were prepared following procedures outlined in Hajibabaei et al. (2005), and sequences or “DNA barcodes” will be published on BOLD (http://www.barcodinglife.org) in the “Bees of Canada” project.

Notes on using the key, and on identifying leafcutter bees

           One of the main difficulties in identifying Megachile species, especially females, is that many keys are based on mandibular dentation.  In old individuals, mandibles are often so badly worn that the shape of the teeth and even their number are difficult to distinguish (Figure C).  A similar problem occurs when identifying dirty specimens or specimens with the mandibles closed.  Therefore, we have tried to provide a key in which dentition is not so heavily relied on, but for obvious reasons it remains a useful character for separating some species (and even some subgenera).  Colour images of female mandibles (Plate 1), male genitalia (Plate 2), lateral images of females and males (Plate 3) and distribution maps are provided for all species.  For difficult specimens (including those females in which mandibles are not visible or useful), users of the key are asked to read through the couplets fully and follow both alternatives within the dichotomy.  Full descriptions and diagnoses are also provided to facilitate accurate identification, and these should also be read for additional morphological characters and distribution information.  A standard format is used to facilitate quick comparison of specific characters; these are numbered consistently within descriptions of the head, mesosoma and metasoma.

Figure C


Table 1.  Nesting biology summary of the genus Megachile in Canada and Alaska.

Species Nesting substrate Reference
M. addenda Sandy soil Medler & Lussenhop 1968; Ivanochko 1979; Cane et al. 1996
M. angelarum Cavities, trap-nests Barthell et al. 1998; Frankie et al. 1998
M. anograe Soil Hobbs & Lilly 1954; Bohart & Youssef 1972; Mitchell 1973
M. apicalis Cavities, trap-nests Barthell et al. 1998
M. brevis Cavities, hollow plant stems, soil, rolled leaves Hicks 1926; Michener 1953; Hobbs & Lilly 1954; Pengelly 1955; Ivanochko 1979; Baker et al. 1985; Packer 1987; Reed 1871
M. campanulae Cavities, trap-nests Medler 1966; Krombein 1967; Baker et al. 1985
M. casadae Unknown (probably soil)  
M. centuncularis Cavities, trap-nests Krombein 1967; Sheffield et al. 2008
M. circumcincta Soil Latter 1906
M. coquilletti Cavities, trap-nests, probably soil? Bohart 1957; Barthell et al. 1998
M. dentitarsis Soil Hobbs & Lilly 1954; Bohart 1957
M. ericetorum Cavities, plant stems Banaszak & Romasenko 1998
M. fidelis Cavities, trap-nests Barthell et al. 1998; Frankie et al. 1998
M. fortis Soil Ivanochko 1979; Neff & Simpson 1991
M. frigida Rotting logs, trap-nests Hobbs & Lilly 1954; Pengelly 1955; Stephen 1956; Jenkins & Matthews 2004
M. gemula Poplar logs, hollow twigs Peck & Bolton 1946; Fye 1965; Medler & Lussenhop 1968
M. gentilis Cavities, trap-nests Snelling 2003; Kim 1992
M. inermis Cavities, trap-nests, rotting logs Stephen 1956; Medler 1958; Sheffield et al. 2008
M. lapponica Cavities, trap-nests Sheffield & Westby 2007
M. latimanus Soil Mitchell 1936; Bohart 1957; Ivanochko 1979
M. lippiae Probably soil Hicks 1926; Hobbs & Lilly 1954 (as M. cleomis)
M. manifesta Unknown (probably soil)  
M. melanophaea Soil Hobbs & Lilly 1954; Pengelly 1955
M. mellitarsis Unknown  
M. mendica Trap-nests; soil Medler 1965; Krombein 1967; Baker et al. 1985; Williams et al. 1986
M. montivaga Soil; old stems Ivanochko 1979; Hicks 1926; Hobbs & Lilly 1954; Baker et al. 1985
M. onobrychidis Probably soil and/or cavities? Bohart 1957
M. parallela Soil Fischer 1951; Hobbs & Lilly 1954
M. perihirta Soil Sladen 1918; Hicks 1926; Hobbs & Lilly 1954; Bohart 1957; Ivanochko 1979
M. pugnata Cavities, trap-nests, rotting logs Medler 1964; Hobbs & Lilly 1954; Sheffield et al. 2008
M. relativa Cavities, trap-nests Medler & Koerber 1958; Sheffield et al. 2008
M. rotundata Cavities, trap-nests, soil in vertical banks Krombein 1967; Sheffield et al. 2008
M. sculpturalis Cavities, trap nests, Xylocopa nests Mangum & Sumner 2003
M. sublaurita Soil Bohart & Youssef 1972
M. subnigra Unknown (probably soil)  
M. texana Soil Krombein 1953; Hobbs & Lilly 1954; Eickwort et al. 1981
M. umatillensis Sandy soil Bohart & Youssef 1972
M. wheeleri Sandy soil, trap-nests at ground level Hobbs & Lilly 1954; Gordon 2000



Abstract | Introduction | Key to Genera | Species | Acknowledgements | References | PDF | Cite this Article