Megachile is a diverse genus that includes species with a wide range of body forms, although they generally exhibit a stocky build and robust head. They range in body length from 5–21 mm (Droege 2015). Most Megachile have nonmetallic black bodies with pale apical bands of hair on their terga, although some species may lack hair bands or have yellow to red hair (Michener 2007).
(modified from Michener 2007)
Some bees in the tribes Lithurgini and Megachilini look superficially similar to Megachile, but can be distinguished by a combination of the diagnostic characters above (Michener 2007).
M. apicalis was introduced from the Old World around 1930 and can be found on both North American coasts, although it is more widespread in the west (Droege 2015). It effectively pollinates yellow starthistle (Centaurea solstitialis), an invasive noxious weed introduced from the Mediterranean, and may be assisting in its spread (Droege 2015; Wilson and Carril 2016). It may compete for nesting sites with native bees (Barthell et al. 1998).
M. chlorura is from the Philippines and was first reported in 1988 from Oahu, Hawaii. It has been noted to prefer Asteraceae (Snelling 2003).
M. concinna was indirectly introduced from Africa in 1940 to the West Indies and Mexico. It has a scattered, uncommon distribution through southern and western North America (Droege 2015).
M. diligens was originally described from Hawaii in 1879; however, in the eastern hemisphere and Samoan islands other subspecies have been described, although the validity of such has been questioned (Snelling 2003). It may not be a native of Hawaii but a long ago introduced species from the South Pacific, possibly being introduced during Polynesian colonization of the Hawaiian archipelago. It can be found on Hawai’i, Kauai, Molokai, and Oahu islands (Snelling 2003).
M. ericetorium is a European and Asian species found in North America since 2000, and has been found in Ontario and New York (Droege 2015). The status of establishment is currently unknown.
M. fullawayi was first collected in Hawaii in 1919 and was introduced from Southeast Asia. It can be found on Midway, Niihau, and Oahu islands (Snelling 2003).
M. gentilis is found on Kauai, Maui, Molokai, Oahu islands and is adventive from western North America, having been introduced sometime before 1899 (Snelling 2003).
M. lanata is native to China and India and was likely introduced indirectly to the continental U.S. sometime during the 18th or 19th century from northern South America or the West Indies (Droege 2015). It is currently found in Florida and Hawaii.
M. rotundata, known as the alfalfa leafcutter bee, is widespread in the Palearctic and was accidentally introduced to the U.S. sometime before 1940 (Pitts-Singer and Cane 2011; Droege 2015; Wilson and Carril 2016). It was later found to be an important pollinator and subsequently commercialized for alfalfa seed production and has been investigated as an alternative pollinator for other crops (Pitts-Singer and Cane 2011; Droege 2015; Wilson and Carril 2016). Not all effects of this introduction are positive, however, and it may increase the incidence of chalkbrood disease (an infection by Ascosphaera aggregata) to native Megachile spp. when they co-occur in fields. Although it is widespread in the U.S., it is rarely found outside agricultural settings (Pitts-Singer and Cane 2011).
M. sculpturalis is from eastern Asia and was first found in the U.S. in 1990 (Laport and Minckley 2012). It is currently found in Quebec, Canada and the eastern and central U.S., where it can displace native bees (Laport and Minckley 2012; Droege 2015). It is expanding its range westward (Wilson and Carril 2016).
M. timberlakei was originally described from Hawaii in 1920, but its native range has been called into question (Snelling 2003). It has been suggested that it was actually introduced in the South Pacific, since multiple related species occur in Southeast Asia. It is a commonly found widespread species that is found on Hawai’i, Kauai, Lanai, Maui, Oahu, and Midway islands (Snelling 2003). Additionally, it has become a recent invasive in the Galápagos.
M. umbripennis is a resin bee with a widespread native distribution through Southeast Asia into the South Pacific (Krombein 1950; Snelling 2003). It was introduced to Hawaii sometime before 1899, possibly before European arrival. It is present on Hawai’i, Kauai, Maui, Midway, Molokai, and Oahu islands (Snelling 2003). It has also recently arrived to Florida, although the extent of its establishment is not currently known.
Species of Megachile can range from specialists to narrow generalists (Wilson and Carril 2016). For example, M. davidsoni specializes on native golden eardrops (Ehrendorferia chrysantha) in California. Megachile fortis forages on Helianthus spp. sunflowers. Some species forage only on evening primrose (Oenothera spp.) or Fabaceae (Wilson and Carril 2016).
While Megachile spp. are solitary, they may sometimes nest in aggregations. Nests are built inside various preexisting holes in wood, rock crevices, inside dead or occasionally living hollow plant stalks, or using man-made objects like cavities in concrete, between bricks, inside copper tubing, or even keyholes (Wilson and Carril 2016). Some are selective where they find their nesting sites. Megachile inimica nests primarily in mesquite trees (Prosopis spp.), while M. gentilis prefers nesting inside hollow twigs (Wilson and Carril 2016). Most species nest aboveground, but some nest belowground (e.g., below the soil surface in sandy soils) (Wilson and Carril 2016). Females harvest leaves, flower petals, and even plastic by cutting circular pieces and then use these materials to line and partition nest cells by chewing the edges and pasting them together in way that is comparable to papier-mâché (Wilson and Carril 2016). Some species are more fastidious in the materials they use for nest construction. For example, M. montivaga uses Clarkia sp. flower petals, and M. subparallela prefers tick-trefoil (Desmodium sp.) (Wilson and Carril 2016). Some, such as members of subgenus Chelostomoides, are known to use sap or resin to build nests and are commonly known as resin bees (Michener 2007). Some groups, like members in subgenus Chalicodoma, use mud, sand, or pebbles that are glued together and coated over to waterproof the cell (Michener 2007). The habit of nesting in preexisting holes contributes to their movement by humans, since they may nest in wooden shipping pallets, inside shipping containers, or directly inside trade products.
Megachile is a very large genus of bees with cosmopolitan distribution. It can be found throughout North America in a wide variety of habitats (Mitchell 1933). There are more species of adventive Megachile globally than any other bee genera. Twelve species of introduced Megachile occur in the continental U.S. and Canada; six of these species have been introduced to Hawaii (Russo 2016).