Megachile (Megachile) relativa Cresson, 1878

Megachile relativa Cresson, 1878: 126 . Mitchell 1927b: 179. Jensen 2003: 195. Fultz 2005: 82. Drons 2012: 59.

Megachile (Anthemois) relativa; Mitchell 1935b: 162.

Megachile (Megachile) relativa; Mitchell 1962: 129. Butler 1965: 5. Hurd 1979: 2056. Ivanochko 1979: 153. Sheffield and Westby 2007: 178. Scott et al. 2011: 55. Sheffield et al. 2011: 51. Kuhlman and Burrows 2017: 13. Reese et al. 2018: 22. Delphia et al. 2019a: 25. Sheffield and Heron 2019: 70. Engel 2020: 10.

Megachile aspera Mitchell, 1924: 158 .

Diagnosis. The female of M. relativa can be identified by its 5-toothed mandibles (Fig. 7G), yellow-orange scopal setae on S2–6, and golden pubescence on T6. The females are most similar to M. centuncularis (see M. centuncularis above). The males of M. relativa cannot be reliably separated from males of M. lapponica in Montana based on external morphology or by examining the genitalia (see Taxonomic Challenges; Sheffield & Westby 2007). Males of M. lapponica / M. relativa can be recognized by the absence of a procoxal spine, small median triangular tubercle on the clypeal margin, narrow brown to black probasitarsi (Fig. 8A), evenly spaced 3-dentate mandibles, and T6 with sparse setae, not tomentose. Megachile lapponica / M. relativa is most similar to M. centuncularis (see M. centuncularis above).

Notes. In Montana, this species has been collected widely but with only a few eastern localities, reflecting the general lack of collecting in the region (Fig. 1 AD). Photographs, a full morphological description (but see Taxonomic Challenges), and notes on its biology can be found in Sheffield et al. (2011). This species readily accepts trap nests (Jensen et al. 2003).