Osmia: Select species diagnoses

This resource is not a substitute for expert determination. Its modest aim is to:
  • Use detailed photos that will help users (like me) fine-tune their skill in assessing traits;
  • Model the process of using the scarce resources to inform diagnosis;
  • For the chosen species, provide the evidence that leads me to a provisional determination.
  • Tip: Make use of cf. determinations. Say you have an unknown, labeled as "Osmia sp." You compare this to the diagnoses here, and suspect that your specimen is Osmia nemoris. However doubt remains; it is consistent with O. nemoris, but you lack the evidence (or knowledge) to declare that determination for sure. So you label the specimen Osmia cf. nemoris. This is far more informative than "Osmia sp," and it will simplify a later re-examination by you or anyone else. You could also reference the criteria used here with a hyperlink to "https://beeglossary.org/osmia_exemplars.html#nemoris" (or other species name after the # sign).

Sources consulted

The specimens examined here were collected by me, or made available from the USGS Bee Lab, Lauren Ponisio's Lab at University of Oregon, Institute for Applied Ecology, Quamash EcoResearch, or the Rufus Isaacs lab at Michigan State University.

Uncredited images are by David Cappaert. I grant permission for any non-commercial use. I've supplemented these with the excellent photos by the USGS/BIML team, and by the Bees of Canada page (Images courtesy of Margarita Miklasevskaja at PCYU with funding from NSERC-CANPOLIN). These public domain images are a huge gift to bee taxonomy.

Related taxonomy pages

Glossary of bee terms
Guide to Andrena on DiscoverLife
Key to Andrena subgenera of the PNW
Endangered plants of the PNW
Key to Lasioglossum of the PPP
Photo blog, 2024, 2025

Note about the DiscoverLife guide: I have spent at least hundreds of hours with the Osmia Temp and Andrena guides. I have also spent a few hours explaining to critics why I believe these are excellent tools. The guides were constructed from existing dichotomous keys, expert knowledge, and specimen examination. So it is incumbent on a critic to point to where these sources are suspect, with the result that the user will declare a species ID that is incorrect. This does in fact happen, but unlike an Osmia key from 1939, the DL key can be updated and illustrated. The greater "problem" in DL is that the trait scoring is incomplete (western species!) and/or overly broad. I.e., the guide is conservative by design. You may not narrow your list beyond a handful of possibles. So you will look elsewhere for finer distinctions. In the context of this Osmia diagnosis page, I extensively use the DL guide because:

  1. The interactive nature of the guide permits a user to screen options against traits the user can confidently assess. By contrast, a dichotomous key (e.g., Sandhouse) may require evaluating vague descriptors and comparison to reference material not available. E.g., a couplet that offers alternatives like "hairs dense and long" vs "shorter and sparser" may not be actionable.
  2. The DL guide can quickly rule out many alternatives that don't have to be considered. See above for many examples.
  3. The guide includes species pages. Ideally these include descriptors from literature (true for the Andrena guide), but in most cases it does provide a range map, host associations, and photographs, all of which might help support or reject a diagnosis.
  4. The DL guides encompass an approach that welcomes participation by a broad community, including citizen scientists. A potential drawback of this is that lowering a barrier might mean lowing the standard. Thus I create documents like this one.

Update January 31, 2026
David Cappaert, Quamash EcoResearch, cappaert@comcast.net

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