these very small, metallic bronze bees have been active among the Red Mason Bees for about 2 weeks now
No interactions present.
This could equally be Osmia leaiana. Although the females are very distinct, males are very similar and can only be distinguished by characters (like the texture of the declivity of basal abdominal segment)that can't be seen in a photo. In my experience, leaiana is consistently larger, but clearly you need to know the relative sizes first before you can use this as a clue to ID in the field.
I collected a small very blue bee from this location this morning, which I assume is a female due to the colour. What can I look for under the microscope to distinguish caerulescens from leaiana? Is there a key I can use?
Edit: (from BWARS) "females.. O. caerulescens is generally smaller, dark blue with a black scopa under the abdomen and O. leaiana is generally larger, dark brown, with a bright orange scopa under the abdomen. "
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Lat/Lng: 51.5559, -0.0242
OS grid ref: TQ370859