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I had similar problems during some recent surveys, although fortunately I didn't have to distinguish between these species.
I was told it is not possible to distinguish female workers in the field. The only visible difference is colour but this is not reliable. Despite this some people will still claim they can tell the difference...
However, it is possible to identify them if they are male (lack pollen baskets and top part of tarsi are hairless). Male lucorum have a yellow nose, whereas male terrestris have a black nose.
I am no expert but this is what I was told by someone who is and so I hope it is helpful :)
Thanks! Not sure my little camera is up to photographing bees' noses though - and I'm not sure how that would go down with my wife as a reason to upgrade :-)
The yellow nose is visible from a distance with the naked eye. Happy nose spotting!
Lat/Lng: 52.0, 1.1
OS grid ref: TM1645