The roost has been active in the cool overnight for past 4 days. Bees always return to the same dry stalk even though there are (approx) 20 similar close by on the same plant.
Update 5 weeks after roost first seen: 2-5 bees nightly return to the same group of burrs at the tip of this one stalk. There have only been about 2 nights recently where none show up but this morning 5 are at home. I'm fascinated as the stalk has basically no protection from rain, wind, heat. The bees have been undeterred by the few storms we had earlier in Feb, or by the heat we now have.
My last update looks to have been the kiss of doom! Roost was abandoned on 4 Mar even though near sunset 10+ bees had gathered there, were active, but chose not to settle overnight. Or did some attack convince them to move away?