
Chlosyne ehrenbergii
This was a quick grab shot with my tele, so it is far from ideal.
I had never managed to ID it and saw it only once.
Ian Morton on Flicker gave me the name and it seems to be it. I suppose the habitat description that you find on the web is wrong.

The White-rayed Checkerspot, White-rayed Patch or Ehrenberg's Patch is a butterfly of the Nymphalidae family. It is found in Mexico.
Adult females are highly gregarious, clustering together in groups on Buddleia bushes. Adults feed on flower nectar of various flowers, including daisies and other Asteraceae species.
The larvae feed on ''Buddleia'' species.