Yes dubia need to be dusted. Dust with calcium 5 days a week and dust with vitamins the other two days. It seems to me that Dubia don't get thickly coated with calcium powder like crix do, but they do get enough of the dust on them to supplement your dragon.
I put my Dubia in a little plastic box, like a small tupperware container and sprinkle on some dust then sort of sift them up and down to cover them with calcium or vitamin dust before feeding them to Spike. That seems to work well for us.
Eric