Looks like barnacles, but it’s the egg sac for tiny, parasitic (but beneficial) braconid wasps. These are most likely Cotesia congregates. The adult wasp lays eggs directly onto a mature Tomato Hornworm, the eggs hatch and the baby parasitic wasps eat the hornworm. It’s the Circle, the Circle of Life.
albert vick 
I had these on one of my tomato plants. I was glad to see it was mostly nature doing what nature does.