From John Sandbakken (Executive Director, National Sunflower Association):
"From the looks of it you have a pest. Not sure but would guess it was some moth or seed maggot."
I Googled pictures of seed maggots and I never saw any of those in my garden, and I also did not notice any moths, but that does not really mean much -- could be possible. In any event, a pest does not bother me as much as a possible virus,