The culprit is a night feeder. Go out with a flash light and you should find a green caterpillar munching away. This larvae should turn into a cocoon then a butterfly or moth in a few weeks. You can usually hand pick the fellow before too much damage is done. Look in the soil at the base of the sunflower to see if you find the larva or some slugs or snails. Fresh and succulent growth is attractive to many different types of pests this time of year but your sunflower should still be blooming soon - I see a beautiful bud in the photo you sent so your summer will be full of blooms. Keep growing, Marianne Binetti