It can be tomato worms or horn worms or cutworms. They eat leaves and they have the color of the stems and may be they are on the stems. Picking them off is the way to remove them. A nail spike beside the plant repels cutworms. Some garlic cloves can repel some bugs. The seven dust is good. Plant garlic cloves by your tomato plants to get rid of the worms. Check for some tiny grasshopper, they can also be a culprit. Put some black pepper on the leaf to get rid of grasshopper. Good Luck