What is the main diet of ducks?

Mallard Ducks love worms - night crawlers! The other items: cracked corn, duckweed, and natural lake vegitation. We noticed they will eat the roots of the aquatic plants we put in their pond. Also, make sure they have access to grit (small stones). If outside, they will pick this up on their own. They love to forage for insects in the wet grass while we cleaned their pond.