This is a focus-stacked image of a yellow jacket's moth parts staged at 7.5X magnification. Their diets vary during their lifecycle.

Adults crave sugars and carbohydrates and they can be found foraging around the following foods:

Fruits, Flower, Nectar, Tree Sap, Fruit juices, and other foods rich in sugar. They collect these foods by sticking their long tongues (paraglossa) into the food source. A word of caution is to check your can of soda or beer before putting it in your mouth if you happen to be outdoors, for you may find a yellow jacket.

Yellowjacket larvae feed on proteins so they are good in controlling garden pests such as the following:

Flies, crickets, aphids, caterpillars, cabbage worms, grubs, moths, spiders, meat, and fish. For their larval stage, yellow jackets need protein. Adults hunt other insects and bring them back to the colony to feed the young, which are chewed and conditioned in preparation for larval consumption (hence the chewing mandibles). That’s also how gardeners benefit from yellowjackets in their yards! (just be sure to look into your soda and beer cans when outdoors).

They also eat dead insects also and it is estimated that in a 2,000-square-foot yard, yellow jackets consume more than two pounds of bugs in the form of live or dead insects.