I need a tremendous variety of native foods.
Box turtles are truly omnivorous. They eat anything you imagine--and other things. On one hand, they find wild strawberries delectable; on the other, entrails left by a successful hunter (of any species) are a smorgasbord. Day after day, turtles will consume the same fungi, the slightest taste of which would gag a hardy outsdoorsman. Box turtles even will clean up droppings left by other wildlife along a forest path. In short, the more natural processes, the more food from which to choose.
Sweet, rotting fruit is perfect box turtle food. Within their home range, turtles remember where raspberry, blackberry, gooseberry, and similar plants grow. They travel to take advantage of seasonal fruit as it falls. Encourage wild berry plants, especially in well-defined thickets that keep human hands and raccoon paws out. Remember: turtles can eat only what hangs long enough to rot and plop down around them.
Box turtles are effective hunters. They strike with surprising speed over the last inch to their target. Speed, paired with remarkable fluidity of movement and astonishing patience, make them a match for almost any insect. They can approach a target invertebrate over many inches, at a pace nearly too slow to discern.
Given a choice, box turtles relish katydids above other prey; common earthworms rank well. Preferred habitat--including variably wet grass or forest floor--provides these readily. Retain patches of heavy leaf litter that sport numerous spiders. The spiders' success is proof of insect variety and productivity.
Natural patches of forest and meadow also produce many fungi. These may recur for years in the same vicinity. Box turtles learn where certain mushrooms flourish, and when, just as they do for a berry snack. One turtle may return day after day to crop new caps from the previous night. When good feeding conditions develop, maintain that character, even if it means doing nothing.
If you feed domestic animals outdoors, box turtles may take advantage. That's dangerous. (Please understand: this is not a recommendation to let pets roam outside. Free-range cats and dogs kill an enormous quantity of wildlife.) If you set out smelly pet food daily over a period of years, individual wild turtles may exploit it. Browsing marks them with fishy, meaty odors of the food, and they may die as a result. Many a turtle leg has been lost to a raccoon who smelled something yummy in the leaves at dusk.
For more box turtle stewardship guidelines, see Habitat from Humanity.