Marsh flies are a diverse family that provide valuable ecosystem services, including the biological control of mollusks that are agricultural pests and vectors of animal and human parasitic diseases. In addition, some species may serve as important ecological bioindicators. Despite the extensive research on this family, most have centered on larval diet and behavior, as this is the life stage primarily used for biological control; virtually nothing is known about the natural dietary components of adult marsh flies. Our study aimed to close this knowledge gap by examining the dietary range and preference of adult marsh fly species. Individual flies were provided with five food choices in cafeteria-style food choice trials, consisting of crushed snail, freshly killed slug, glucose solution, honey-yeast mixture (the standard laboratory rearing diet), or water. Sciomyzidae at family level displayed significant differences in food selection (P = 0.0212), with carbohydrates (honey-yeast and glucose solution) significantly preferred over protein options (mollusk tissue) or the water control (P < 0.001). This suggests that marsh flies may naturally maintain a carbohydrate-rich diet. Because many plants typical at field sites produce little or no nectar, a second experiment aimed to determine the source of these carbohydrates in nature. When presented with honeydew harvested from aphids (Hemiptera: Auchenorrhycha), Tetanocera elata Fabricius (Diptera: Sciomyzidae) individuals were observed to feed on dry honeydew and honeydew solution significantly more frequently than the water control (P < 0.001 and P = 0.01969, respectively), suggesting that honeydew may play an important role in adult marsh fly diet.