The diet of D. sylvestris larvae was determined based on close analysis of larval pellets and observation. It was found that the wasp larvae consumed both other insects and nectar from flowers. The queen finds the insects or nectar and returns to the nest to feed them to each of the larvae during the queen nest before workers take over foraging and feeding activities. At this time, workers begin feeding the larvae, using the same types of food. Insects included many types of flies, including but not limited to march flies (Bibionidae) and crane flies (Tipulidae). The type of insects that were chosen as the main food source depended on the time of the season. These wasps prefer to feed on flies and tend to avoid ants or spiders. Queens were also observed feeding larvae nectar from flowers one drop at a time by putting her tongue and their mouths together. This nectar comes from flowers of plants such as blackberry, raspberry, and Rhododendron. Nectar seems to be a very important part of the larval diet as both queen and workers took more trips and more time collecting it than either insects or pulp for the nests. Experiments on larvae have shown that the sugar in the nectar is a necessary energy source and that without it larvae will quickly become unable to survive. Therefore, both insects and nectar are an important part of the larval diet.
