The Crabronidae includes a large number of nest-providing predators, but also a few ectoparasitoids that provision their larvae with such large prey that they need only a single host. The Crabronidae show that the distinction between parasitoid and predator is sometimes quite fuzzy, especially if the same species sometimes provide its larva with a single host, and sometimes with two, if they are too small. These observations also apply to the Sphecidae below.

Source: O'Neill, K.M. 2001. Solitary wasps: Behavior and Natural History, p. 58.