Common and widespread across much of North America, greater bee flies (Diptera: Bombyliidae: Bombylius major) are usually found in and around woodlands. Adults are typically encountered from March through May feeding on the nectar of early spring wildflowers. Their larvae, in contrast, are parasitoids of solitary bees.
Female bees will repeatedly dig holes, provision them with food, and then lay a single egg inside each one. Bee fly females will locate these nests and lay one of their own eggs near the opening. The bee fly egg will hatch and the larva will eat the bee larva as well as the provisions the female bee left for her young. After pupating the bee fly will emerge as an adult.
This insidious behavior is great for the bee flies because it ensures their reproductive success. At the same time it is not so common that the bee populations suffer much as a whole.