To determine the effect of short-term temporal and small-scale spatial variation in availability of specific prey groups, field studies of prey use by a population of the robber fly Efferia staminea were undertaken. In one study, the appearance of mating swarms of winged males of the ant Formica subpolita was associated with a rapid increase in the proportion of E. staminea observed feeding, and an increase in the proportion of these ants taken as prey. The change in diet occurred over the same time scale as the change in the activity of the ants. When the swarms were absent from the same area, the fewer E. staminea observed feeding utilized a greater diversity of prey taxa and sizes. The proportion of conspecifics in prey records during swarms of F. subpolita was only one-tenth of that during non-swarm intervals, suggesting that high alternative prey availability decreases the incidence of cannibalism in this species. In the second study, E. staminea used a wider diversity of prey on an area of grassland with native vegetation than on a nearby area of grassland that had been reseeded with the grass Agropyron intermedium as part of a range-management program. In the latter area, a large population of crambine moths supplied a major portion of the robber flies' diet. The results of this population-level study illustrate the fine scale over which the composition of the diet of E. staminea varies, and show that the diet of a generalist predator is a function of the temporal and spatial scales over which sampling occurs. The implications of the data for interpreting the composition of the diet, population dynamics, and impact upon prey communities of robber flies are discussed.