The searching behaviour of Diaeretiella rapae , the main parasitoid of the cabbage aphid. Brevicoryne brassicae , was analysed using whole Brussels sprouts plants. The spatial configuration of the plant and the distribution of aphids and their cues on it have served as a template on which the Diaretiella foraging strategy has been moulded by natural selection. The individual plant is thus an "elementary unit of foraging" (EUF) within the wasp environment. The EUF may form a basic element in the environment of many foraging insects: its importance of understanding insect foraging strategies is discussed. none