It has long been observed that bumble bees (Bombus spp.) can distinguish between more and less rewarding owers of the same plant species without actually sampling the reward available. Typically, the bee hovers briey next to a ower with its antennae extended and nearly touching the corolla, and then either proceeds to land and attempt to feed, or instead rejects the ower without landing and moves on (Figure 13.1). In some circumstances the bee may be directly assessing the reward level, or perhaps examining correlates of reward, such as ower size and symmetry (Brink and deWet 1980; Stanton and Preston 1988; Cresswell and Galen 1991; Møller 1995; Møller and Eriksson 1995). However, there is now strong evidence that perhaps the most important cues used by bees to decide whether to probe or reject a ower are chemical clues left by bees on previous visits (Cameron 1981; Marden 1984; Kato 1988; Schmitt and Bertsch 1990; Goulson et al. 1998; Stout et al. 1998).