How do solitary bees forage in patches with a fixed number of food items?

Efficient foragers avoid returning to food sources that they had previously depleted. Bombus terrestris bumblebees use a counting-like strategy to leave Alcea setosa flowers just after visiting all of their five nectaries. We tested whether a similar strategy is employed by solitary Eucera sp. bees that also forage on A. setosa. Analyses of 261 videorecorded flower visits showed that the bees probed five nectaries in 26% of visits, which was the most common case. However, this proportion did not differ significantly from that of six-nectary visits. Probe durations that preceded flower departures were generally shorter than those followed by an additional nectary visit in the same flower. Assuming that probe durations correlate with nectar volumes, this suggests that flower departure frequencies increased after probing of low-rewarding nectaries. The flowers' spatial attributes were not used as departure cues, but the bees may have left flowers in response to scent marks on previously visited nectaries in a small fraction (<14%) of visits. These findings suggest that a combination of a reward-based patch-leaving rule and scent marking contribute to Eucera's efficient foraging. While a counting-like strategy cannot be completely ruled out for Eucera, it seems less exact than in Bombus, and may be mediated by different cognitive processes. Thus, Eucera resemble bumblebees in avoiding most revisits to already visited nectaries, but probably use different foraging strategies to do so. This difference may reflect lower learning capabilities of solitary bee species compared to social ones.