Osmia bees (Hymenoptera: Megachilidae) can detect nectar-rewarding flowers using olfactory cues

Nectar-foraging bees frequently face the choice of which flowers to visit and which to avoid. One possible mechanism by which bees could discriminate between flowers before visiting them is by detecting nectar via its odours. To test this idea, we observed visits by solitary bees in the genus Osmia (Megachilidae) to flowers of Penstemon caesius (Scrophulariaceae) in the San Bernardino Mountains of southern California. We observed that free-foraging Osmia bees visited flowers containing nectar seven times more frequently than they visited nectar-depleted flowers. To test whether bees could detect the presence of nectar via odour cues, we compared floral preferences between trials where we blocked the olfactory capabilities of bees by coating their antennae with nontoxic silicone and where bees foraged with uncovered antennae. We randomly assigned the order of the silicone treatments and attempted to test 32 bees at P. caesius arrays containing nectar-depleted flowers, nectar-depleted flowers with added water and nectar-rewarding control flowers. Bees with uncovered antennae visited more than twice as many control flowers as they did either group of nectar-depleted flowers. In contrast, bees foraging with silicone-covered antennae visited all treatment flowers equally. Bees that completed both trials visited nectar-rewarding control flowers twice as frequently while foraging with uncovered antennae as they did while foraging with silicone-coated antennae. These results are consistent with the idea that solitary Osmia bees are capable of perceiving nectar volatiles to identify nectar-rewarding Penstemon flowers.