The Oenothera genus contains about 125 species of flowering plants; pink evening primrose, beach evening primrose, and sundrops are especially common in Texas. Their flowers open primarily in the evening, and are pollinated by a variety of bees and moths.
Because the pollen grains of these flowers are loosely linked by threads of a substance called viscin, only bees with specialized pollen-transporting hairs can gather their pollen effectively. Viscin, a clear, tasteless, sticky substance not only holds the pollen grains together, it also helps attach the pollen to visiting insects.
Some botanists theorize the plants evolved in this way to allow the pollen to stick onto insects that aren’t necessarily designed to carry pollen—especially nearly hairless creatures such as beetles. Other plants in the Onagraceae also have viscin threads, although some—like those found in Fireweed (Epilobium angustifolium) — can only be seen with magnification.
Beyond the viscous pollen strings Oenothera species have developed to aid their pollination, at least one of their kind has evolved another trick for tempting insects to stop by. Scientists have found that Oenothera drummondii, the pretty beach evening primrose, can increase the sugar content of its nectar within three minutes of its flowers being vibrated by visiting bees.
How the scientists figured that out I can’t say, but its very improbability makes me smile. It seems some flowers actively invite bees to drop by for a sip of nectar, as well as a little packet of pollen to go.