What makes an efficient pollinator?
Flower visitors vary greatly in their efficiency as pollinators: the likelihood that an animal will deliver a viable pollen grain to another flower of the same plant species. An efficient pollinator gathers a lot of pollen from each flower it visits, and it visits several flowers of the same
species before switching to other species. Inefficient pollinators pick up few pollen grains; or eat most of them right away; or deliver them to the wrong plant species. Pollinator efficiency is determined by the structure of an insect and its behavior.
Efficient pollinators are covered in dense setae (hairs or bristles) that serve as “pollen Velcro”, and may be concentrated on the face and “shoulders” (upper side of the thorax or scutellum) in species that plunge head-first into flowers, and on the underside of the abdomen of species that tend to walk on top of composite flowers or umbels, like leaf-cutter bees (Megachilidae). Some bees and flies are completely covered in hair (e.g. bumblebees and bee-flies), while most of the hover-flies have only very fine, short, sparse fuzz that can best be seen with a microscope. Some bees have evolved specialized structures such as scopae (bristly combs) and corbiculae (pollen baskets) on their hind legs. Other bees simply store pollen in their mouths the way squirrels pack nuts and seeds in their cheeks.
Andrena caerulea, American Camp, San Juan Island
On average, individual flies pick up less pollen than bees; but flies are more numerous, and with no nests to provision, they eat less of the pollen they transport, and travel farther with it.
Faithfulness, or constancy is the probability that a flower visitor will fly directly from one flower to another of the same species. If other species of flowers are visited along the way, most pollen will be delivered to the wrong address and will never germinate. Some bee species are specialists on a single flower. Most, like bumblebees, will work one type of flower until it is finished for the season, then switch to another. This is very efficient. Flies are more likely to eat whatever they encounter, but they prefer yellows and composites, so there are often only two or three suitable species in bloom at a time.