Flies are the second-greatest pollinators after bees. As pollinators, flies are undervalued, understudied and underappreciated, according to John Whittlesey, owner of Canyon Creek Nursery and Design.
Whittlesey spoke at the Northern California Botanists’ Symposium last year. He said he started doing macrophotography several years ago, taking photos of “whatever ends up on a flower,” and saw lots of flies. He discovered that flies are very photogenic, and he wondered what they were doing.
Bees gather pollen to feed their larvae; flies don’t. Whittlesey learned that syrphid flies required the protein, carbs, lipids, vitamins and minerals found in pollen for normal ovarian development and possibly to initiate sperm production.
In addition, flies use the sugars in flower nectar as fuel for flight, and they seek mates, find shelter and lay eggs near flowers. Because flowers are a few degrees warmer than the surrounding air, flies also settle on flower heads for “solar basking,” according to Whittlesey, to warm themselves. At higher elevations, he observed flies on flowers earlier in the day than bees, because flies can tolerate cooler temperatures than bees. On the tidy tips wildflower, for instance, Whittlesey observed seven different flies. On serviceberry flowers, he found six different flies.
On flowers whose stamens were longer than the pistils, Whittlesey observed some flies hanging onto the stamens and bringing pollen up to their mouth parts. Delicate as the stamens were, the flies were even more delicate. Other flies hovered and licked the pollen off the stamens.
Much less research has been done on fly than bee pollination. One study in the Rockies found that bees were more effective pollinators than flies, but because there are more flies than bees, and therefore more flower visits, the flies actually transferred more pollen.
To be effective pollinators, they have to transfer pollen regularly from the male to the female parts of a flower, Whittlesey said, whether between plants of the same kind, between flowers on the same plant or within the same flower.
Although 70 families of flies regularly visit 555 flowering plant species, Whittlesey identified three main families of flies that commonly appear on flowers:
Syrphids, also known as hover flies or flower flies
The 6,000 species of syrphids are the most important pollinating group other than bees. They collect nectar, pollen, or both. Those species with shorter mouth parts can get pollen from open flowers, such as yarrow or daisies. Species of syrphid flies with longer mouth parts also feed on tubular flowers.
Bee flies are nectar eaters and “eminent pollinators,” Whittlesey said, and some species have special adaptations for eating pollen. Like bees and syrphid flies, they also hover around flowers. A study of bee flies on triteleia, a native bulb, found that bee flies visited the flowers regularly.
Tachinid, or bristle flies
Tachinid flies are “abundant and conspicuous” nectar eaters, Whittlesey noted. Their bristly bodies collect pollen, but their importance as pollinators is unknown, he added.
Another group of flies, the robber or assassin flies, hang out near flowers to hunt and feed on other insects.
One of Whittlesey’s most interesting discoveries was the amount of mimicry among flies. There are flies that look almost exactly like specific bees or wasps, down to the orange spot on the abdomen.