Meet the Pollinators: Flies?

By Jill Eisenstein, Master Gardener Volunteer

What is your first reaction when you see a fly? Shoo it, swat it, move away from it? I had a Family and Consumer Sciences teacher in middle school that showed us a graphic close-up video of a fly regurgitating on food; unforgettable. So – annoying at best, repulsive at worst, that describes flies, right? Time for a reset.

Flies are important pollinators. Ubiquitous little creatures, they live in nearly every environment on earth and are second only to bees in terms of importance for pollination. Research indicates they help pollinate more than 100 types of crops and hundreds of species of flowers. One crop you can thank them for is chocolate; a tiny midge (Forcipomyia squamipennis) is the primary pollinator of cacao!

Flies may not have the charm of hummingbirds, the grace of butterflies or the wow factor of giant silk moths, but they bring distinct advantages to the pollinator world. They tolerate a wide range of temperatures and will be out in the rain and wind that might keep bees and butterflies at home. They also tend to forage more widely; with no nests and no young to feed, they have no need to stay close to home.

The flies responsible for pollination in this part of the world are primarily houseflies, blowflies, hoverflies and biting midges. All of them go through complete metamorphosis – egg, larva, pupa and adult. Adults become the pollinators, but some flies are garden helpers even in the larval stage. These larvae are predators, feeding on insects such as scales or aphids and some, like tachinid and syrphid flies, help provide biological control of garden pests by parasitizing them.

Flies pollinate plants in ways similar to bees. They visit flowers to drink nectar for energy or get nutrients for sexual maturation; many are hairy and trap pollen on their head and thorax. Like bees, some flies are generalists while some visit specific flowering plants. Researchers at Penn State found the generalists preferred flower species in the Asteraceae, Rosaceae and Apiaceae families.

Two fly families stand out in the pollinator world: syrphid flies (family Syrphidae) and blowflies (family Calliphoridae). According to biologist Romina Radar and colleagues from Australia, New Zealand and the US, the most observed flies on flowers are adult syrphid flies, also known as “flower flies” and sometimes called “bee mimics”. They use their long tongues to drink nectar. Their yellow and black striped bodies mimic bees, but two ways to distinguish them are 1) flies have only one set of wings while bees have two, and 2) the flight pattern of these flies is unique; in fact, it’s what gave them their common name “hoverflies”. There are more than 400 species of hoverflies in the northeastern US alone. Blowflies, on the other hand, are usually metallic looking and noisier in flight. Although some adults seek nectar, blowflies have sponge-like mouthparts and tend to visit flowers that imitate their preferred fare -- rotting meat, dung, blood and carrion.

One of our earliest spring flowers, Skunk cabbage (Symplocarpus foetidus), relies on such carrion flies for pollination and emits a strong putrid odor to attract them. If you visited wetlands in February, you probably observed it had already pushed its stunning curl of deep purple right through snow on the frozen forest floor. These flowers emerged long before most bees had become active, but the flies got there! Later-blooming flowers such as pawpaws (Asimina triloba), Stinking benjamin (Trillium erectum), and Dutchman’s pipe vine (Aristolochia macrophylla) also attract their pollinator flies with putrid odors and meat-like colors. Jack-in-the-pulpit flowers have a fungus-like smell that attracts fungus gnats.

Some researchers have turned toward flies in hopes that they might become another managed pollinator source like honeybees to help with world food supplies. One researcher in Australia is using blowflies to pollinate mango trees and is providing piles of rotting meat to keep the bees in the orchard. The smell is wretched, but said one researcher, at least the flies don’t sting.